Neurology: A Clinician's Approach (Cambridge Medicine (Paperback)), 1st Ed.

11. Focal limb weakness

Anatomy

Asymmetric limb weakness generally conforms to one of several common patterns (Table 11.1). The ability to recognize these patterns and the three to five most common causes of each pattern is a powerful clinical tool that will help in localizing and diagnosing most patients with focal limb weakness. However, any pattern-matching algorithm has limitations, and a firm grounding in the anatomy of the motor system (see also Chapter 10) becomes necessary when evaluating patients with asymmetric weakness, especially for those patients with atypical or multifocal deficits.

The motor pathways that control the extremities begin in the precentral gyrus of the contralateral motor cortex (Figure 11.1). Fibers controlling the leg are located medially within the cortex, while those controlling the arm are found superiorly and laterally. From the cortex, motor fibers descend through the subcortical white matter and the posterior limb of the internal capsule. The motor pathways continue in the cerebral peduncle and then in the ventral pons, crossing within the pyramids of the medulla. Motor fibers descend predominantly through the lateral corticospinal tract to reach the anterior horn cells. The nerve roots are derived from the anterior horn cells, and form the brachial or lumbosacral plexi, which give rise to the named nerves. These nerves innervate the muscles via the neuromuscular junctions. Although focal weakness may be derived from any of these structures, problems at the level of the neuromuscular junction and the muscles usually lead to proximal or generalized rather than focal weakness of the extremities.

Shoulder weakness

C5–6 radiculopathy

Pain and paresthesias are usually the most prominent symptoms of cervical radiculopathy. Because there is a substantial degree of overlap among adjacent myotomes, weakness secondary to radiculopathy is often subtle and involves arm abduction, arm flexion, and external rotation. Biceps and brachioradialis reflexes may be diminished or lost. Sensation is decreased over the lateral shoulder and arm. Patients with C5–6 radiculopathy substantial enough to produce weakness should undergo evaluation and treatment as discussed in Chapter 17.

Brachial plexopathy

It is important to maintain a healthy skepticism when considering the possibility of a brachial plexopathy, as lesions of the plexus are rare in comparison with radiculopathy and musculoskeletal conditions such as rotator cuff tendonitis. Brachial plexus lesions occur most commonly in the setting of trauma, cancer, and idiopathic brachial neuritis. They are usually quite painful, and for this reason, they are discussed in more detail in Chapter 16. Although memorizing the structure of the brachial plexus is among the most time-consuming tasks in learning neuroanatomy, detailed anatomic knowledge is often not necessary because most plexus lesions conform to one of three common patterns:

1. Upper trunk brachial plexopathy produces proximal arm weakness involving the suprascapular (infraspinatus/external rotation), axillary (deltoid/shoulder abduction), and musculocutaneous (biceps/elbow flexion) nerves. Sensory signs and symptoms involve the lateral shoulder, arm, forearm, and hand. It is often difficult to distinguish weakness due to an upper trunk lesion from a C5–6 radiculopathy: both imaging and electrophysiological studies are often necessary.

2. Lower trunk brachial plexopathy produces weakness of the hand muscles and sensory symptoms involving the medial hand, forearm, and arm. Patients with lower trunk plexopathies often have accompanying Horner’s syndrome.

Figure 11.1

Figure 11.1 Schematic of the motor system.

Table 11.1 Patterns of focal limb weakness

Table 11.1

3. Pan-plexopathy produces widespread weakness and sensory disturbances in the upper extremity. It is among the most common patterns of plexus lesions, especially in the context of trauma or neoplastic infiltration.

Periscapular weakness (scapular winging)

Long thoracic neuropathy

The long thoracic nerve innervates the serratus anterior, which stabilizes the scapula. A lesion of this nerve leads to medial scapular winging. This can be elicited on physical examination by examining the patient from behind while they attempt to do a pushup against a wall with their arms adducted to their trunk. The medial border of the scapula will elevate away from the posterior thoracic wall. Although isolated long thoracic neuropathy is a mononeuropathy, it is commonly produced by conditions that lead to brachial plexopathy such as idiopathic brachial neuritis.

