US - Elevated ONSD: Ultrasound of the Month
/THE CASE PRESENTATION...
The patient is a 20 year old obese female with no significant past medical history who presents to the emergency department with a headache. She notes that it is a constant and unrelenting headache present over the prior 2 weeks described as a pressure sensation with associated blurriness of her vision, photophobia, nausea, and vomiting. She denies any infectious symptoms. She has no prior history of migraine headaches. She has a positive family history of brain aneurysms. She had been seen at her primary care doctor three days prior and outside hospital emergency department two days prior, where she underwent CT head without contrast and CTA of her head and neck, which were all within normal limits. She was taking Excedrin without relief of her headache. A lumbar puncture (LP) was performed but due to technical difficulty the successful attempt was in a seated position and thus an accurate opening pressure could not be obtained. CSF analysis showed no signs of subarachnoid hemorrhage or meningitis.
And now for the ultrasound images…
+ What do you see on ultrasound?
Bedside ocular ultrasound was performed to evaluate the Optic Nerve Sheath Diameter (ONSD), a surrogate for increased intracranial pressure (ICP). Increased ICP is transmitted to the subarachnoid space surrounding the optic nerve, causing optic nerve sheath expansion (1). Images were obtained using a linear probe placed in a transverse plane over the orbit. The Optic Nerve Sheath Diameter (ONSD), the length from outer wall to outer wall of the optic nerve, was measured 3mm deep to the retina bilaterally. The ONSD was above the limits of normal bilaterally at 7.76 mm, 6.16 mm (left) and 6.86 mm, 6.44mm (right). There were also convex appearing, elevated optic discs protruding into the hypoechoic posterior vitreous, indicating papilledema.
+ What is your diagnosis?
Idiopathic Intracranial Hypertension (IIH), formerly known as Pseudotumor Cerebri.
The patient’s clinical history was highly suggestive of IIH and bedside ocular ultrasound demonstrated increased ONSD bilaterally, in addition to sonographic papilledema, as evidenced by the convex appearing optic discs. Neurology and Ophthalmology were both consulted in the Emergency Department where she had visualized 2-3+ papilledema on dilated retinal exam. The patient was subsequently started on diuretic therapy. Two days later she underwent fluoroscopic-guided LP and was found to have an opening pressure of 52 cm of H2O. 30 mL of CSF was removed and she noted significant improvement of her headache and vision.
IIH is characterized by an increase in ICP without a demonstrated precipitant. Cerebrospinal fluid (CSF) analysis should be normal with the exception of an elevated opening pressure, and neuraxial imaging should be performed to rule-out any underlying tumor or other pathology (eg. dural venous thrombosis, mass lesion). It most commonly affects females of childbearing age and obesity is a major risk factor. Patients often present with daily headaches associated with vision changes, as was seen in the patient above. Treatment is aimed at weight loss via diet and exercise, as well as prescribing oral acetazolamide, which inhibits CSF production. In patients with refractory symptoms despite initial therapy, bariatric surgery, CSF shunting, and optic nerve fenestration surgeries may be considered.
+ Ultrasound Pearls
Ultrasound has emerged as a reliable tool for bedside diagnosis of increased ICP. Bedside ocular ultrasound should be considered as an adjunct to physical examination when there is concern for papilledema or increased ICP. It is often quite difficult to perform a non-dilated fundoscopic exam in the Emergency Department, especially if the patient is experiencing photophobia. In experienced hands, ultrasound can be used to evaluate for increased ONSD and sonographic papilledema.
A prospective, blinded observational study in 2008 by Kimberly et al showed that bedside ocular ultrasound performed by experienced ultrasonographers correlated well with direct measurements of ICP by ventriculostomy in ED and ICU patients, and confirmed that an ONSD >5mm was 88% sensitive and 93% specific for an ICP >20 cm H2O (2). Compared to other non-invasive measures of ICP, ONSD measurements correlates most closely with invasive ICP measurement (3). When specifically evaluating for IIH, Lochner et al published an article this year in the Journal of Neurology showing the use of ocular ultrasonography as a means of following a patient’s clinical history over time, and showed that sonographic papilledema and ONSD improved over time, which correlated well with patient reported improvement in headache (4).
To set up for ocular ultrasound, we recommend placing the patient in the supine position. A Tegaderm should be carefully applied to the patient's closed eye ensuring that no air bubbles have been trapped underneath. Sterile ultrasound gel should then be placed on the Tegaderm. A sterilized linear transducer should be utilized. It is important to brace the hand on the patient’s cheek or nose so to avoid inadvertent pressure transmission to the eye. Obtain an axial view of the orbit and freeze the screen when the optic nerve is visualized, seen as a hypoechoic linear structure beginning at the posterior orbit and extending deep into the far-field (appears similar to a shadow). Mark 3mm posterior to the orbit, then measure the ONSD from outer wall to outer wall.
Similar to Kimberly et al., we recommend taking three separate measurements on three separate static images per eye, and taking the average ONSD for each eye. When performing ultrasound of the orbit, you may also evaluate for sonographic papilledema, seen in our patient as raised optic discs that are convex in appearance and protruding into the posterior vitreous hypoechoic space. This is also called the crescent sign. The only absolute contraindication to orbital ultrasound is concern for an open globe injury.
The cutoff for an ONSD measurement that represents increased ICP has evolved with time. The literature is variable, however, 5mm is concerning for increased ICP (>20 cm H2O), 5-6mm is a gray zone, and >6mm represents elevated ICP.
Authored by Shaun Harty, MD
Peer Reviewed by Lori Stolz, MD, RDMS
References
Hansen HC, Helmke K. The subarachnoid space surrounding the optic nerves. An ultrasound study of the optic nerve sheath. Surg Radiol Anat. 1996;18(4):323–8.
Kimberly HH, Shah S, Marill K, et al. Correlation of optic nerve sheath diameter with direct measurement of intracranial pressure. Acad Emerg Med. 2008;15(2):201–4
Robba C, Cardim D, Tajsac T, et al. Ultrasound non-invasice measurement of intracranial pressure in neurointensive care: A prospective observational study. PLoS Med 14(7): e1002356.
Lochner P, Fassbender K, Lesmeister M, et al. Ocular ultrasound for monitoring pseudotumor cerebri syndrome. Journal of Neurology. 2018;265(2):356-361
