ONSD and Idiopathic Intracranial Hypertension

27F no PMH presenting with a nontraumatic headache and with normal vitals

1. What are the normal values for the ONSD?

Measured from the inner-edge to inner-edge, 3 mm posterior to the retina, for the upper limit of normal of the ONSD is:
– up to 4 mm in infants
– up to 4.5 mm in children
– up to 5 mm in adults
Measurements above 5 mm (bilaterally) classically correspond with elevations in intracranial pressure above 20 mmHg

2. What is visualized in picture B?

Crescent Sign – The separation of the optic nerve sheath from the nerve due to increased subarachnoid fluid
Nipple Sign – Protrusion of the ONSD into the vitreous chamber, >0.6 mm correlates with papilledema
Differentiation between acute and chronic elevations in ICP may be aided by the crescent sign, the sonographic correlate of papilledema

3. What is the significance of the ONSD:ETD Ratio?

While taking multiple measurements and averaging them is useful for accuracy, studies have shown upper limit ONSD ranges from 4.8 to 6.2mm.
There has been increasing evidence that the ONSD:ETD Ratio is more accurate.
One study in patients with TBI was able to predict intracranial hypertension with a threshold ratio value of 0.25 with a sensitivity of 90% and a specificity of 82.3%.

In our patient’s case, the ONSD:ETD Ratio is 0.318. Confirmatory LP was performed and found to be elevated confirming her diagnosis of Idiopathic Intracranial Hypertension.