Ultrasonography of the Optic Nerve Sheath

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A 77-year-old man presented to the ED with no past medical history after being found on the floor of his shower. His blood sugar and vital signs were normal, but his Glasgow coma score was 6, and he was intubated for airway protection. His physical exam revealed midrange sluggishly reactive pupils, clear breath sounds, normal heart rate without murmurs, no signs of trauma, and occasional random extremity movements. The chest x-ray, EKG, CBC, PT, PTT, and cardiac enzymes were normal. Ultrasound of the patient's optic nerve sheath was performed to look for signs of elevated intracranial pressure. The ultrasound in Figure 1 is of his eye and the optic nerve extending posteriorly to the globe. Figure 2 shows the patient's eye as seen by CT scan. The measurement is of the optic nerve sheath diameter (ONSD), which has been shown in multiple small studies to correlate well with elevated intracranial pressure. (J Neurosurg 1997;87:34; Amer J Ophthalmol 1993;116:548; Doc Ophthalmol 1968;24:289.) Anatomically the optic nerve is an extension of the central nervous system, and is encased in a meningeal sheath. This optic nerve sheath is actually an extension of the dura mater and arachnoid meningeal layer. Under the meningeal sheath, cerebrospinal fluid (CSF) fills the subarachnoid space surrounding the optic nerve. In 1968, Hayreh performed studies in rhesus monkeys showing that the subarachnoid space surrounding the optic nerve sheath communicates directly with the intracranial cavity. (Doc Ophthalmol 1968;24[2]:289.) He did this by placing rubber balloons in the subarachnoid space, and documented variability of the ONSD with changing CSF pressures.Fig. 1: This ultrasound shows the eye and the optic nerve extending posteriorly to the globe.Fig. 2: The patient's eye as seen by CT scan.Using ultrasound to measure the optic nerve sheath diameter can be used in head trauma, intracranial bleeding, CNS tumor, hydrocephalus, and pseudotumor cerebri More specifically, as he increased the pressure in the balloons there was an immediate increase in the diameter of the optic nerve sheath, which decreased with a reduction in balloon pressure. German researchers further expounded on these studies by performing simultaneous lumbar punctures on humans at two specific segments of the spinal cord. (J Neurosurg 1997;87:34.) This allowed them to infuse and remove set amounts of fluid while recording the CSF pressures in real time. They then repeated the experiment, measuring the ONSD at specific CSF pressures. They found a direct relationship between the CSF pressure and the dilation of the optic nerve sheath until a diameter of 7 mm was reached. The conclusion of these studies was that increased intracranial pressure (ICP) will increase the optic nerve sheath diameter. CT radiographic findings of increased ICP such as midline shift from mass effect of 3 mm or greater, collapse of the third ventricle, hydrocephalus, effacement of the sulci with evidence of significant edema, and abnormal mesencephalic cisterns have been well validated in previous studies. Blaivas et al compared the ONSD of 14 patients with elevated ICP with the ONSD of 14 patients with normal CT scans. (Acad Emerg Med 2002;9:791.) They found that at an ONSD greater than 5 mm, ultrasound has a sensitivity of 100% and a specificity of 95% for increased ICP when compared with CT. In this study, the positive predictive value of ultrasound was 93% and the negative predictive value was 100% for documenting increased ICP. The applications for which ultrasound of the ONSD can be used are numerous, including head trauma, intracranial bleeding, CNS tumor, hydrocephalus, pseudotumor cerebri, and hypertensive emergency. The ultrasound is performed by using a high-resolution linear array probe in the range of 9-11 MHz. The probe is applied to the closed eyelid with coupling gel and three measurements of the ONSD 3 mm posterior to the retina are taken and averaged between both eyes. Current literature shows that bedside sonography of the ONSD can be performed after a one-hour lecture and a one-hour training session. Measurements of the ONSD are not limited to adults. Studies performed in pediatrics have demonstrated that the upper limit of normal for the ONSD is 4.5 mm in children 1 to 15 years and 4.0 mm in infants up to age 1. (Brit J Ophthalmol 2002;86;1109.)

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