Cataract Surgery, 3rd Edition

PART V – Special Techniques for Cataract Extraction

Chapter 33 – Nanophthalmos, Relative Anterior Microphthalmos, and Axial Hyperopia

Richard K. Parrish II, MD,
Kendall Donaldson, MD, MS,
Marianne B. Mellem Kairala, MD,
Richard J. Simmons, MD


Contents

  

   

Classification and Terminology

  

   

Preoperative Assessment

  

   

Intraocular Lens Assessment in Eyes with Extremely Short Axial Length

  

   

Modifications in Cataract Surgery Technique in Microphthalmic Eyes

  

   

Postoperative Monitoring

  

   

Complications of Surgery

CHAPTER HIGHLIGHTS

  

>>   

Anatomic classification of small eyes

  

>>   

Uveal effusion syndrome

  

>>   

Angle closure and the lens

  

>>   

Surgical techniques

  

>>   

Intraocular lens power calculation and piggyback lenses

Understanding the anatomic differences in small eyes gives surgeons a framework for surgical planning in these challenging cases.

Classification and terminology

Microphthalmos – the result of developmental arrest of ocular growth during gestation – usually occurs sporadically, but can be inherited in an autosomal-dominant or recessive pattern. It is often associated with systemic diseases, including genetic or environmental disorders, or those of unknown causes.[1,][2] Evaluation of microphthalmic patients should be interdisciplinary, with special attention given to the history of the disease and examination of other family members.

Because microphthalmos is a heterogeneous condition, with many possible and overlapping presentations, the authors prefer to use an anatomic classification for these eyes, based on anterior chamber depth and total axial length (Tables 33-1 and 33-2).[3,][4]


Table 33-1   -- Relationship between axial length and anterior-chamber depth

Axial length  

Shorter

Longer

   Simple microphthalmos =

Relative anterior microphthalmos

Nanophthalmos

Colobomatous microphthalmos

Complex microphthalmos

Axial hyperopia

Normal

Modified from Holladay J: Achieving emmetropia in extremely short eyes with two piggyback posterior chamber intraocular lenses, Ophthalmology 103:1118-1123, 1996; Auffarth GU, Blum M, Faller U et al: Relative anterior microphthalmos: morphometric analysis and its implications for cataract surgery, Ophthalmology 107:1555-1560, 2000.

 


Table 33-2   -- Anatomic classification of short eyes

  

2.1 

Short anterior-chamber (AC) depth with short axial length

  

2.1.1 

  Nanophthalmos (simple microphthalmos)

  

2.1.2 

  Colobomatous microphthalmos

  

2.1.3 

  Complex microphthalmos

  

2.2 

Short AC depth with normal axial length

  

 

Relative anterior microphthalmos

  

2.3 

Normal AC depth with short axial length

  

 

Axial hyperopia

 

Short anterior chamber depth with short axial length: nanophthalmos, colobomatous, and complex microphthalmos

Duke-Elder[5] in 1964 described three categories of microphthalmos: simple microphthalmos, or nanophthalmos, which is a short eye with no other associated morphologic anomalies; colobomatous microphthalmos, related to incomplete closure of embryonic fissure; and complex microphthalmos, not related to closure of the fissure, but associated with systemic anomalies and other anterior and posterior malformations of the eye.

Nanophthalmos (Simple Microphthalmos)

Nanophthalmos is a rare condition characterized by a total axial length that is at least two standard deviations below the mean for age or less than 20.5 mm.[6] There are no systemic or other ocular morphologic abnormalities.[5,][7] Many authors, including Duke-Elder and Naumann, have synonymously used the terms “simple microphthalmos” and “nanophthalmos.”[5,][6,][8,][9]

Although microphthalmos in general is a fairly common ocular malformation found in all races,[1] nanophthalmos is a very rare form. Most cases of nanophthalmos are sporadic, but recently genetic studies of several nanophthalmic pedigrees showed autosomal recessive inheritance.[10] In 1998, the first human gene locus associated with autosomal-dominant nanophthalmos was identified on chromosome 11 and named NNO1.[11] A family history of nanophthalmos is often unknown, but patients sometimes report relatives, without a specific gender predilection, who were blind from unknown cause, consistent with angle-closure glaucoma.

Younger patients usually present for evaluation of poor visual acuity related to a high refractive error, which usually is fully correctable with corrective lenses. Consequently, these patients often remain undiagnosed until middle age, when complications most commonly develop. If one or more clinical features are present, the diagnosis of nanophthalmos should be considered. If left untreated, this condition often results in blindness.

Clinical features

The most prominent clinical features seen in nanophthalmos are described here and are summarized in Table 33-3.


Table 33-3   -- Nanophthalmic clinical features

Short axial length

Moderate to high axial hyperopia

Small cornea

Thick sclera

Shallow anterior chamber

Thickened choroid

Marked iris convexity

Angle closure glaucoma*

Normal or increased lens thickness

Uveal effusions*

High lens/eye volume ratio

Exudative retinal detachment*

*

May develop these features in the late course of the disease.

 

Nanophthalmos is a bilateral disease in which the eyes are deeply set with narrow palpebral fissures (Figure 33-1).

  

 

Figure 33-1  Nanophthalmos. Eyes deeply set with narrow palpebral fissures.

