• When the patient has clinical signs of meningitis, but the spinal fluid has fewer than 500 WBC, predominantly lymphocytes, protein below 200, and normal CSF glucose, the gram stain will seldom show any bacteria. This is usually called aseptic meningitis, because of the absence of bacteria. Cultures still need to be taken in the event the patient has compromised reaction or the infection is still early. Antibiotics should usually not be initiated unless there is other evidence to suggest a bacterial infection.
• When suspecting aseptic meningitis, depending upon the setting, skin tests for TB, a chest x-ray, a reagin test for syphilis, fungal cultures, and India ink preparations should be done. Viruses are the most common cause of this picture in military age groups.
• Viral meningitis, usually caused by enteric viruses, tends to occur in local epidemics in the late summer and runs a course lasting about 10 days to 2 weeks. Treatment consists of supportive care, analgesics, hydration, and enteric isolation. Preventive medicine technicians should look for spread of illness and pay particular attention to latrines, food service, and berthing areas.

Partially Treated Meningitis

Unfortunately, this is all too common a scenario: a patient has been recently treated with an antibiotic or sulfa drug for some minor condition and now returns for medical care because of systemic symptoms to include a headache, nausea, vomiting, photophobia, and stiff neck. The spinal fluid shows a few hundred lymphocytes, moderate elevation of protein, a negative gram stain, and negative cultures. Does the patient simply have viral meningitis, or does he or she have bacterial meningitis obscured but not adequately treated with antibiotics? If there is a major medical treatment facility nearby, this patient must be transferred for evaluation and treatment. If there is currently an epidemic of typical viral meningitis at the command or in the community, it is reasonable to discontinue the patient’s antibiotics, examine the patient every day and repeat their LP in 1 or 2 days for signs of infection. If the patient cannot be quickly referred to a treatment facility and if there is not a local outbreak of viral meningitis to account for the patient’s illness, the GMO must contact an infectious disease specialist for consultation and start the patient on high doses of ceftriaxone or penicillin for 2 weeks.

The Lumbar Puncture

• Spinal fluid is the only source to identify the organisms in meningitis, so a lumbar puncture must be done for the diagnosis. Spinal fluid is also necessary for some viral studies, chemical studies in some degenerative diseases, cytologic studies in lymphoma and leukemia, diagnosis of Guillain-Barre syndrome, adjunctive data in the diagnosis of multiple sclerosis, and therapeutic relief of pressure in benign intracranial hypertension, and some forms of hydrocephalus.
• The sterile tray and collection tubes should be ready and the manometer and stopcock assembled before the patient is touched. Assemble all equipment out of the view of the patient. Standard aseptic technique requires the use of surgical gloves, mask and safety glasses, sterile scrub of the patient's back and a suitable drape with an aperture about 15 to 20 cm in diameter for the procedure. The patient should lie on his or her left side for a right-handed operator, on the right side for a left-handed operator. A firm surface is important to avoid sagging of the spine, which may make it more difficult to reach the subarachnoid space. With the patient's neck and knees bent to flex the lumbar spine, the operator should palpate the interspace between the fourth and fifth lumbar vertebrae, and hold the tips of his or her left index and middle finger on either side of the interspace. The patient's back should be perfectly vertical.
• Many operators use a local anesthetic such as lidocaine to numb the skin before placing the needle. Others, however, including this author, find that the use of anesthetic on the skin does not spare the total amount of discomfort for the patient, but it does prolong the procedure. With the needle absolutely parallel to the floor, and the bevel facing up, the needle is gripped at the hub and the tip of the needle is firmly inserted into the interspinal space between L4 and L5, and directed toward the patient's chin. Proceeding deliberately but not timidly, the needle should be advanced at least 3 cm before slowing. When the needle perforates the meningeal sac, there is a detectable soft pop. At this point the trochar is removed and fluid will dribble out of the needle.
• The pressure should not be recorded until the patient stretches back out and relaxes. Increased abdominal pressure from the flexed posture and groaning will give a misleading elevated pressure. The normal pressure is less than 200 mm. If the patient appears agitated or is sobbing, the pressure should not be recorded until the patient is as relaxed as possible in the situation. After the pressure is recorded, the manometer can be put aside, a closing pressure provides no useful information.
• The first tube of fluid is the least contaminated and it is from this tube that gram stain and cultures should come. The last tube should have the fewest red cells from the insertion of the needle, so it should be the one used for cell counts and cytology if desired. CSF protein and glucose should be measured from a middle tube. It is seldom necessary to take more than 6-10 cc.
• If the needle strikes bone within the first few centimeters, it should first be directed more toward the head, and if that fails then very slightly more caudad. Once the pop is encountered if no fluid comes out of the needle, it should be very slowly retracted with trochar out while rotating it gently. As it is pulled away from the anterior rim of the space, fluid will emerge.
• If fluid cannot be obtained despite these measures, the operator should go the next interspinal space, L3-L4.

The Difficult LP: Recommendations

If after three attempts, you cannot reach the subarachnoid space, it is best to stop and ask another colleague to try. The most skillful at this task in most clinics or hospitals are the anesthetists and anesthesiologists, then the neurologists and neurosurgeons. Here are a few tricks to try if it is difficult to reach the spinal fluid. Aseptic technique still is required while following these steps.

• Perform the procedure with the patient sitting and hunched forward at the shoulders.
• The landmarks are more easily seen in this position, and it is easier to direct the needle at the midline. Once the needle is inserted, leave it in place and lay the patient again in a lateral position to measure the pressure.
• Use an 18-gauge needle. Most kits now come equipped with a standard 20-gauge needle. The larger needle is firmer and allows fluid to egress more easily.
• Consider sedation with a short acting benzodiazepine such as midazolam to relax the patient and ease muscle guarding.

Important points

• Measure the pressure after the patient has settled down - and do not forget to record it! If the puncture was done in the sitting position, and you cannot lay the person back on his/her side, then record the pressure from the level of the base of the skull.
• Use the first tube for cultures. It will be the least contaminated.
• If there is blood in the sample, spin one of the tubes in a centrifuge. If the blood is caused by your efforts, the supernatant fluid will be clear. If the supernatant fluid is blood-tinged, that indicates hemolysis, suggesting the cells have been in the fluid for at least 30 minutes.
• Check the cell count (preferably review the slides yourself along with the lab tech).
• Check the protein and glucose.
• Freeze some fluid immediately, in the event you later determine a need for an additional laboratory testing.

Reviewed by CDR Siefert, MC, USN, Emergency Medicine Department, Naval Medical Center San Diego, San Diego, CA. (1999).