Spinal accessory neuropathy

The other common cause of asymmetric scapular winging is spinal accessory neuropathy. This scapular winging is characterized by lifting of the superolateral border of the scapula away from the posterior thoracic wall upon attempted abduction and external rotation of the arm. In addition to scapular winging, spinal accessory neuropathy produces weakness of ipsilateral shoulder shrug (trapezius) and contralateral head turning (sternocleidomastoid). Common sites of spinal accessory nerve entrapment include the jugular foramen (in which case the glossopharyngeal and vagus nerves will also be involved; see Chapter 8) and the posterior cervical triangle.

Hand and finger weakness

Ulnar neuropathy

The ulnar nerve innervates most of the intrinsic hand muscles, and is therefore the mononeuropathy most likely to lead to hand muscle weakness. The usual cause of ulnar neuropathy is compression of the nerve at the elbow, a site at which it is relatively exposed to wear-and-tear trauma. Less common sites of ulnar nerve damage are in the forearm (especially in patients with diabetes1), and at the wrist in cyclists or in heavy older men who rest their weight on their wrists while using walkers. Numbness and paresthesias are usually present in the fourth and fifth digits, but in some cases, sensory complaints are completely absent. On examination, the hypothenar eminence and first dorsal interosseous (the fleshy muscle between the thumb and index finger) may be atrophic. Weakness of abduction of digits two and five is tested appropriately by applying finger pressure from the side and not by forcefully squeezing the fingers together. Flexion of the distal interphalangeal joints of digits four and five may be weak. Sensory loss is classically noted in the fifth digit and in the ulnar half of the fourth digit on both the palmar and dorsal aspects of the hand. Confirm the localization of a suspected ulnar neuropathy with nerve conduction studies. Mild ulnar neuropathies may respond to conservative measures such as wearing an elbow pad. More severe ones, particularly those that produce prominent weakness, are likely to require surgical release and transposition of the ulnar nerve.

Motor neuron disease

Focal hand weakness and wasting are often the first symptoms of amyotrophic lateral sclerosis, a disorder discussed in more detail in Chapter 10.

C8–T1 radiculopathy

A C8 or T1 radiculopathy may produce focal hand weakness, usually in association with neck pain and paresthesias radiating into the arm and hand. In general, the C8 and T1 nerve roots are relatively protected from degenerative spine disease, making this cause of hand weakness uncommon.

Central causes of hand weakness

Two common stroke syndromes lead to prominent hand weakness. The first is infarction of the contralateral cortical hand area, which produces weakness resembling an ulnar or radial neuropathy.2 The second is the dysarthria–clumsy hand syndrome, which results from lacunar infarction in the subcortical white matter, internal capsule, or pons.3 Contralateral cerebral cortical or subcortical lesions such as neoplasms, abscesses, or demyelinating lesions may also produce hand weakness. Many patients with parkinsonism come to neurological attention advertised as having hand weakness. Careful examination, however, reveals that the actual problems are bradykinesia and rigidity rather than weakness (Chapter 13).

Wrist and finger drop

Wrist and finger drop are special kinds of hand weakness that develop acutely and are characterized by weakness of extension at the wrist and fingers. The most common causes of wrist and finger drop are stroke and radial neuropathy.

Stroke

Middle cerebral artery infarction is the most common cause of hand and arm weakness secondary to stroke. Because the cortical representations of the hand and face are adjacent to each other in the motor homunculus, hand weakness is often (but not always) accompanied by facial weakness.

Radial neuropathy

“Saturday night palsy” is the classic radial neuropathy: an intoxicated patient, after a night of heavy sleep with their arm draped over a chair, awakens and finds that they cannot extend their wrist or fingers. Intoxication is not a prerequisite for acquiring a pressure palsy of the radial nerve, and it may develop in any state of prolonged immobilization such as surgery or even normal deep sleep. Other than compression of the radial nerve, the other common etiologies for radial neuropathy include vasculitis related to mononeuropathy multiplex and multifocal motor neuropathy with conduction block (see below).

Differentiating between stroke and radial neuropathy

The following examination techniques may help to distinguish between stroke and radial neuropathy:

1. Most compressive radial neuropathies occur at the level of the spiral groove, distal to the branches that innervate the triceps. Thus, a patient with a compressive radial neuropathy usually has profound weakness of wrist and finger extension with preserved arm extension. A patient with stroke may have triceps weakness in addition to wrist drop.