 

 

The eyes are uniformly small, usually about two-thirds of the normal volume, but have an increased crystalline lens to total eye volume ratio. The crystalline lens can be normal or can have a slightly increased anteroposterior length (Figure 33-2). Nanophthalmos is typically associated with microcornea, with corneal diameters between 9.5 and 11 mm. The central and peripheral anterior chamber depths are also very shallow, from less than 1 mm to 2.7 mm.[6] Pupils usually dilate poorly, and eyes have a wide amplitude of intraocular pulse pressure, the mechanism of which is unknown. The disproportion between lenticular and ocular volume contributes to the shallowing of the central anterior chamber and to the marked peripheral convexity of the iris, and is responsible for the appearance of “Vesuvio iris” (Figure 33-3). Early in life, the angle can remain wide open despite the central shallowing of the anterior chamber, but later the peripheral anterior chamber progressively shallows and closes the filtration angle. High hypermetropia with a range between +7.25 and +20.00[6]typifies this condition. Depending on the corneal and lens refractive power, nanophthalmic patients can have lower degrees of hyperopia or, very rarely, myopia.[2,][5–8,][12–14]

  

 

Figure 33-2  Schematic drawing of nanophthalmic eye showing the main ocular findings. A, short axial length; B, reduced corneal diameter; C, shallow anterior chamber; D, normal or increased lens thickness and high lens/eye volume ratio; E, thickened uveal tract with intrachoroidal or suprachoroidal peripheral uveal effusions; F, thickened scleral wall.

 

 

  

 

Figure 33-3  Gonioscopic view of a nanophthalmic eye showing marked iris convexity, “Vesuvio iris.” Note the patent surgical iridectomy (arrow).

 

 

Nanophthalmic eyes are also characterized by very thick choroid and sclera. In normal eyes the mean combined sclerochoroidal thickness is approximately 1.01 mm, but nanophthalmic patients have values between 0.75 and 4 mm (mean of 2.78 mm), as described in a series of 32 eyes of 16 patients with nanophthalmos.[6]

Retinal findings in nanophthalmic patients include chorioretinal folds, macular hypoplasia, cystic macular degeneration, retinal pigmentary degeneration, retinitis pigmentosa, and disc drusen.[15–18] The distinguishing feature of nanophthalmic eyes is the abnormally thick and inelastic sclera.[8,][19,][20] Ultrastructural and histochemical studies demonstrate abnormal scleral collagen fibers (Table 33-4) and an alteration in the metabolism of glycosaminoglycans and fibronectin production by scleral cells.[21–28] It has been speculated that the restriction of normal eye growth by the thick sclera is the underlying pathophysiology of nanophthalmos.


Table 33-4   -- Scleral abnormalities in nanophthalmic eyes

Collagen fibers

Variations in the size of collagen fibers[23,][27,][28]

Increased fraying and splitting of collagen fibers[23]

Disordered lamellar arrangement of collagen bundles[22,][29]

Extracellular matrix

Glycosaminoglycan deposits[22–24]

Increased fibronectin content[28]

 

Untreated nanophthalmic eyes are likely to develop gradual progressive narrowing of the anterior chamber angle, formation of peripheral synechiae, and elevation of intraocular pressure (IOP). Nanophthalmic eyes characteristically develop spontaneous uveal effusions in the late course of the disease and may be associated with exudative retinal detachment (Figure 33-4).

  

 

Figure 33-4  Proposed pathogenesis of uveal effusion and retinal detachment in nanophthalmic eyes. RD, Retinal detachment; RPE, retinal pigment epithelium.

 

 

Brockhurst described the association of nanophthalmos and spontaneous uveal effusion in 1974.[8] The pathophysiology of nanophthalmic uveal effusion has been postulated by Brockhurst to be related to the compression of vortex veins and by Gass, to be associated with the reduced scleral permeability to proteins. In both cases, the primary cause is the abnormal sclera found in nanophthalmic eyes.[13,][19,][27,][30–32]

Nanophthalmic uveal effusion (intrachoroidal, suprachoroidal, or both)[33] can occur spontaneously, and usually develops between the fourth and seventh decades, or acutely after intraocular surgery. Uveal effusion can lead to choroidal and exudative retinal detachment spontaneously or after intraocular procedures. The choroidal detachment causes retinal pigment epithelium dysfunction, which may promote leakage of serous fluid into the subretinal space and lead to an exudative retinal detachment. The progressive accumulation of noninflammatory fluid in the choroid leads to an increase in the sclerochoroidal[34] thickness and can be noted by ultrasound or magnetic resonance imaging.

Chronic angle-closure glaucoma usually develops painlessly in middle age and is associated with a progressive increase in intraocular pressure. As the crystalline lens enlarges anteroposteriorly with age, relative pupillary block increases, with progressive shallowing of the central and peripheral anterior chamber, narrowing and gradually closing off the angle.[33] Angle closure is usually precipitated by an anteriorly located peripheral annular uveal effusion or exudative retinal detachment, which causes the forward rotation of the ciliary body, forward movement of the peripheral iris, and consequent increase in the relative pupillary block.[9,][33] Pupillary block, an early component of this angle closure, is not the sole mechanism of angle closure. Therefore, a patent peripheral iridotomy, which eliminates pupillary block, does not prevent progressive choroidal effusion that may result in further angle closure.

