Madtown Sono

Young female with headache

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By Johnny Duan, MD
Edited by Hani I. Kuttab, MD

Clinical Case

A young female with no significant medical history presents to the emergency department (ED) with a headache intermittently present for the past week. She also endorses blurry vision, black spots in her vision, dizziness, trouble walking, nausea, and vomiting. On physical exam, she has difficulty with leftward gaze. A bedside ocular ultrasound was performed (Figures 1 and 2).

Ocular ultrasound of the left eye
Figure 1. Ocular ultrasound of the left eye

Ocular ultrasound of the right eye
Figure 2. Ocular ultrasound of the right eye

Diagnosis

Idiopathic Intracranial Hypertension (Pseudotumor Cerebri). Bedside ultrasound showed dilatation of the optic nerve sheath diameter (ONSD) (Figure 3). MRI obtained in the ED confirmed bilateral optic nerve sheath dilatation and signs of severe papilledema. Lumbar puncture was performed with an opening pressure of >55cm H2O. The patient was subsequently admitted for Neurology consultation and was initiated on acetazolamide. Two weeks following ED presentation, the patient reported significant improvement in symptoms.

Measurement of the optic nerve sheath diameter (OSND)
Figure 3. Measurement of the optic nerve sheath diameter (OSND)

What is the OSND?

The optic nerve is surrounded by the optic nerve sheath, which is contiguous with the meninges. High intracranial pressure (ICP) can be transmitted along the optic nerve sheath, which, in response, may dilate. The point of maximum dilation is at the ampulla of the optic nerve sheath, which lies 3mm deep to the optic disk.

How do I obtain the OSND?

  • Cover the eye with a occlusive dressing (tegaderm) and apply plenty of gel. You can also perform the exam without a dressing but with the use of surgilube, not ultrasound gel (as this may be irritating to the eye).
  • Using a high frequency linear probe, obtain an on-axis view of the optic nerve. You can change the probe angle or ask the patient to look in a different direction to achieve this. The ONSD is the inner-to-inner diameter of the hyperechoic CSF surrounding the hypoechoic optic nerve at the ampulla of the ONS— so, when measuring, you want to be sure to measure 3mm deep to the retina (as in Figure 3 above).
  • Other tips:
    • Anchor your hand on the patient’s face to avoid applying significant pressure to the globe.
    • Avoid imaging through the lens – refraction often distorts the ONSD.
    • Use high gain to minimize the “blooming artifact.”
    • Use the highest measured ONSD value from both eyes – this may have greater diagnostic value.
    • Be mindful of the mechanical index (MI). “As low as reasonably achievable” (ALARA) approach is best. Use the dedicated ocular settings on your machine, if available. An MI of <0.23 is considered standard.
Axial alignment of ultrasound probe with optic nerve
Figure 4. Axial alignment of ultrasound probe with optic nerve (from Newman et al., 2020)

In adults, <5mm is considered normal.

Strengths and Limitations

Ocular ultrasound is non-invasive and can be easily performed at the bedside by ED physicians and trainees. Increased ICP is a factor in many pathologies, ranging from benign (idiopathic intracranial hypertension) to critical (encephalitis, herniation). If the patient’s acuity or the next diagnostic step is unclear, ONSD measurement could potentially guide further workup.

Though there is consensus that ONSD has excellent test characteristics (sensitivity/specificity is typically 85%-95%), the optimal upper limit of normal varies between studies (5-6mm). ONSD responds to acute increases in ICP, but the biomechanics of this are not fully understood. Correlation between ONSD and CT or MRI may show up to 10% intermodal variation, although this does not seem to impact the validity of ONSD in screening.

Teaching Points

ONSD may provide value in the ED setting as a low-cost, reliable, and noninvasive test. However, its limitations preclude its use as a gold standard. When an ambiguous patient presentation or limited information challenges the clinician, ONSD may provide a valuable additional data point and guide further workup or consultation.

References

  1. Lin J, Chen AE, Lin EE, Hsia S, Chiang M, Lin K. Point-of-care ultrasound of optic nerve sheath diameter to detect intracranial pressure in neurocritically ill children – A narrative review. Biomed J 2020; 43:231-239.
  2. Agrawal A, Cheng R, Tang J, Madhok DY Comparison of Two Techniques to Measure Optic Nerve Sheath Diameter in Patients at Risk for Increased Intracranial Pressure. Crit Care Med 2019; 47:e495-e501.
  3. Naldi A, Provero P, Vercelli A, Bergui M, Mazzeo AT, Cantello R, Tondo G, Lochner P. Optic nerve sheath diameter asymmetry in healthy subjects and patients with intracranial hypertension. Neurol Sci 2020; 41:329-333.
  4. 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 2016; 54:35-38.
  5. Marchese RF, Mistry RD, Binenbaum G, Liu GT, Scarfone RJ, Woodford AL, Chen AE. Identification of Optic Nerve Swelling Using Point-of-Care Ocular Ultrasound in Children. Pediatr Emer Care 2018; 34:531-536.
  6. Kishk NA, Ebraheim AM, Ashour AS, Badr NM, Eshra MA. Optic nerve sonographic examination to predict raised intracranial pressure in idiopathic intracranial hypertension: The cut-off points. Neuroradiol J 2018; 31(5): 490-495.
  7. Koziarz A, Sne N, Kegel F, Nath S, Badhiwala JH, Nassiri F, Mansouri A, Yang K, Zhou Q, Rice T, Faidi S, Passos E, Healey A, Banfield L, Mensour M, Kirkpatrick AW, Nassar A, Fehlings MG, Hawryluk GWJ, Almenawer SA, Bedside Optic Nerve Ultrasonography for Diagnosing Increased Intracranial Pressure. Ann Intern Med 2019; 171:896-905.
  8. Hassen GW, Bruck I, Donahue J, Mason B, Sweeney B, Saab W, Weedon J, Patel N, Perry K, Matari H, Jaiswal R, Kalantari H. Accuracy of Optic Nerve Sheath Diameter Measurement by Emergency Physicians using Bedside Ultrasound. J Emerg Med 2015; 48(4): 450-457.
  9. Bhargava V, Tawfik D, Tan YJ, Dunbar T, Haileselassie B, Su E. Ultrasonographic Optic Nerve Sheath Diameter Measurement to Detect Intracranial Hypertension in Children With Neurological Injury: A Systematic Review. Pediatr Crit Care Med 2020; 21:e858-e868.
  10. Newman WD, Hollman AS, Dutton GN, Carachi R. Measurement of optic nerve sheath diameter by ultrasound: a means of detecting acute raised intracranial pressure in hydrocephalus. Br J Ophthalmol. 2002;86(10):1109-1113. doi:10.1136/bjo.86.10.1109.