The Central Nervous System: Infections

Classified according to the infected tissue (1) Meningeal infections (meningitis), which may involve the dura primarily (pachymeningitis) or the pia-arachnoid (leptomeningitis) (2) Infections of the cerebral and spinal parenchyma (encephalitis or myelitis). In many cases, both the meninges and the brain parenchyma are affected to varying degrees (meningoencephalitis).

Meningeal Infections Acute Leptomeningitis Acute inflammation of the pia mater and arachnoid. Caused by infectious agents; rarely, release of keratinaceous contents from an intradural epidermoid cyst or teratoma causes a chemical meningitis. When the term meningitis is used without qualification, it means leptomeningitis.

Etiologic Agents in Bacterial Meningitis. Streptococcus pneumoniae Most common agent in patients over age 40 years 30–50% of cases in adults 10–20% of cases in children 5% of cases in infants Neisseria meningitidis Most common agent in patients aged 5–40 years 25–49% of cases in children aged 5–15 years 10–35% of cases in adults Haemophilus influenzae Most common agent in patients aged 1–5 years 40–60% of cases in children aged 1–5 years 2% of cases in adults

Acute Viral Meningitis 10,000 cases per year in the United States 90% of these occur in patients under 30 Mild, benign illness, which rarely causes death. Enteroviruses, mumps virus, and lymphocytic choriomeningitis (LCM) virus. An acute meningitis occurs in 10% of patients (HIV) infection

Tuberculous Meningitis Typically chronic; however, in the early stages there may be an exudative phase that resembles acute meningitis

Routes of Infection of the Meninges Bloodstream spread accounts for the majority of cases; The primary entry site of the organism may be the respiratory tract (N meningitidis, H influenzae, S pneumoniae, C neoformans, many viruses), skin (bacteria causing neonatal meningitis), Intestine (enteroviruses).

From direct spread of organisms from an infected middle ear or paranasal sinus, especially in childhood. May be associated with skull fractures, Especially those at the base of the skull Lumbar puncture. Organisms may also gain entry through the intact nasal cribriform plate (eg, free-living soil amebas in stagnant swimming pools).

Pathology Grossly, the leptomeninges are congested and opaque and contain an exudate. Microscopically, acute meningitis is characterized by hyperemia, fibrin formation, and inflammatory cells. In bacterial meningitis, neutrophils dominate in acute viral meningitis, neutrophils are rare and lymphocytes dominate In acute tuberculous meningitis, there is an inflammatory exudate that contains increased numbers of both neutrophils and lymphocytes.

Pyogenic meningitis, showing obliteration of the gyri of the brain surface by the purulent exudate.

Clinical Features Acute meningitis presents with fever and symptoms of meningeal irritation,(headache, neck pain, and vomiting.) Physical examination reveals neck stiffness and a positive Kernig sign (due to reflex spasm of spinal muscles, a consequence of irritation of nerves passing across the inflamed meninges)

Bacterial meningitis is a serious disease with considerable risk of death Viral meningitis is usually a mild, self-limited infection. Tuberculous meningitis has an insidious onset and a slow rate of progression but is frequently a severe illness with a fatal outcome if not treated

Encephali tis BacterialViralTuberculo us Brain Abscess PressureRaised May be very high Gross appearan ce ClearTurbidClearClear; may clot Clear ProteinSlightly elevated HighSlightly elevated Very highElevated GlucoseNormalVery lowNormalLowNormal ChlorideNormalLowNormalVery lowNormal or low

CellsLymphoc ytes or normal Neutroph ils Lymphoc ytes Pleocytos is 2 Pleocytos is Gram stain NegativePositive in 90% Negative Occasion ally positive Acid–fast stain Negative Rarely positive Negative

Bacteria l culture Negativ e Positive in 90% Negativ e Occasio nally positive Mycoba cterial culture Negative PositiveNegative Viral culture Positive in 30% or less NegativePositive in 70% Negative

Chronic Meningitis Facultative intracellular organisms such as mycobacterium tuberculosis, fungi, and treponema pallidum. It is now relatively uncommon in the United States More prevalent in parts of africa, india, south america, and southeast asia

Pathology & Clinical Features caseous granulomatous inflammation with fibrosis Marked fibrous thickening of the meninges The entire brain surface is involved, with the basal meninges more severely affected in cases of tuberculosis. The causative agent may be identified in tissue sections specially stained for acid-fast bacilli and fungi

Complications of chronic meningitis include (1) Obliterative vasculitis (endarteritis obliterans), which may produce focal ischemia with microinfarcts in the brain and brain stem; (2) Entrapment of cranial nerves in the fibrosis as they traverse the meninges, resulting in cranial nerve palsies; and (3) Fibrosis around the fourth ventricular foramina, causing obstructive hydrocephalus

Insidious onset with symptoms of diffuse neurologic involvement, Including apathy, somnolence, personality change, and poor concentration. Headache and vomiting are less severe than in acute meningitis, Fever is often low-grade. Focal neurologic signs and epileptic seizures result from ischemia, cranial nerve palsies, or hydrocephalus

Diagnosis & Treatment Lumbar puncture Serologic tests for syphilis performed on both serum and CSF are positive in meningeal syphilis. Culture is commonly positive in cases caused by tuberculosis and fungal infection Skin tests for tuberculosis and fungal infection are positive unless the patient is anergic.