Colobomatous Microphthalmos

Colobomatous microphthalmos results from involution of the primary optic disc or incomplete closure of the embryonic fissure,[5] which is usually closed by the sixth week of gestation. The typical colobomatous defect is located inferonasally and is frequently associated with other ocular anomalies. According to the grade of involvement regarding the ocular and surrounding tissues, these anomalies can vary from a small iris coloboma or choroidal coloboma to clinical orbital cysts, with extrusion of intraocular tissues through the sclera.[1,][4]The visual pathways and the occipital cortex may also be hypoplasic.[5]

Complex Microphthalmos

Complex microphthalmos is associated with other syndromes (Table 33-5)[35] and further anatomic malformations of the anterior or posterior segment of the eye, but it is not related to the incomplete closure of the embryonic fissure. This is a heterogeneous group of conditions in which the microphthalmia is secondary to the other ocular abnormalities.[4,][5]


Table 33-5   -- Syndromes associated with microphthalmia

Trisomy 13 (Patau's syndrome)

Chromosome 18 deletion syndrome

Congenital rubella

Hallermann-Streiff syndrome

LSD (lysergic acid diethylamide) embryopathy

Goldenhar's syndrome

Oculodentodigital syndrome

Pierre Robin syndrome

Oculocerebrorenal syndrome

Focal dermal hypoplasia

Francois’ syndrome

Ullrich's syndrome

Modified from Ritch R: Glaucoma related to other ocular disorders. In Richt R, Shields M, editors: The secondary glaucomas, St Louis, 1982, Mosby, pp 55-57.

 

Congenital cataracts are common[1] in these patients and are usually associated with poorly developed or defective retinal and optic nerve structures, which further compromise the visual prognosis.

Short anterior chamber depth with normal axial length: relative anterior microphthalmos

In 1980, Naumann coined the term ‘relative anterior microphthalmos’ (RAM) to describe smaller than usual eyes that did not fit into any classification. He described eyes with an axial length greater than 20 mm and a horizontal corneal diameter between 9 and 11 mm, but with disproportionately smaller anterior segment volumes.

The terminology and classification used for RAM and microcornea are confusing. Microcornea is a classification based exclusively on the dimension of the cornea (smaller than 10 mm) and can occur as part of the simple, colobomatous, and/or complex microphthalmos.

RAM refers to those eyes with a normal axial length and a disproportionate smaller anterior segment, despite the corneal diameters, which may be in the microcornea or in the lower normal range. These eyes show no other morphologic macroscopic malformations and can easily be overlooked at slit-lamp examination. RAM is more common than nanophthalmos and high hyperopia (Table 33-6).


Table 33-6   -- Anatomic parameters in relative anterior microphthalmos vs. nanophthalmos

Ocular Parameters

Relative Anterior Microphthalmos

Nanophthalmos

Corneal diameter

Average: 10.7 mm

Average: 10.3 mm

Range: 9–11 mm

Range: 9.5−11 mm

Anterior chamber depth

Average: 2.2 mm

Average: 1.46 mm

Range: 0.98−3.70 mm

Range: 1−2.7 mm

Anteroposterior lens thickness

Average: 5.05 mm

Average: 5.18 mm

Range: 3.49−6.46 mm

Range: 4.20−7.26 mm

Total axial length

Average: 21.92 mm

Average: 17 mm

Range: 20.29–23.89 mm

Range: 14.5−20.5 mm

Refractive error

Average:−0.13 diopters

Average: +13.60 diopters

Range: −6.0–+7.5 diopters

Range: +7.25− +20.00 diopters

Modified from Auffarth GU, Blum M, Faller U et al: Relative anterior microphthalmos: morphometric analysis and its implications for cataract surgery, Ophthalmology 107:1555-1560, 2000.

 

Like nanophthalmic patients, RAM patients are at high risk of developing chronic angle-closure glaucoma as a result of the crowded anterior segment; however, these eyes do not have scleral abnormalities or uveal effusion. If pupillary block is untreated, then angle-closure glaucoma will progress.

Normal anterior chamber depth with short axial length: axial high hyperopia

A third group of microphthalmic eyes are characterized by high hyperopia but normal anterior chamber depth, despite the short axial length. These eyes usually do not have the same complications as the two previous groups and the morphology of the anterior chamber is normal. The main issue in this group of patients is the high refractive error.

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Preoperative assessment

The potential hazards of cataract surgery or glaucoma surgery in eyes with nanophthalmos and, to a lesser extent, RAM, demand the identification of patients with these conditions preoperatively to allow for the necessary prophylactic measures to be taken, thus, minimizing the likelihood of serious complications during cataract surgery and in the postoperative period.

Nanophthalmos

Evaluation for Potential Nanophthalmic Glaucoma

Eyes with small corneal diameters and refractive errors of greater than 8 diopters (D) of hypermetropia, and axial lengths of less than 20.5 mm, should be evaluated for other features of nanophthalmos before intraocular surgery is performed, and special attention should be given to evaluation of potential nanophthalmic glaucoma.

