Meningitis - How to Diagnose it

Meningitis, an inflammation of the membranes that cover the brain and spinal cord, can be caused by infection with a bacterium or virus.

Children who have partially treated meningitis or develop it while on antibiotics have modified signs and symptoms, and the diagnosis is usually delayed.

Bacterial meningitis is a life-threatening illness that results from bacterial infection of the meninges. Beyond the neonatal period, the 3 most common organisms that cause acute bacterial meningitis are Streptococcus pneumoniae, Neisseria meningitidis,and Haemophilus influenzae type b (Hib). Since the routine use of Hib, conjugate pneumococcal, and conjugate meningococcal vaccines in the United States, the incidence of meningitis has dramatically decreased.

Examination
Upon arrival at the emergency department, the child’s temperature, blood pressure, respiratory rate, pulse, and oxygen in the blood may be checked. After quickly checking the child’s airway, breathing, and circulation, the doctor completely examines the child to look for a focal source of infection, to assess any alteration in mental status, and to determine the presence of meningitis. If meningitis is suspected, several tests and procedures are needed to determine the diagnosis.

Investigations

Lumbar Puncture
The most important laboratory study is examination of CSF. The lumbar puncture (LP) should include opening and closing pressure in the cooperative patient.

• Cell count
• Gram stain
• Culture and sensitivity
• Glucose
• Protein and antigen
• Acid-fast bacillus
• Fungal stains

Lumbar puncture is done by positioning the patient, usually lying on the side, applying local anesthetic, and inserting a needle into the dural sac (a sac around the spinal cord) to collect cerebrospinal fluid (CSF). When this has been achieved, the “opening pressure” of the CSF is measured using a manometer. The pressure is normally between 6 and 18 cm water (cmH2O); in bacterial meningitis the pressure is typically elevated. The initial appearance of the fluid may prove an indication of the nature of the infection: cloudy CSF indicates higher levels of protein, white and red blood cells and/or bacteria, and therefore may suggest bacterial meningitis.

Bacterial meningitis

• White blood cell (WBC) counts over 1000/mm3 usually are caused by bacterial infections. Counts of 500-1000/mm3 may be bacterial or viral and need further evaluation.
• Gram stain may aid in diagnosis, but the diagnosis may be missed in up to 30-40% of cases of culture-proven disease.
• The protein concentration usually is elevated in bacterial meningitis, but it also is elevated by a traumatic tap.
• The glucose is usually reduced in bacterial meningitis. Normal CSF glucose should be greater than two-thirds that of the serum glucose. Levels less than 50% of serum are suggestive of bacterial meningitis.
• Latex agglutination tests are available to test for S pneumoniae, H influenzae,group B Streptococcus, and N meningitidis. A negative result, however, does not rule out bacterial infection.
• In cases where antibiotic administration leads to CSF sterilization, polymerase chain reaction (PCR) may have a role in identifying the pathogen. PCR is able to identify the pathogen quickly and accurately.

Viral meningitis

• The WBC count in viral meningitis is usually below 500/mm3, with greater than 50% lymphocytes.
• The protein may be elevated.
• The glucose level may be normal or low.
• Gram stain results are negative.

Traumatic LP

• If bleeding occurs during the procedure and the CSF is contaminated with blood, the interpretation becomes more difficult.
• Formulas to adjust for white blood cell count in the CSF analysis have not increased the specificity or sensitivity in traumatic lumbar puncture.
• In any situation when a traumatic LP occurs and the interpretation is difficult, it is better to treat and wait for the results of the CSF culture.

Contraindications for Lumbar Puncture
A lumbar puncture (LP) may be contraindicated in some of the following conditions:

• unstable patients with hypotension or respiratory distress who may not be able to tolerate the procedure,
• brain abscess, brain tumors or other cause of raised intracranial pressure,
• and occasionally infection at the lumbar puncture site.

Laboratory Studies

• Complete blood count (CBC) with differential
• Blood cultures
• Coagulation studies
• Serum glucose
• Electrolytes
• Bacterial antigen studies can be performed on urine and serum; they can be useful in cases of pretreated meningitis. However, a negative bacterial antigen study result does not rule out meningitis.

CT scan brain
Occasionally, a CT of the brain is done before the spinal tap if other problems are suspected by the doctor.
A CT scan is sometimes necessary if any of the following is present or suspected:

• Trauma
• Increased brain pressure
• Neurologic problem
• Lack of fever
• Brain abscess or tumor