Meningitis Children

No bacteria on gram film are seen in up to 40% of bacterial meningitis even without prior antibiotics. Normal white cell counts and glucose in meningitis have been described but only where LP has been done within 12 hours of onset of symptoms. 57% of aseptic meningitis cases in 1 study had a neutrophil predominance. Acutely unwell children without meningitis can have white cell counts up to 16, with neutrophils up to 48%, and low glucose (seen in 25%) even without CNS infection. Correcting for blood staining does not improve diagnostic accuracy.

Fear is of raised intracranial pressure, leading to coning. Abnormal respiratory pattern eg Cheynes Stokes is one sign, focal neurological signs esp pupil abnormalities and decerebrate/decorticate posturing are other indicators. Double vision would only be a contraindication if there was impairment of eye movements suggesting a cranial nerve palsy, or fixed deviation. In meningococcal septicaemia (suggested by a petechial rash), the yield from cultures is higher for blood/skin scraping culture and PCR, and lumbar puncture does not add much (but if meningitis is the predominant clinical symptom, then LP would be indicated). Cardiovascular instability eg hypotension demands resuscitation and stabilization prior to LP, as the procedure itself may increased oxygen demand. Impaired conscious level is not an absolute contraindication, but if there is progressive deterioration in coma score then raised intracranial pressure should be suspected and LP deferred. If comatose (ie GCS less than 8), then clinical assessment is of limited value and CT should be done first.

CT scan does not estimate intracranial pressure, and coning has been reported after normal CT. On the other hand, if it shows an intracranial lesion eg tumour, haemorrhage then you will be more suspicious of raised pressure. Antibiotics do reduce the yield of positive cultures but will not correct all CSF abnormalities and PCR may still be positive so urgent treatment is preferred.
Latex antigen tests have suboptimal sensitivity and specificity and in 1 study had only 7% sensitivity for culture negative clinically suspected meningitis.

Big overlap in CSF values for viral and bacterial meningitis. CSF protein over 1.5 or glucose under 2 are predictive of bacterial cause. CSF white cell count not very reliable until you get up to values of 10 000+! Blood is also useful (and presence of bacteria, obviously).

Bacterial meningitis can present with fever and seizures, but the prevalence of meningitis is falling with the introduction of Hib, MenC and Pneumococcal vaccines. In the typical patient with febrile convulsion, you would have to do over 200 LPs to detect one case of meningitis. Seizures have been reported to increase protein counts (up to 0.5) and neutrophil %(up to 55%), but not total white cell count so probably not clinically significant.

HSV encephalitis can occur in primary infection, as reactivation or as re-infection. Aciclovir reduces mortality from 70% to 30%.

Various EEG abnormalities are described - often just non-specific. MRI is better than CT and usually shows temporal or frontal lesions, but can be normal early on. Normal CSF and negative PCR are well described in first few days of illness esp where symptoms mild and child is under 1yr.

Enterovirus epidemics occur in summer/autumn. Wide regional variations world wide in predominant causes. Arboviruses are important globally - esp tick borne encephalitis (common in forests of mainland Europe), West Nile Fever (now endemic in US), Japanese encephalitis (seen across Asia).
ADEM is post infectious (or post vaccination), does not usually manifest as a febrile illness, and spinal cord signs if present will be a big clue. The MRI changes are focal rather than diffuse, tend to be multiple (whereas in viral encephalitis there may only be 1 or 2), and often affect brainstem and cerebellum (less common in viral encephalitis)