Indentation or compression gonioscopy can be performed with a mirrored goniolens, such as Zeiss or Sussman. The extreme iris convexity in microphthalmic eyes and indentation of the small cornea that can produce Descemet's folds, obscures visualization of the angle. A child's 12 mm Koeppe lens, bilaterally inserted, offers a clear view with simultaneous comparison of both eyes and allows for examination by multiple observers without moving the lens;[33] however, this requires special equipment and proficiency with this skill.

Ultrasound biomicroscopy (UBM) may be used to document the relationship between the anterior chamber structures and the anatomy of the angle to identify peripheral choroidal effusions (Figure 33-5).

  

 

Figure 33-5  Ultrasound biomiscroscope image of nanophthalmic eye showing the anatomy of the angle. Note the thickened choroidal layer.

 

 

B-scan ultrasonography may be used to verify the thickness of the sclera and choroid, as well as the presence of uveal effusions that can be difficult to detect by clinical exam.

In view of the high risk of angle-closure glaucoma, it is important to evaluate and document the optic disc in all eyes, irrespective of the IOP. Optic nerve visualization and photography may not be possible due to poor dilation in nanophthalmic eyes. If the angle is extremely narrow, laser iridotomy should be performed before attempting pharmacologic mydriasis. In these cases, documentation of the disc may be achieved by drawings or imaging techniques that do not require dilation, such as confocal scanning laser ophthalmoscopy and scanning laser polarimetry.

In nanophthalmic patients, the prophylactic and therapeutic approach depends on the status of this condition, which may be divided into five stages (Table 33-7).[33]


Table 33-7   -- Stages of nanophthalmic glaucoma

Stage 1

Narrow angles; nanophthalmos recognized/no elevation of intraocular pressure

Stage 2

Progressive narrowing of the angle/angle closure threatened but not present

Stage 3

Progressive narrowing of the angle/angle partially closed

Stage 4

Extensive angle closure/increased intraocular pressure controlled with medical therapy

Stage 5

Angle closed by synechiae/intraocular pressure uncontrolled by medical therapy

 

Therapy of Nanophthalmos

In 1980, Brockhurst[30] described the successful use of vortex vein decompression by the dissection of a large partial-thickness sclerectomy around the vortex veins for the treatment of nanophthalmic uveal effusions. In 1983, based on his hypothesis that the primary cause of the idiopathic uveal effusion syndrome was the impairment of transscleral outflow of proteins by the abnormal sclera, Gass[19] proposed a new surgical technique: a quadrantic equatorial partial-thickness sclerectomy. The successful use of this scleral-thinning procedure, without vortex-vein decompression, in patients with nanophthalmic uveal effusion, supports the hypothesis that the barrier effect of sclera is more important than the vortex-vein obstructing effect.[31,][36–38]

Uveal effusion usually resolves within several months of surgical intervention.[19,][31,][38] However, recurrence of uveal effusion several months or years after the first scleral thinning procedure has been reported, in which case, repetition of the surgery proved to be successful.[31,][36] Recurrence can be explained by the wound healing at the sclerectomy, with the scar tissue blocking the bypass created surgically.[39] Recently, new techniques have been proposed to minimize the scarring of the sclerectomy sites and to promote long-term control of uveal effusion. Akduman, Adelberg, and Del Priore[40] described the successful use of topical mitomycin-C (0.3 mg/mL for 21/2 min) over the sclerectomy sites, and Krohn and Seland[41] reported the use of absorbable gelatin film to cover the scleral bed after the procedure in order to maintain a low resistance to transscleral outflow and prevent scarring.

In stage 1 and 2, nanophthalmic eyes with anterior chamber angles that are judged to be narrow and occludable angles with indentation gonioscopy, prophylactic laser iridotomy is indicated. Uveal effusions after prophylactic laser trabeculoplasty and retinal photocoagulation in nanophthalmic patients have been reported.[36,][42]

The thicker than normal iris in nanophthalmic eyes makes penetration of the stroma with neodymium:yttrium-aluminum-garnet (Nd:YAG) laser usually more difficult than in other eyes with chronic narrow angle configuration. To facilitate perforation, argon laser burns should be applied to thin the proposed iridotomy site before final treatment with the Nd:YAG laser.[43]

Although successful iridotomies eliminate pupillary block, the peripheral iris can remain convex and the angle narrow in these eyes. Despite the patent iridotomy, the angle may progressively narrow (stage 3). In this stage, laser gonioplasty, or peripheral iridoplasty, can be used to flatten the peripheral iris, widen the angle, and prevent the development of peripheral anterior synechiae. Gonioplasty in some cases may cause separation of early peripheral anterior synechiae.

To perform gonioplasty, after miosis is achieved with 2% pilocarpine hydrochloride, a mirrored contact gonioscopic lens is used to visualize the peripheral iris and to apply low-power argon laser burns. With a spot size of 250–500 μm and a laser power of 200 mW, and a duration or 200 ms, the power is slowly increased until a localized iris burn with contraction is observed. Gonioplasty is applied to 3–4 clock hours at a time (Figures 33-6 and 33-7, Table 33-8), and caution is taken not to over-treat, as that could cause injury to the dilator muscle fibers and result in mydriasis. Widening of the angle achieved by gonioplasty may gradually lessen, in which case repeated iridoplasty may be of value. The procedure may be performed multiple times until it is no longer beneficial. In many cases, the anterior chamber angle may be prevented from closing for several years (Figure 33-8).

