Grossly there is widening of the optic nerve sheath, measurement demonstrates significant widening. This finding was consistent with the subsequent CT brain which demonstrated a massive intracranial haemorrhage.
This case highlights the potential use of optic nerve scanning, helping to provide a quick, bedside answer when other imaging modalities are not so easily available.
Elevated Intracranial Pressure – Optic Nerve Sheath Measurement
The evaluation of the optic nerve sheath diameter is a simple non-invasive procedure, which is a useful tool in the assessment of elevated intracranial pressure. Ocular ultrasound for evidence of increased intracranial pressure has been described in cadavers. Recently, Blaivas et al. described its use among adult patients in the emergency department with suspected elevated intracranial pressure (EICP).(2)
Patients with altered level of consciousness may be suffering from increased intracranial pressure from a variety of causes. EICP may be present in emergency department patients with head injury and also in those with spontaneous intracranial bleed. Physical examination has significant limitations if the patient is unconscious, or intubated and paralyzed. Papilledema from EICP may be delayed after ICP elevation, by up to several hours. A rapid, bedside and noninvasive means of detecting EICP is desirable when conventional imaging methods are unavailable.
The eyes often reflect disease states elsewhere in the body. The optic nerve attaches to the globe posteriorly and is wrapped in a sheath that contains fluid. The optic nerve sheath is contiguous with the dura mater and has a trabeculated arachnoid space through which cerebrospinal fluid slowly percolates. The relationship between the optic nerve sheath diameter (ONSD) and ICP has been well established. Evaluation of the optic nerve sheath diameter (ONSD) can detect EICP. On ultrasound a normal optic nerve sheath measures up to 5.0 mm in diameter. The ONSD is measured 3 mm posterior to the globe for both eyes. A position of 3 mm behind the globe is recommended because the ultrasound contrast is greatest, the results are more reproducible (Figure 6). Two measurements are averaged. An average ONSD greater than 5 mm is considered abnormal and elevated intracranial pressure should be suspected.
Dr. Sanjay Patel
Acute and Emergency Care Centre, Khoo Teck Puat Hospital. Singapore.
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