Tintinalli's Emergency Medicine - Just the Facts, 3ed.

150. CENTRAL NERVOUS SYSTEM AND SPINAL INFECTIONS

O. John Ma

MENINGITIS

EPIDEMIOLOGY

images The median age of illness has risen to 39 years. The overall incidence of bacterial meningitis has declined.

images Changes in epidemiology have mirrored vaccination practices in adults and children against Haemophilus influenzae, Staphylococcus pneumoniae, and Neisseria meningitidis.

images There is an increasing prevalence of ceftriaxone-and penicillin-resistant S. pneumoniae strains in the community.

images Military barracks and college dormitories are environments in which clusters of N. meningitidis cases occur.

PATHOPHYSIOLOGY

images Infection begins with entrance of bacteria into the sub-arachnoid space, usually by upper airway inoculation, and is followed by dissemination into the bloodstream and invasion across the blood-brain barrier. Direct inoculation is also possible from infection of parame-ningeal structures (eg, otitis media, brain abscess, and sinusitis), neurosurgery, and traumatic or congenital communications with the exterior.

images The brain becomes edematous through several mechanisms: (1) There is reduced cerebrospinal fluid (CSF) drainage through interference with flow and absorption by arachnoid granulations. The increased quantity of CSF results in periventricular edema and hydro-cephalus. (2) There is disruption of the blood-brain barrier, which allows entry of protein and water. These mechanisms lead to ischemia as intracranial pressure exceeds cerebral perfusion pressure.

CLINICAL FEATURES

images In classic and fulminant cases of bacterial meningitis, the patient presents with fever, headache, neck stiffness, photophobia, and altered mental status. Seizures may occur in nearly 25% of cases.

images The presenting picture, however, may be more nonspecific, particularly in the very young and elderly. Confusion and fever may be symptoms of meningeal irritation in the elderly.

images Inquire about recent antibiotic use, which may cloud the clinical picture in a less florid case. Other key historical data include living conditions (eg, military barracks, college dormitories), trauma, immunocom-petence, immunization status, and recent neurosurgi-cal procedures.

images Assess for meningeal irritation with resistance to passive neck flexion, Brudzinskis sign (flexion of hips and knees in response to passive neck flexion), or Kernig’s sign (contraction of hamstrings in response to knee extension while hip is flexed).

images Examine the skin for the purpuric r ash characteristic of meningococcemia. Percuss paranasal sinuses and examine ears for evidence of primary infection at those sites.

images Document focal neurologic deficits, which are present in 25% of cases.

DIAGNOSIS AND DIFFERENTIAL

images When the diagnosis of bacterial meningitis is entertained, performing a lumbar puncture (LP) is mandatory. At a minimum, send CSF for Gram’s stain and culture, cell count, protein, and glucose. Typical CSF results for meningeal processes are listed in Table 150-1.

images Additional CSF studies to be considered are latex agglutination or counterimmune electrophoresis for bacterial antigens in potentially partially treated bacterial cases, India ink or serum cryptococcal antigen in immunocompromised patients, acid-fast stain and culture for mycobacteria in tuberculous meningitis, Borrelia antibodies for possible Lyme disease, and viral cultures in suspected viral meningitis.

images Other laboratory tests should include a complete blood count, blood cultures, basic metabolic panel, and coagulation studies.

images LP can be performed safely if intracranial mass lesions and coagulopathy are unlikely based on clinical grounds. Table 150-2 reviews suggested criteria for obtaining computed tomography (CT) of the head prior to LP when meningitis is suspected.

images The differential diagnosis includes subarachnoid hemorrhage, meningeal neoplasm, brain abscess, viral encephalitis, cerebral toxoplasmosis, and other infectious meningitides.