  

 

Figure 33-6  Gonioplasty burns from 1 to 2 clock hours.

 

 

  

 

Figure 33-7  Appearance of nanophthalmic eye following successful opening of the entire angle with argon laser gonioplasty, done in multiple sessions.

 

 


Table 33-8   -- Laser settings for gonioplasty in nanophthalmos

Power

200–500 mW

Spot size

250–500 μm

Time

0.2–0.5 s

Applications

Three to five burns per clock hour

Each session

3–4 clock hours

 

  

 

Figure 33-8  Nanophthalmic eye showing the narrow, but open angle maintained with multiple sessions of argon-laser gonioplasty.

 

 

If gonioplasty fails to maintain a functioning open angle, before IOP is permanently elevated, sclerectomies may be employed to widen the angle by reducing the amount of fluid in the suprachoroidal space. The technique has been used since the 1980s and is illustrated in Figures 33-9A–E.

   

 

Figure 33-9  A, After a radial incision is created in both lower quadrants, a full-thickness triangular scleral flap 4 × 4 × 4 mm is created, with its apex at 3.5 mm from the limbus, over the pars plana. B, Flap is dissected posteriorly to expose the underlying choroid. C, Cyclodialysis spatula is then advanced to both sides of the sclerectomy, tangentially to the sclera, to drain any existing fluid from the suprachoroidal spaces. D, Full-thickness scleral flaps are excised completely, leaving two full-thickness triangular sclerectomies in each lower quadrant. E, Conjunctival incisions are closed with one or two 10-0 interrupted nylon sutures.

 

 

At stage 4, when angle closure is extensive and the IOP is substantially elevated, the authors opt to treat with topical aqueous humor suppressants, such as beta blockers, carbonic antihydrase inhibitors, and alpha agonists until they become ineffective. Stage 5 is defined by extensive synechial closure of the angle at which time further laser iridotomy, laser gonioplasty, and medical therapy are ineffective. At this point, two IOP-lowering strategies are available: filtering surgery or a cycloablative procedure.

In stage 5, when glaucoma is such that trabeculectomy can be temporarily deferred, the authors prefer to perform prophylactic anterior sclerectomies (as shown in Figures 33-9A–E) in the two lower quadrants, 4 to 6 weeks before the filtering procedure to allow recovery before trabeculectomy. However, if the IOP is so severely elevated that immediate surgery is necessary, anterior sclerectomies may be performed at the time of the trabeculectomy. We modified our trabeculectomy procedure to minimize hypotony in nanophthalmic eyes, using high-viscosity viscoelastic in the anterior chamber to prevent sudden decompression of the anterior chamber, replacing sutures in the flap to allow rapid closure and making scleral flap closure tighter than usual.

Cyclodestructive procedures are generally used as a last resort when other attempts to control IOP have failed.

Phacoemulsification in nanophthalmic patients may be performed when IOP is well controlled and should be performed after, or simultaneously with, prophylactic sclerectomies. The use of early phacoemulsification as a therapy for narrow angles and as a prophylaxis for angle-closure glaucoma in nanophthalmos is hypothetical and controversial. It can be considered in the face of progressive narrowing of the angle despite other treatments, before extensive synechiae are formed.

Colobomatous and complex microphthalmos

Despite different pathophysiologies, colobomatous and complex, microphthalmos can be assessed in the same manner because the common and more important feature is the presence of associated ocular or systemic pathologic conditions. In both cases, the final visual acuity is extremely variable and depends on the grade of disorganization of ocular tissues and integrity of the visual pathway. The presence of congenital cataracts is common in both diseases[1] and is usually associated with poorly developed or defective retinal and optic nerve structures that will further compromise the visual prognosis. For these reasons, cataract surgery is not always indicated in these patients. In children, the health and systemic prognosis of the child should be carefully considered, and the pediatrician should perform a thorough evaluation before the decision to perform cataract surgery is made.

In viable eyes with some expectation of visual acuity improvement, some surgeons[44] recommend using corneal diameter measurements to determine surgical planning as follows:

  

   

Eyes with corneal diameters of less than 5 mm probably should not have cataract surgery unless the cataracts are bilateral. In such cases, surgery without an intraocular lens (IOL) is indicated. Surgery should be performed in the eye with the largest cornea and longest axial length or the one presenting the most normal ocular structures. The correction of aphakia in these cases should be done with spectacles.

  

   

If the corneal diameter is between 6 and 9 mm, a rigid gas-permeable contact lens with a reduced diameter could be adapted.

  

   

In complex microphthalmic eyes with corneal diameters greater than 9 mm, a posterior chamber IOL should be considered.

Patients with choroidal colobomas are at a higher risk for retinal detachment and should be warned of the symptoms.[1] Therefore, any retinal area susceptible to retinal detachment should be treated prophylactically before surgery.

