Chapter VI-7. Hydrocephalus—Use of CT before LP

A 79-year-old man presented to the ED with weakness, anorexia, and lethargy of two day’s duration. His next door neighbor called for an ambulance.

In the ED, he complained of a mild headache, but did not have abdominal pain, vomiting, diarrhea, cough, or urinary symptoms.

On examination, he was an elderly man who was lethargic and oriented only to person and place. His verbal responses and responses to simple commands were slow. His vital signs were normal aside from a temperature of 100.9°F (rectal).

His pupils were equal and reactive and extraocular motion was normal. Funduscopic examination was limited by cataracts. His neck was supple. His heart, lung, and abdominal examinations were normal. Neurological examination revealed normal strength, sensation, and reflexes in all extremities.

Given the patient’s low-grade fever without an identified source, slowed mentation and complaint of headache, meningitis was a diagnostic consideration. Because his fundi could not be visualized, a noncontrast head CT was performed prior to lumbar puncture (Figure 1).

• There is no midline shift or mass effect.
• Is it safe to perform a lumbar puncture?

(NB. Because the CT slice orientation was not parallel to the skull base, the anterior portions of the lower slices (8–10) extend into the orbits.)

See patient outcome.

Intracranial lesions causing mass effect are important to identify prior to performing a lumbar puncture (LP) in patients suspected of having meningitis because they increase the risk of post-LP herniation. The risk of herniation is greatest when a mass causes unequal pressure between intracranial compartments.

Although contrast CT is better than noncontrast at detecting intracerebral masses such as tumors and abscesses, a lesion that is causing significant mass effect can usually be detected on a noncontrast scan. Therefore, noncontrast CT is generally felt to be an acceptable means of excluding an intracranial mass prior to LP.

In this patient’s CT, there is no evident distortion of brain parenchyma and no midline shift. However, a sizable intracranial mass is present. Had the patient undergone lumbar puncture, fatal herniation would have been the likely outcome.

This patient’s CT is notable for ventricular enlargement (Figure 2). Ventricular enlargement is an expected finding in elderly patients and is usually due to cerebral atrophy (loss of brain tissue). Atrophy is also termed ex-vacuo hydrocephalus.