TABLE 150-1 Typical Spinal Fluid Results for Meningeal Processes

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TABLE 150-2 Some Suggested Criteria for Obtaining Head CT before Lumbar Puncture for Suspected Meningitis


Altered mental status or deteriorating level of consciousness

Focal neurologic deficit

Seizure

Papilledema

Immunocompromised state

Malignancy

History of focal central nervous system disease (stroke, focal infection, tumor)

Concern for CNS mass lesion

Age >60 y


EMERGENCY DEPARTMENT CARE AND DISPOSITION

images Upon presentation of the patient with suspected bacterial meningitis, perform the LP expeditiously. Initiate empiric antibiotic therapy as preparations for LP are made. Antibiotic therapy administered up to 2 hours prior to LP will not decrease the diagnostic sensitivity if CSF bacterial antigen assays are obtained along with CSF cultures.

images However, if the patient meets any of the criteria in Table 150-2, order a head CT scan prior to LP in order to determine the possible risks for transtentorial or tonsillar herniation associated with LP. In these cases, empiric antibiotic therapy must be initiated prior to CT. Always initiate antibiotic therapy in the ED and never delay its administration for neuroimaging or LP.

images Empiric treatment for bacterial meningitis is based on the likelihood of certain pathogens and risk factors (Table 150-3).

images Steroid therapy (dexamethasone 10 milligrams IV 15 minutes prior to antibiotic administration) has proven to be beneficial in adults. Its precise role in the ED, where emergency physicians rarely manage known cases of bacterial meningitis and appropriately administer antibiotics prior to confirmed diagnosis, remains unclear.

images Avoid hypotonie fluids. Monitor serum sodium levels to detect the syndrome of inappropriate antidiuretic hormone or cerebral salt wasting. Treat hyperpyrexia with acetaminophen. Correct coagulopathy using specific replacement therapies.

images Treat seizures with standard modalities. Treat increased intracranial pressure with head elevation and mannitol.

images Manage viral meningitis, without evidence of encephalitis, on an outpatient basis provided the patient is nontoxic in appearance, can tolerate oral fluids, and has reliable follow-up within 24 hours. However, it remains a diagnosis of exclusion; unless the diagnosis of viral meningitis is obvious, admission is warranted.

TABLE 150-3 Guidelines for Empirical Treatment of Bacterial Meningitis in Adults or with No Organisms on Gram’s Stain

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ENCEPHALITIS

EPIDEMIOLOGY

images Viral encephalitis is a viral infection of brain parenchyma producing an inflammatory response. It is distinct from, although often coexists with, viral meningitis.

images In North America, viruses that cause encephalitis are the arboviruses (including the West Nile virus), herpes simplex virus (HSV), herpes zoster virus (HZV), Epstein-Barr virus, cytomegalovirus (CMV), and rabies.

images Arboviruses can account for up to 50% of cases during epidemic outbreaks. The most common arboviral encephalitides in the United States are the La Crosse encephalitis, St. Louis equine encephalitis, western equine encephalitis, eastern equine encephalitis, and West Nile virus.

images Herpes simplex virus type 1 (HSV-1) is typically seen in older children and adults as a reactivation disease. Herpes simplex virus type 2 (HSV-2) is seen in neonates as a result of perinatal transmission.

PATHOPHYSIOLOGY

images Mosquitoes and ticks transmit arboviruses. The bite of an infected animal transmits rabies. Impaired immune status plays a role in herpes zoster and CMV encephalitis.

images Neurologic dysfunction and damage are caused by disruption of neural cell functions by the virus and by the effects of the host’s inflammatory responses. Gray matter is predominantly affected, resulting in cognitive and psychiatric signs, lethargy, and seizures.

CLINICAL FEATURES

images Encephalitis should be considered in patients presenting with any or all of the following features: new psychiatric symptoms, cognitive deficits (aphasia, amnestic syndrome, acute confusional state), seizures, and movement disorders. Headache, photophobia, fever, and meningeal irritation may be present.

images Assessment for neurologic findings and cognitive deficits is crucial. Motor and sensory deficits are not typical.

images Encephalitides may show special regional trophism. HSV involves limbic structures of the temporal and frontal lobes, with prominent psychiatric features, memory disturbance, and aphasia. Some arbovi-ruses predominantly affect the basal ganglia, causing chorea-athetosis and parkinsonism. Involvement of the brain stem nuclei leads to hydrophobic choking characteristic of rabies encephalitis.

images Symptoms of West Nile virus infection include fever, headache, muscle weakness, and lymphadenopathy.

images Most infections are mild and last only a few days. More severe symptoms and signs consist of high fever, neck stiffness, altered mental status, tremors, and seizures.

images In rare cases (mostly involving the elderly), the infection can lead to encephalitis and death.