2. Testing extension of the interphalangeal joints may also help to differentiate between the two localizations. The prime extensors at these joints are the lumbricals, which are innervated by the median (digits two and three) and ulnar (digits four and five) nerves. Thus, extension of the distal interphalangeal joints should be preserved in a patient with radial neuropathy, but may be weak in a patient with stroke.

3. The pattern of sensory loss is also helpful in differentiating radial neuropathy from stroke. In a patient with radial neuropathy, sensory loss affects

Box 11.1 Postpartum leg weakness

Leg weakness following delivery occurs in approximately 1% of women.4 The etiologies are usually benign, and self-resolving, but some may be more serious:

• Femoral neuropathy is produced by excessive thigh abduction and external rotation during delivery. It produces weakness of hip flexion and knee extension with sensory loss in the anteromedial thigh and leg. Despite common belief, the femoral nerve is not actually compressed by the baby’s head, as the nerve does not enter the true pelvis.

• Obturator neuropathy is usually the result of compression of the nerve by the baby as it descends through the pelvis. It leads to weakness of thigh adduction and sensory loss in the medial thigh. Risk factors for compression include prolonged labor and cephalopelvic disproportion.

• Lumbosacral plexopathy is also caused by the descent of the baby through the pelvis. Due to the positioning of the component nerves within the pelvis, the peroneal nerve is often affected out of proportion to the other nerves of the lumbosacral plexus.

• Peroneal neuropathy may also result from compression of the fibular neck against stirrups during delivery. Deficits include weakness of foot dorsiflexion and eversion with sensory changes on the dorsum of the foot and lateral leg.

• Conus medullaris and cauda equina syndromes (Chapter 17) are exceedingly rare complications of epidural anesthesia. Because many women have back pain after delivery, new neurological deficits are often ascribed to the injection. Careful neurological examination, and in some cases imaging of the lumbosacral spine, however, discloses that procedure-related lesions of the conus medullaris or cauda equina are almost never responsible.

• Central nervous system processes, particularly superior sagittal sinus thrombosis, are important causes of postpartum leg weakness, especially when associated with headache, encephalopathy, or seizures. Women with suspected venous sinus thrombosis should undergo MRI and magnetic resonance venography (MRV) of the brain. Venous sinus thrombosis is discussed further in Chapter 19.

Peripheral nerve and plexus lesions acquired during delivery are usually self-resolving and should be managed with the expectation that they will improve. Patients with atypical examination findings may require imaging studies of the pelvis, lumbosacral spine, or brain. If symptoms do not improve by 3 weeks postpartum, nerve conduction studies and EMG help to localize the lesion and guide prognosis.

the dorsal hand, while in stroke sensory loss is typically greater in the palm.

Should any difficulty remain in distinguishing between radial neuropathy and ischemic stroke, head MRI with diffusion-weighted imaging or electromyography (EMG) should be performed.

Hip and proximal leg weakness

L2–3 radiculopathy

Spinal stenosis commonly involves the L2 or L3 nerve roots in older adults. Pain and paresthesias radiating from the back into the hip and thigh are usually more prominent in patients with upper lumbar radiculopathies, but in some cases, proximal leg weakness is the chief complaint. Because spinal stenosis is such a common radiological finding in older patients, it is often difficult to determine whether it is the true cause of weakness or if it is only an incidental finding. Upper lumbar radiculopathy is discussed in greater detail in Chapters 16 and 17.

Lumbosacral plexopathy

Like brachial plexopathy, lumbosacral plexopathy is usually associated with exquisite pain, and is therefore discussed in greater detail in Chapter 16. Common causes of lumbosacral plexopathy include trauma (including childbirth, see Box 11.1), neoplastic infiltration, diabetic amyotrophy, and retroperitoneal hematoma.

Foot drop

Weakness of the tibialis anterior, the principal dorsiflexor of the foot, results in foot drop. From distal to proximal, possible responsible lesion sites include the peroneal nerve, sciatic nerve, lumbosacral plexus, L4–5 nerve roots, and the CNS. In most cases, the cause is in the peripheral nervous system, and the localization of foot drop is determined by the presence or absence of weakness of other lower extremity muscles (Table 11.2).

