Ultrasound measurement of the optic nerve sheath diameter is a simple non-invasive procedure, which is a useful tool in the assessment of elevated intracranial pressure.
Early identification of elevated ICP (intracranial pressure) is critical to ensuring timely and appropriate management. However, there are few diagnostic methods available for detecting increased ICP in an acutely ill patient, which can be performed quickly and non-invasively at the bedside.
The optic nerve sheath is a continuation of the dura mater of the central nervous system. Optic nerve sheath diameter (ONSD) correlates closely with ICP and can be viewed by ocular ultrasound. Pressure changes within the intracranial cavity affect the diameter of the optic nerve sheath.
The normal optic nerve sheath is up to 5 mm in diameter. ONSD is higher in the presence of a raised ICP (>20 mmHg).
To measure ONSD, take the measurement 3 mm posterior to the globe for both eyes , then take the of the average two measurements. If the average ONSD is >5 mm, suspect raised ICP.
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