32 yo overweight female presents with a headache. This headache is similar to prior headaches, which she gets frequently. The rest of her story seems consistent with a migraine and her physical exam, including a through neuro exam, is normal. However, you still feel concerned because she’s in the right demographic for idiopathic intracranial hypertension. You think the optic disc looked okay, but if you’re anything like me you’re not super confident in that assessment. What is another modality you could use to assess for increased intracranial pressure? How good of a test is it?
Ocular ultrasound is a great test to assess patients for increased ICP. It’s noninvasive, has good test characteristics and is probably easier than a fundoscopic exam. Use the high frequency linear probe and with an ample amount of chilled gel, place the probe over the closed eye. Be sure to visualize the lens or iris to ensure you are on-axis. Measure 3mm back from the globe along the optic nerve. Then measure the width of the nerve. Using a cutoff of >6mm will yield a sensitivity of 74-96% and specificity of 80-100%. Studies have used various cutoffs between 4.8-6mm yielding a range of test characteristics but most agree that this is a good test for ruling out increased ICP.
Check out this podcast of Chris Fox for more applications of ocular ultrasound: http://www.ultrasoundpodcast.com/2012/04/episode-26-ocular-ultrasound-with-chris-fox/