A 22 year-old female with a medical history of migraines and obesity who presents to the Emergency Department with a headache. Her headache was gradual in onset, but has progressively worsened and has not been relieved with NSAIDs. She describes the headache as dull and pounding with photophobia and phonophobia. She also notes associated nausea but no vomiting, and no associated neurologic complaints. She does note that she thinks during a previous ED visit at an outside hospital she was told she may have “high pressure in her head” but she does not remember the details and is not on any medication for it. A bedside ocular ultrasound was performed and the following images were obtained:
An LP was performed and the opening pressure was mildly elevated at 33 cm H2O that decreased to 18 cm H2O with removal of 20 cc of fluid. The remainder of labs and imaging were normal. The patient was diagnosed with Idiopathic Intracranial Hypertension and started on Diamox with outpatient neurology and ophthalmology follow-up. On follow-up she reports resolution of headaches.
The ultrasound images were obtained using the 7.5 MHz high frequency linear probe. The ophthalmic setting was used given its lower thermal index, thus decreasing the risk of injury to the eye from heat exposure. For patient comfort, a non-adhesive sterile dressing was placed over eye prior to application of a copious amount of ultrasound gel. The ultrasound probe was placed lightly over the gel in a transverse orientation with probe marker to the patient’s right and with careful attention not to exert pressure on the globe. The probe was angled superiorly and inferiorly slowly until the optic nerve came into view as a black stripe extending posteriorly from the rear of the globe. The nerve sheath diameter was then measured 3 mm posterior to the optic disc. For a more accurate value, an average of 2-3 measurements can be taken.
The relationship between intracranial pressure and optic nerve sheath diameter has been well established. In general, an optic nerve sheath diameter of >5.0 mm is considered abnormal, however there remains some controversy about the exact diameter of the optic nerve sheath that best predicts elevated intracranial pressure. For example, one prospective blinded observational study by Kimberly et al. in 2008 found that an optic nerve sheath diameter >5.0 mm correlated with an elevated intracranial pressure with a sensitivity of 100% and a specificity of 63%. Another 2013 study by Amini et al measured the optic nerve sheath diameter of 50 atraumatic patients undergoing lumbar puncture and found that an optic nerve sheath diameter greater then 5.5 mm correlated with an elevated ICP (>30 mm Hg) with a sensitivity and specificity of 100%. Finally, a third study by Qayyum found a sensitivity of 100% and specificity of 75% for a cutoff of 5.0 mm.
In general, measurement of optic nerve sheath diameter appears to be a useful adjunct for detection of elevated intracranial pressure due to the fact that it is non-invasive, easily performed at bedside, and has good sensitivity for detecting increased intracranial pressure. Though a normal optic nerve sheath diameter does not exclude a life-threatening intracranial process, an elevated optic nerve sheath diameter can further support a diagnosis of elevated intracranial pressure when in the right clinical context.
Faculty Reviewer: Dr. Kristin Dwyer
Blaivas, M. , Theodoro, D. and Sierzenski, P. R. (2003), Elevated Intracranial Pressure Detected by Bedside Emergency Ultrasonography of the Optic Nerve Sheath. Academic Emergency Medicine, 10: 376-381.
Dubourg J, Javouhey E, Geeraerts T, Messerer M, Kassai B. Ultrasonography of optic nerve sheath diameter for detection of raised intracranial pressure: a systematic review and meta-analysis. Intensive Care Med. 2011 Jul;37(7):1059-68.
Irazuzta JE, Brown ME, Akhtar J. Bedside Optic Nerve Sheath Diameter Assessment in the Identification of Increased Intracranial Pressure in Suspected Idiopathic Intracranial Hypertension. Pediatr Neurol. 2015 Aug 28.
Qayyum H, Ramlakhan S. Can ocular ultrasound predict intracranial hypertension? A pilot diagnostic accuracy evaluation in a UK emergency department. Eur J Emerg Med. 2013;20(2):91–97.
Shevlin C. Optic Nerve Sheath Ultrasound for the Bedside Diagnosis of Intracranial Hypertension: Pitfalls and Potential. Critical Care Horizons 2015; 1: 22-30.
Tayal VS, Neulander M, Norton HJ, et al. Emergency department sonographic measurement of optic nerve sheath diameter to detect findings of increased intracranial pressure in adult head injury patients. Ann Emerg Med. 2007 Apr;49(4):508-514.