DIAGNOSIS AND DIFFERENTIAL

images Findings on CT or magnetic resonance imaging (MRI) and LP aid in the ED diagnosis of encephalitis.

images Neuroimaging, particularly MRI, not only excludes other potential lesions, such as brain abscess, but may display findings highly suggestive of HSV encephalitis if the medial temporal and inferior frontal gray matter is involved.

images Findings of aseptic meningitis are typically found on CSF examination.

images For the West Nile virus, the most widely used screening test is the IgM ELISA for detecting acute antibodies.

images The differential diagnosis includes brain abscess; Lyme disease; subacute subarachnoid hemorrhage; bacterial, tuberculous, fungal, or neoplastic meningitis; bacterial endocarditis; postinfectious encephalomyelitis; toxic or metabolic encephalopathies; and primary psychiatric disorders.

EMERGENCY DEPARTMENT CARE AND DISPOSITION

images Admit the patient suspected of suffering from viral encephalitis. Treat patients with suspected HSV or HZV encephalitis with acyclovir 10 milligrams/kg IV every 8 hours. Treat patients with suspected CMV encephalitis with ganciclovir 5 milligrams/kg IV every 12 hours.

images Manage potential complications of encephalitis—seizures, disorders of sodium metabolism, increased intracranial pressure, and systemic consequences of a comatose state—with standard methods.

images There is no specific treatment for the West Nile virus infection. In more severe cases, intensive supportive therapy is indicated. The primary prevention step is advocating the use of insect repellant containing DEET when people go outdoors during dawn or dusk.

BRAIN ABSCESS

EPIDEMIOLOGY

images The incidence of brain abscess has progressively declined over the past century, reflecting the effect of antibiotics on predisposing conditions, such as otitis media.

PATHOPHYSIOLOGY

images A brain abscess is a focal pyogenic infection. It is composed of a central pus-filled cavity, ringed by a layer of granulation tissue and an outer fibrous capsule.

images Three known routes are available for organisms to reach the brain: hematogenously (33%); from contiguous infection of the middle ear, sinus, or teeth (33%); or by direct implantation after neurosurgery or penetrating trauma (10%). The route is unknown in 20% of cases.

CLINICAL FEATURES

images Since patients typically are not acutely toxic, the presenting features of brain abscess are nonspecific. Presenting signs and symptoms include headache, neck stiffness, fever, vomiting, confusion, or obtundation. The presentation may be dominated by the origin of the infection (eg, ear or sinus pain).

images Meningeal signs and focal neurologic findings, such as hemiparesis, seizures, and papilledema, are present in less than half the cases.

DIAGNOSIS AND DIFFERENTIAL

images Brain abscess can be diagnosed by a CT scan of the head with contrast, which demonstrates one or several thin, smoothly contoured rings of enhancement surrounding a low-density center and in turn surrounded by white matter edema.

images LP is contraindicated if a brain abscess is suspected or after the diagnosis has been established. Routine laboratory studies are usually nonspecific. Blood cultures should be obtained.

images The differential diagnosis includes cerebrovascular disease, meningitis, brain neoplasm, subacute cerebral hemorrhage, and other focal brain infections, such as toxoplasmosis.

EMERGENCY DEPARTMENT CARE AND DISPOSITION

images Decisions on antibiotic therapy for brain abscess are dependent on the likely source of the infection (Table 150-4).

images Neurosurgical consultation and admission are warranted since many cases will require surgery for diagnosis, bacteriology, and definitive treatment.

TABLE 150-4 Guidelines for Empiric Treatment of Brain Abscess Based on Presumed Source

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For further reading in Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 7th ed., see Chapter 168, “Central Nervous System and Spinal Infections,” by Keith E. Loring and Judith E. Tintinalli.