Relative anterior microphthalmos

Indications for cataract extraction in eyes with relative anterior microphthalmos are the same as for normal eyes. Auffarth et al.[4] found that lens removal alone leads to a significant IOP reduction and a decrease in the number of glaucoma medicines in eyes with RAM. The main complication after cataract surgery in these eyes is iridovitreal block. Because of the increased incidence of cornea guttata, transitory or permanent corneal edema can occur following cataract surgery. Another risk factor for cataract surgery in this group is the increased incidence of the pseudoexfoliation syndrome.[4]

Axial hyperopia

Phacoemulsification surgery in high hyperopic patients is carried out using the standard surgical technique. In addition to cataract extraction, clear lensectomy for refractive correction in the absence of significant opacity of the lens may be another indication for phacoemulsification. Other techniques for refractive correction in low-to-moderate hyperopic patients include laser in situ keratomileusis (LASIK), photorefractive keratectomy, and laser thermal keratoplasty.[45]

Clear lens extraction in high hyperopic eyes was first described by Lyle and Jin[46] in 1994, in six patients with refractive errors between +4.25 and +7.87, with axial lengths greater than 20.5 mm. No complications were reported during surgery or in the postoperative period. Refractive lensectomy for high hyperopia may have several advantages over other refractive techniques. It has the ability to correct higher refractive errors, offers a higher predictability of refractive outcome, does not cause irregular astigmatism, and it is less likely to result in regression hyperopia.[47] The risks of retinal detachment, loss of accommodation, and of endophthalmitis, in addition to the difficulty in obtaining an optimal IOL selection without using a piggyback IOL implantation, have prevented the widespread acceptance of this treatment for high hyperopia.

Regardless of the indication for phacoemulsification, either cataract extraction or refractive correction, there may be difficulties in the calculation of the implant power[48,][49] because of the disproportion between anterior and posterior segments. In addition, extremely high hyperopic eyes require high-power IOLs that are often not available. Piggyback implantation of two IOLs has been used in these eyes with mixed results.

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Intraocular lens assessment in eyes with extremely short axial length

Axial length measurement

The measurement of axial length is a critical determinant of the IOL power and a final refractive result in any patient. This is particularly true in extremely short eyes, where a minor error in axial length measurement can lead to a large and unexpected refractive error.[3] An accurate axial length determination can be a challenge in these eyes because most ultrasound biometry devices are calibrated with the average velocities of normal-sized eyes, and some still have limitations of axial length range, not measuring values of less than 21.5 mm.[49] Because of the short axial length, the posterior wall echo can be of great intensity, sometimes making it necessary to reduce the gain to obtain a clear echo (Figure 33-10).[48] Any flattening of the corneal surface during applanation biometry can also account for significant IOL calculation errors. Immersion biometry and/or optical biometry with partial coherence inferometry (PCI) (IOL Master, Zeiss Humphrey Systems) may be useful in these cases to obtain more accurate results.

  

 

Figure 33-10  Biometry of a nanophthalmic eye. Total axial length is 16.98 mm, lens thickness is 5.05 mm, and anterior chamber depth is 2.77 mm.

 

 

Lens power calculation

IOL power calculations for extremely short eyes remain a problem for the cataract surgeon. This can be attributed to the poor predictability of the older formulas. Current third-generation formulas (SRK/T, Holladay II, Hoffer Q, and Haigis) that take into account variables such as anterior chamber depth, corneal diameter, and lens thickness, have improved refractive predictability.[3,][49–51] Theoretical formulas have been shown to be more accurate than empirical ones in microphthalmic eyes.[52,][53] In a study of IOL calculations, Inatomi et al.[52] demonstrated that all of the formulas tested still showed a tendency for residual hypermetropia. A recent study of short eyes (axial length below 22 mm) showed that the Hoffer Q formula was significantly more accurate than the SRK-T in determining the correct IOL power.[54] This study, by Gavin and Hammond, is the largest retrospective and prospective examination of lens power (41 eyes) in this group of patients (axial length range 21.96–20.29 mm with a mean of 21.51 mm).[54] Until newer and more accurate formulas are available, calculations for eyes with an axial length of <22 mm should be made using more than one formula for comparison and should be weighted toward the results of the Hoffer Q formula.

Implant choice

Implant choice is another challenge for the surgeon, because rigid polymethylmethacrylate (PMMA) IOLs with a power greater than +45.0 D and foldable lenses greater than +40.0 D are not available.[54,][55] Implanting two PMMA posterior chamber lenses to achieve the desired total lens power was proposed by Gayton[56] in 1993, when +35.0 D was the maximum power available. These piggyback implantations later proved to offer better optical quality and cause less spherical aberration than a single lens with such a high dioptric power.[3] In 1996, Shugar[57] implanted two acrylic foldable lenses in six patients. He combined the advantages of small-incision surgery and the increased biocompatibility of the acrylic material. Acrylic lenses are most suitable for the higher power IOL that is implanted posteriorly in planned piggyback implantation, because the high refractive index of acrylic lenses allows them to be thinner and flatter than PMMA or silicone lenses. To avoid interlenticular membrane formation from the growth of lens epithelial cells between the IOLs, many surgeons advocate that the anterior piggyback IOL should have a silicone optic, be of low power, and be placed in the sulcus.

Gills and Cherchio[58] calculate the lens power by dividing the total power equally between the two lenses, whereas others prefer to place two-thirds of the lens power in the more posterior lens and one-third anteriorly.[49] This last option offers more advantages. By placing the more powerful lens posteriorly within the bag, spherical aberrations can be reduced. Inserting the least powerful lens in the sulcus facilitates access to the lens in case an exchange for a different power lens is necessary at a later date.

