What place does measuring the optic nerve sheath with ultrasound have in an average ED? What placed doesn't it have? Take patients with altered mental status, for instance. These patients are challenging for most emergency physicians because of the lack of history and limited physical exam. But evaluation of the optic nerve sheath in these patients, particularly in those without obvious trauma, may be helpful in rapidly pointing toward elevated ICP as a cause of that mental status.
This procedure is particularly advantageous for patients who tend to develop elevated ICP, such as those with idiopathic intracranial hypertension or ventricular shunts. These patients, to their detriment, may receive multiple CT scans over a lifetime because imaging is typically performed whenever these patients present with a headache. Evaluating the optic nerve sheath for signs of elevated ICP in these patients may save repeated radiation dosing.
The optic disc and even the optic nerve sheath itself are easily visualized with ultrasound, allowing an indirect evaluation of ICP. (Image 1.) The optic nerve is an extension of the central nervous system, and is surrounded by subarachnoid fluid within the sheath covering the nerve. When ICP is elevated, the sheath distends and enlarges. This phenomenon appears to happen very quickly, and can be visualized with ocular ultrasound. A high-frequency transducer can be used and placed on the closed eyelid with a substantial amount of gel between the eyelid and the transducer.
Multiple studies have determined that the width of the optic nerve sheath in the presence of normal ICP is less than 5 mm in adults, measured in transverse 3 mm posterior to the optic disc. (Image 2.) This stepoff of 3 mm allows for the greatest contrast and more reproducible results. The greater the deviation above 5 mm, the stronger the correlation with elevated ICP. The optic nerve sheath can appear somewhat different from different angles, so two measurements should be obtained and averaged.
Studies have demonstrated a relationship between increasing ICP and the size of the optic nerve sheath through direct measurement of ICP, either through invasive monitoring such as with a ventriculostomy or through lumbar puncture. Similarly, a recent study by Singleton, et al, in the American Journal of Emergency Medicine found that the size of the optic nerve sheath in patients with idiopathic intracranial hypertension decreased as patients had therapeutic lumbar puncture. (2014 Dec 19; 10.1016.) Although these studies demonstrate a linear relationship, it does not yet appear possible to quantify the ICP based on optic nerve sheath diameter.
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