Table 11.2 Distinguishing among peripheral nervous system sources of foot drop

Table 11.2

Peroneal neuropathy

Peroneal nerve damage results in weakness of dorsiflexion and eversion. Although foot inversion (mediated by the tibial nerve and tibialis posterior) is spared, it often appears to be involved in patients with severe dorsiflexion weakness. To properly test for inversion weakness in patients with severe dorsiflexion weakness, the foot must be passively dorsiflexed: in patients with peroneal neuropathy, it will become immediately obvious that inversion is much stronger than eversion. Damage to the peroneal nerve usually occurs as a result of trauma or prolonged immobilization. Sometimes, there is no clear precipitant. In patients without a relevant history of trauma, consider the possibilities of mononeuropathy multiplex, and multifocal motor neuropathy with conduction block.

Sciatic neuropathy

The sciatic nerve divides into the common peroneal and tibial nerves at the knee. Weakness should therefore involve dorsiflexion, plantarflexion, foot inversion, and foot eversion. Many cases of sciatic neuropathy, however, preferentially involve the peroneal fascicles while sparing the tibial fascicles. Sciatic neuropathy is distinguished from L5 radiculopathy by sparing of hip abduction. In such cases, a sciatic neuropathy may only be distinguishable from a peroneal neuropathy by EMG. The etiologies of sciatic neuropathy, like peroneal neuropathy, include trauma, immobilization, mononeuropathy multiplex, and multifocal motor neuropathy with conduction block.

L5 radiculopathy

Most L5 radiculopathies are associated with lower back pain. The pattern of motor abnormalities helps to distinguish L5 radiculopathy from peroneal or sciatic neuropathy when pain is absent: an L5 radiculopathy would be expected to produce weakness of dorsiflexion, inversion, eversion, and hip abduction, while sparing plantarflexion. L5 radiculopathy should be confirmed with MRI of the lumbosacral spine. Strongly consider surgery for patients with structural disc disease sufficient to produce a foot drop.

Motor neuron disease

Isolated foot drop may be the first abnormality in amyotrophic lateral sclerosis (ALS). Careful examination may disclose weakness of other muscles, fasciculations, and hyperreflexia, which support the diagnosis. Confirmation of the diagnosis by EMG is usually required for patients with ALS.

Central nervous system localizations

While more widespread lower extremity weakness might be expected with CNS disease, medial frontal lesions may also produce isolated or predominant foot drop. Clues to a CNS source include the presence of headache and, in some cases, transcortical motor aphasia (Chapter 3). Anterior cerebral artery stroke, dural sinus thrombosis, tumor, and hemorrhage are among the most common CNS etiologies of foot drop.

Treating foot drop

Treatment of foot drop begins by addressing the underlying cause. In many cases, however, supportive care including physical therapy and wearing an ankle foot orthosis to keep the foot passively dorsiflexed while the patient recovers is the only available treatment option. It is important to counsel patients on the prolonged (up to 2 years) and sometimes incomplete recovery from foot drop due to peripheral nerve lesions. Electromyography is often helpful for prognostication.

Hemiparesis and hemiplegia

Hemiparesis essentially always reflects dysfunction of the CNS. Common causes include ischemic stroke, hemorrhage, tumor, demyelination, or abscess. In exceptional cases, multiple simultaneous peripheral nervous system lesions may produce multifocal problems that masquerade as hemiparesis. The following five rules help to pinpoint the site of pathology:

1. Cerebral cortical lesions producing simultaneous weakness of the face, arm, and leg should always be accompanied by some behavioral manifestation. It would be unusual for a right-handed patient with a left cortical lesion, for example, to have severe hemiparesis without aphasia.

2. Large subcortical lesions, particularly within the internal capsule, produce severe hemiparesis that is usually unassociated with behavioral deficits. The most common etiology of hemiplegia secondary to an internal capsule lesion is infarction in the territory of the medial lenticulostriate arteries (Chapter 21).