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Modifications in cataract surgery technique in microphthalmic eyes

Standard large-incision extracapsular cataract extraction in eyes with increased IOP results in a sudden drop in pressure to atmospheric values, which can lead to dilation of the choroidal vascular bed increasing the risk of intrachoroidal effusion, suprachoroidal or intrachoroidal hemorrhage, or expulsive hemorrhage. This technique is dangerous in eyes with short anterior-chamber depth, and specifically in nanophthalmic eyes, in which the inelasticity of the sclera makes these complications more likely.

Phacoemulsification allows avoidance of intraocular hypotony during surgery. When performing phaco surgery, these eyes should be carefully prepared and the IOP lowered to normal levels preoperatively. If topical medications and mechanical pressure-lowering devices (e.g., Honan's balloon) are not sufficient to lower the pressure to less than 25 mm Hg, 20% mannitol IV, 1–2 mL/kg body weight, should be employed 15–30 min before surgery. In rare cases, the surgery can be performed under general anesthesia with controlled hypotension, resulting in a reduction in arterial pressure, thus decreasing the risks associated with a dramatic pressure drop.[48]

Topical and intracameral anesthesia may have some advantage over peribulbar and retrobulbar anesthesia, as local infiltration causes an increase in orbital volume, which may precipitate an increase in posterior pressure and vortex vein congestion. Clear corneal incisions in eyes with shallow anterior chamber depths offer the surgeon a better anatomic approach to the lens and allow the employment of smaller and safer incisions.[49] A shorter and more anterior corneal tunnel will help to prevent iris prolapse and facilitate manipulation of the nucleus.[48] A temporal approach is especially useful in these microphthalmic eyes which are typically deeply set in a normal-sized orbit.

Maximal control over intraocular fluid dynamics is critical, and the new technology phaco machines offer great advantages over older ones. Paracentesis should be done carefully and gradually, avoiding the iris and anterior capsule. Intraoperative hypotony should be avoided as much as possible.

In severely hyperopic eyes, RAM, and nanophthalmic eyes, the shallow anterior chambers limit the distance between the anterior capsule and the corneal endothelium resulting in limited workspace for phacoemulsification. These eyes often have low endothelial cell counts and risk corneal decompensation following cataract extraction. The Arshinoff soft-shell technique using a cohesive viscoelastic in the center of the anterior chamber and a dispersive viscoelastic above it, may better stabilize the eye, decrease iris prolapse, and protect the endothelium.[49,][59]

Posterior synechiae and pupillary membranes, if present, should be dissected off the anterior capsule with an iris spatula. A 30-gauge needle can also be placed on the viscoelastic syringe to perform viscodisection while using the cutting edge of the needle to facilitate this process. If viscoelastic substance and pharmacologic agents (including 10% phenylephrine) fail to increase the pupil size, mechanical dilation of the pupil or sphincterotomies may be needed.

The size of capsulorrhexis should be selected in accordance with the IOL plan. For a single implant 5–6 mm is adequate. Generous use of viscoelastic is recommended to hyperinflate and maintain the anterior chamber depth, despite the posterior vitreous pressure. This will help depress and flatten the anterior capsule, thus preventing radial extension of the capsulorrhexis. For piggyback implantation, a larger (6.5–7 mm) capsulorrhexis is preferred so that the border of the anterior capsule does not cover the edge of the IOL.[60]

The use of a Kelman–Mackool phaco tip may facilitate surgical manipulation as its tip is bent downward toward the cataract.[49,][56] It is important to remember to enter the eye with the phacoemulsification handpiece in the irrigation position. Chilled balanced salt solution (BSS) may help to prevent incision burns in these shallow, anterior chambers.[60]

The anterior epinucleus should be removed before phacoemulsification of the nucleus to allow more space for working in the anterior chamber. During nucleus removal, a chopping technique[48,][49,][59] with high vacuum and short pulses of ultrasound may be helpful. The surgeon should start with a lower phaco power and increase it, as dictated by nuclear density, up to an efficient rate. Working in the nucleus at the level of the iris plane or in the posterior chamber avoids the endothelium and helps maintain the integrity of the incision.

The risk of a posterior capsular rupture is increased in these eyes because of the frequent presence of significant posterior pressure, weakened zonules, and floppy capsules.[61] To reduce the incidence of this complication, vacuum should be decreased during the removal of the last pieces of nucleus, and a second instrument should be used to protect the posterior capsule.

Automated irrigation–aspiration should be done thoroughly to prevent interlenticular opacification in the case of piggyback implantation.

If a single IOL is planned, there are no changes in the technique. In the case of piggyback implantation, the first lens should be placed in the bag, with the haptics oriented vertically. The second lens should then be inserted into the sulcus vertically with forceps, while maintaining downward pressure on the optic through the side port with the second instrument. The haptics of the two lenses should remain perpendicular to each other, increasing the separation between the optics and perhaps decreasing the incidence of interlenticular opacification.[56] Special attention should be given to viscoelastic removal from behind and between the two lenses.

The incision should be closed with a 10-0 nylon suture to protect against wound leakage and hypotony in the postoperative period, which could lead to a disastrous outcome, especially in nanophthalmic eyes.

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Postoperative monitoring

Careful observation is required following any anterior segment surgery in microphthalmic eyes. Some surgeons re-evaluate the patient 4–6 hours after surgery for IOP and anterior-chamber-depth evaluation.

