To CT or not to CT? That is the question.

As discussed, the suspicion of increased ICP due to a mass lesion is a contraindication to lumbar puncture. In these cases, a CT scan is recommended prior to LP. Unfortunately, it has been wrongly interpreted by many that all patients require CT scan prior to their LP.

The clinical findings of decreased level of consciousness, focal neurological deficits, or papilledema make CT scan necessary prior to LP. Any of these findings place a patient into a high-risk group for having increased ICP. Having said this, LP's performed on these patients do not always lead to disaster. Studies reviewing LP in patients with known brain neoplasms, hematomas or abscesses found that neurological deterioration with LP occurs in only 0 - 5% of patients. This low rate occurs in patients known to have signs, symptoms and diagnoses of increased ICP with mass lesions!

Given this low complication rate, it has been accepted that patients with a normal level of consciousness, lack of focal neurological findings and absence of papilledema are safe to undergo LP. Many clinicians ask, "What if I cannot reliably see the fundi of my patient?". Papilledema is a late finding of increased ICP and is present in less than half of those with raised ICP. If your patient has a normal level of consciousness, no focal neurological defects, and you cannot visualize the fundi, it is still safe to proceed with the LP.

The safety of this practice has been studied in suspected meningitis. In 1993, the Canadian Medical Association Journal reviewed the need for CT prior to LP in meningitis and found that there were no clinical studies nor anecdotal reports of patients with suspected meningitis and normal neurological exam deteriorating with LP. In 1993, Durando et al reviewed almost 450 LP's in adult patients in suspected meningitis. They found that 5 cases had neurological deterioration after LP, and all these patients had either signs of increased ICP or focal neurological deficits prior to LP. In suspected meningitis, patients with a normal level of consciousness, no focal deficits and an absence of papilledema (or inability to visualize the fundi) should undergo lumbar puncture without prior CT scan. If the clinician finds reason to image the patient prior to LP, and meningitis is suspected, the patient must receive antibiotics prior to CT scan.

Delays in starting appropriate antibiotic therapy, to arrange a scan can result in increased morbidity in the setting of bacterial meningitis. Arranging, performing and interpreting a CT scan will take a minimum of an hour in most cases even if the CT is up and running. If the technician and the radiologist need to come in from home, or the patient requires transfer to another center, it will involve a considerably longer period of time. It is worth repeating that these patients must get antibiotics prior to CT scan!

The safety of LP in suspected SAH is somewhat less clear. As mentioned earlier, large intracranial bleeds have the potential to increase ICP. Patients with large bleeds either die, have decreased levels of consciousness or neurological deficits. Patients with small SAH's often have more subtle presentations. These patients may not have neurological defects. Using the same logic as in meningitis, it is inferred that lumbar puncture is safe in the suspected SAH patient who has a normal level of consciousness, no focal deficits, nor papilledema. Unfortunately, this has neither been proven nor disproven by rigorous studies.