3. A brainstem lesion often produces a “crossed hemiparesis” in which an ipsilateral cranial nerve deficit is accompanied by contralateral hemiparesis:

a. A midbrain lesion produces ipsilateral third-nerve palsy and contralateral hemiparesis (Chapter 6).

b. A pontine lesion produces ipsilateral sixth- and/or seventh-nerve palsy and contralateral hemiparesis (Chapters 6 and 8).

c. A medullary lesion produces ipsilateral twelfth-nerve palsy and contralateral hemiparesis (Chapter 8).

4. Small lacunar infarcts of the contralateral corona radiata, internal capsule, or pons may lead to the syndrome of ataxic hemiparesis (Chapter 21). As its name suggests, the limbs are both clumsy and weak.

5. Hemiparesis that spares the face is generally the result of an ipsilateral cervical spinal cord lesion (Chapter 17).

Multifocal weakness

Multifocal weakness is always due to multifocal nervous system disease. The common localizations of multifocal weakness are the CNS white matter, anterior horn cells, nerve roots, and peripheral nerves. Common conditions that produce multifocal weakness include multiple sclerosis (Chapter 22), motor neuron disease (Chapter 10), and cervical and lumbosacral polyradiculopathy (Chapter 17). In some cases, multiple simultaneous embolic strokes may lead to multifocal weakness. The following are three disorders of the peripheral nervous system that produce multifocal weakness.

Vasculitic mononeuropathy multiplex

Mononeuropathy multiplex is an uncommon diagnosis, usually associated with vasculitis. Although rheumatologists often refer patients with vague pain syndromes for mononeuropathy multiplex evaluation, true mononeuropathy multiplex is not subtle, and patients often have profound multifocal weakness. In many cases, the first mononeuropathy of mononeuropathy multiplex clinically resembles an ischemic stroke rather than a peripheral nerve disorder. The nerves that are most commonly affected include the sciatic, peroneal, radial, and ulnar. The site of nerve involvement is usually at the vascular watershed territories in the middle of the femur or humerus. Attacks of mononeuropathy multiplex are usually painful and result in flaccid weakness. A generalized polyneuropathy often accompanies the motor symptoms (Chapter 15). The diagnosis of vasculitic mononeuropathy multiplex is established by finding lymphocytic infiltration of blood vessels with fibrinoid necrosis on nerve biopsy. Laboratory studies should include erythrocyte sedimentation rate, rheumatoid factor, double-stranded DNA antibodies, anti-Smith antibodies, Sjögren’s antibodies, and cryoglobulins. Mononeuropathy multiplex should be treated with a combination of steroids and cyclophosphamide. Recovery is often very slow and incomplete.

Multifocal motor neuropathy with conduction block

Patients with multifocal motor neuropathy with conduction block (MMNCB) are typically younger to middle-aged men who develop focal arm weakness in the distribution of a named nerve.5 Because the weakness begins so suddenly and is usually not associated with pain, patients are often evaluated for stroke. As the disease progresses, additional motor nerves become involved, and, at this stage, MMNCB may resemble motor neuron disease. Nerve conduction studies confirm the presence of conduction block. Approximately 50% of patients with MMNCB will have antibodies to the ganglioside GM1. Intravenous immunoglobulin is the standard first-line treatment for MMNCB.

Hereditary neuropathy with liability to pressure palsies

Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominantly inherited neuropathy characterized, as its name suggests, by the development of multiple pressure palsies. Recurrent carpal tunnel syndrome, ulnar neuropathy, and peroneal neuropathy are separated in space and time over many years. The diagnosis is established by testing for the responsible mutation in the PMP22 gene. The mainstay of treating HNPP is counseling the patient to avoid activities that precipitate pressure palsies.

References

1. Acosta JAHoffman SNRaynor EMNardin RARutkove SB. Ulnar neuropathy in the forearm: a possible complication of diabetes mellitus. Muscle Nerve 2003;28:40–45.

2. Gass ASzabo KBehrens SRossmanith CHennerici M. A diffusion-weighted MRI study of acute ischemic distal arm paresis. Neurology 2001;57:1589–1594.

3. Fisher CM. Lacunar strokes and infarcts: a review. Neurology 1982;32:871–876.

4. Wong CA. Neurologic deficits and labor analgesia. Regional Anesth Pain Med 2004;29:341–351.

5. Nobile-Orazio ECappelari APriori A. Multifocal motor neuropathy: current concepts and controversies. Muscle Nerve 2005;31:663–680.