Cataract extraction can dramatically improve the IOP in many cases with narrow angles (e.g., RAM). After surgery, eyes with persistent IOP elevation can be treated with laser trabeculoplasty because their angles are now more accessible.[48]

In patients who undergo piggyback IOL implantation, a dilated exam is recommended every 4–6 months for evaluation of interlenticular opacification.[48]

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Complications of surgery

Many complications that arise during and after surgery in microphthalmic eyes are the same as those observed in the routine phacoemulsification of normal-sized eyes. However, they tend to occur with much higher frequency, depending on the disproportion between anterior and posterior chamber depths.[48] Microphthalmic eyes are predisposed to positive vitreous pressure and iris prolapse resulting in technically more challenging cases.[60]

Corneal burns

Microphthalmic eyes with shallow anterior chambers are at greater risk of corneal burns given the proximity of the endothelium to the phacoemulsification tip. The use of chilled BSS has been shown to reduce the incidence of this complication. Inserting the phaco handpiece in the irrigating position with careful positioning of the tip in the anterior chamber prior to phacoemulsification also reduces this complication.

Rupture or disinsertion of posterior capsule

The posterior capsule is thin in microphthalmic eyes and very susceptible to ruptures that can extend dramatically because of the positive vitreous pressure present in these small eyes. Surgery may lead to partial or total zonular dialysis. There is no consensus on the best course of action when implantation of a posterior chamber IOL is not feasible due to lack of capsular support. An anterior chamber IOL is difficult to implant in a shallow anterior chamber and, if implanted, carries with it a high risk of subsequent corneal decompensation. Immediate or subsequent scleral fixation of the implant is an extremely risky maneuver associated with uveal effusions and hemorrhage.[48] This is especially true for nanophthalmic eyes, which are most susceptible to these complications.

Uveal effusions/hemorrhages

As emphasized earlier, nanophthalmos is a special group of microphthalmic eyes distinguished by its abnormal sclera. Uveal effusions in this group of patients can occur spontaneously or may be precipitated by cataract extraction, glaucoma surgery, argon laser trabeculoplasty, and even prophylactic laser iridotomy.[42,][62] Any eye surgery can precipitate or worsen a previous effusion by inflammatory increase of protein leakage or by reduction of transscleral hydrostatic pressure during intraoperative and/or postoperative hypotony. The effusions can be intrachoroidal, suprachoroidal, or both.

Suprachoroidal hemorrhage is more common in nanophthalmic patients. The sudden decompression of the eye during surgery may lead to choroidal engorgement that cannot be handled because of the inelastic sclera.

In case of sudden uveal effusion or hemorrhage, surgery must be interrupted, and tight closure of wounds must be performed. No further intervention is advised until the problem is resolved.

Retinal detachments

Exudative retinal detachment can occur in isolation after surgery or following postoperative uveal effusion if treatment of the latter is delayed or fails. Treatment of exudative retinal detachment consists of performing multiple sclerectomies, as described previously in this chapter (see Figure 33-9A–E).[37,][41,][42,][62,][63]

Angle-closure glaucoma

Many nanophthalmic and RAM eyes have narrow angles with crowded anterior chambers; therefore, the surgeon should be guarded to the possibility that a strong preoperative dilation on the day of surgery may induce a primary angle closure attack in the most susceptible eyes.[60] Secondary angle closure can be caused by sudden peripheral uveal effusion and/or exudative retinal detachment, which causes a forward rotation of the ciliary body, forward movement of the peripheral iris, and pseudophakic pupillary block.[9,][33]

If the aqueous is misdirected to the vitreous instead of to the posterior chamber, malignant glaucoma can occur. Cycloplegic and mydriatic therapy should be initiated, together with steroids, and Nd:YAG laser to the anterior hyaloid face through a patent iridectomy may be attempted. If suprachoroidal effusion is also present, surgical drainage of fluid may be required. Posterior vitrectomy should be kept as a last resort for treatment of this complication.[48]

Interlenticular opacification

Also known as interpseudophakos opacification or interpseudophakos Elschnig pearls, this late complication of piggyback IOL implantation recently became the subject of diverse studies and research.[57,][64,][65] It is characterized by the ingrowth of lens epithelial cells in the space between the two IOLs and results in a hyperopic shift in these patients. It occurs most commonly 1–3 years following the piggyback IOL implantation.

This complication appears to be related to the border apposition of the anterior capsule toward the surface of the anterior lens. To avoid this problem, the authors recommend the use of a larger capsulorrhexis and the insertion of one lens in the bag and the other in the sulcus, instead of inserting both lenses into the bag. Thorough cleaning of epithelial cells from the remaining anterior and posterior capsule should be performed. In patients with microphthalmos and narrow angles, both lenses may be inserted into the bag, as this maximizes the anterior chamber depth and angle dimensions.[57] Some surgeons believe that interlenticular cellular ingrowth is minimized when the optic of the anterior IOL is silicone.

If an interlenticular opacity develops, treatment varies from the use of Nd:YAG in the borders of anterior capsulorrhexis to an IOL exchange. If a lens exchange is necessary due to interlenticular opacification, the surgeon should base the IOL calculation on previous measurements because a hyperopic shift may have occurred.

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