Infections of the nervous system I

Bacterial meningitis

This occurs in 5–10 per 100 000 per year in the developed world. The most common organisms are Haemophilus influenzae, Neisseria meningitidis and Streptococcus pneumoniae. N. meningitidis tends to occur in epidemics. Neonatal meningitis is usually due to Escherichia coli or group B streptococcus. In developing countries, tuberculous meningitis is also common but usually clinically distinct (see below).

Overcrowding and poverty have been shown to be risk factors.

Meningitis can occur with organisms crossing the blood–brain barrier during systemic infection or as a result of a breakdown in the barrier, for example after skull fracture or a neurosurgical procedure. In the latter cases, a wider range of organisms is seen.

The clinical features of meningitis are:

headache

fever

neck stiffness.

There may also be altered consciousness, seizures and focal signs in about 15% of patients. Patients may have a positive Kernig’s sign (Fig. 1), another sign of meningism. Meningococcal meningitis may be associated with a purpuric rash (Fig. 2).

Fig. 1 Kernig’s sign. Flex the leg at the hip and knee and try to extend the knee. Low back pain with this procedure indicates meningeal irritation.

Fig. 2 Meningococcal rash.

Bacterial meningitis is a medical emergency. Treatment should begin as soon as the diagnosis is suspected and should not await investigation results or on occasion investigation. Acute treatment is usually with systemic penicillin and a third generation cephalosporin, though alternatively ampicillin and chloramphenicol may be used.

Blood cultures should be taken immediately. Lumbar puncture is important to confirm the diagnosis and determine the organism. However, if the patient has focal signs, altered consciousness or has had a seizure, then a CT brain scan or MRI needs to be done prior to the lumbar puncture. In severely ill children, lumbar puncture may lead to deterioration and should be avoided. The CSF findings are indicated in Table 1. The Gram stain and culture are usually diagnostic, and can be helped by newer tests for specific bacterial antigens.

The main differential diagnoses are subarachnoid haemorrhage and other meningitic illnesses.