Optic Nerve Disorders
Lisa D. Lystad, MD*, Brandy C. Hayden, BS, Arun D. Singh, MD
KEYWORDS
Papilledema Optic disc drusen
Congenital disc anomalies Giant cell arrteritis
Diseases of the optic nerve are difficult to assess, as tissue diagnosis is usually unavailable. Therefore, various subjective functional tests, such as visual acuity, visual field, and color testing; and objective tests, such as afferent pupillary defect or electrophysiologic evaluation are relied upon to achieve a differential diagnosis. Imaging modalities such as MR imaging and CT generally are required to establish a definitive diagnosis. This article reviews clinical scenarios wherein ultrasonographic imaging provides a readily accessible and inexpensive means for diagnosing and monitoring optic nerve disorders.
TECHNIQUE
The details of ophthalmic ultrasonographic technique are described elsewhere (see the article by Hayden and colleagues, elsewhere in this issue). The basic B-scan imaging technique used for the globe is the same for the optic disc and retrobulbar optic nerve.1,2 Specialized A-scan techniques, however, are used for measuring and evaluating the retrobulbar optic nerve.
Normal Retrobulbar Optic Nerve
Measurements
The optic nerves are usually symmetric and measure the same thickness both anteriorly and posteriorly.3 The optic nerve sheath diameter is measured in two locations, 3 mm posterior to the optic nerve head and as close as possible to the orbital apex (Fig. 1). Normal retrobulbar optic nerves measured at the arachnoid sheath are 2.2 to 3.3 mm in diameter.4 Significant variation, however, can occur in the general population, and it is advised to compare the diameters in both eyes. A difference of 0.5 mm between eyes
frequently indicates an abnormal thickness in one eye and should raise a suspicion for a retro-or- bital mass lesion (Fig. 2).5 There are no significant changes in optic nerve sheath diameter when measured in Trendelenburg or reverse Trendelenburg positions as compared with the supine position in healthy adults.6
30 Test
Increased subarachnoid fluid can be differentiated from thickening of the parenchyma or perineural sheaths with a 30 test.7 The patient fixates in primary gaze (straight ahead position), and the optic nerve perineural sheaths are measured anteriorly and posteriorly. The patient’s gaze then is directed 30 laterally, and the perineural sheaths are measured again. The test is based on the premise that when the eye is fixated laterally, the optic nerve sheaths are stretched and the subarachnoid fluid is spread over a larger area. A decrease in sheath diameter of greater than 10% in lateral gaze, as compared with primary gaze, is considered a positive 30 test and thus indicative of increased subarachnoid fluid.
PAPILLEDEMA
Adults
Kimberly and colleagues8 correlated optic nerve sheath diameters (measured by operators blinded to the intracranial pressure (ICP) measurements) with directly measured ICP in patients who had invasive intracranial monitors placed as part of their treatment. Optic nerve sheath diameters greater than 5 mm correlated with ICP greater than 20 cm water, with a sensitivity of 88% and a specificity of 93%.
Department of Ophthalmology, Cole Eye Institute, Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, OH 44195, USA
* Corresponding author. Cole Eye Institute (Mail code i-32), 9500 Euclid Avenue, Cleveland, OH 44195. E-mail address: lystadl@ccf.org (L.D. Lystad).
Ultrasound Clin 3 (2008) 257–266 doi:10.1016/j.cult.2008.04.010
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Fig.1. Normal optic disc and retrobulbar optic nerve. Fundus photograph shows normal optic disc of the left eye (A). Transverse B-scan shows a cross section of the retrobulbar optic nerve (B). Diagnostic A-scan shows a normal retrobulbar optic nerve diameter measuring 2.8 mm (C, anteriorly and D, posteriorly). The spikes correspond to the arachnoid sheath, and the distance between the arrows gives the optic nerve diameter.
Trauma
CT scan is the gold standard for evaluating acute head trauma. This, however, is not always available or feasible in the immediate evaluation of trauma. Ophthalmic ultrasonography, combining B-scan and use of the 30 test, can provide a means to identify those who may need surgical intervention for acute increased intracranial pressure (Fig. 3).
In a year-long, prospective, blinded observational study, Goel and colleagues9 evaluated 100 consecutive head trauma patients who presented at an emergency department of a tertiary care institution. The median Glasgow coma scale score was 11. Optic nerve sheath diameter using a 7.5 MHz probe was compared with CT scan of the head for evaluation of acute ICP elevation. A mean optic nerve sheath diameter of greater than 5 mm was considered to be indicative of increased ICP. The mean optic nerve sheath diameter in those without increased ICP was 3.5 mm plus or minus 0.75 mm. Of 74 patients who had optic nerve sheath diameter of greater than 5 mm, 59 patients had intracranial hematoma requiring urgent surgical
evacuation. Overall, ultrasonography yielded a sensitivity of 98.6% and a specificity of 92.8% for detection of elevated ICP.
In another study Karakitsos and colleagues10 observed that optic nerve diameter greater than 5.9 mm at presentation and an increase in nerve diameter of 2.5 mm between repeated measurements were associated with a poor clinical prognosis. Optic nerve monitoring, however, had a low predictive value for brain death.
Intracranial pathology
Blaivas and colleagues5 performed a prospective blinded observational study on emergency department patients suspected of having elevated ICP from various intracranial pathologies. He correlated ultrasonographic measurements of optic nerve sheath diameter with CT scan findings. All cases exhibiting a midline shift of 3 mm or more, collapse of the third ventricle, hydrocephalus, abnormal mesencephalic cisterns, or sulcal effacement were considered to have raised ICP. Such patients were predicted correctly by optic nerve sheath diameter greater than 5 mm. The
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Fig. 2. Uveal lymphoma infiltrating the optic nerve. Fundus photo showing diffuse uveal infiltration and normalappearing optic disc (A). Transverse B-scan showing diffuse, low reflective infiltration of the choroid (white arrow), infiltration in Tenon’s space (black arrows), and enlarged optic nerve (arrowheads) (B). Diagnostic A-scan of the left eye shows slightly enlarged retrobulbar optic nerve diameter measuring 3.7 mm (C) compared with contralateral normal retrobulbar optic nerve (D, diameter 5 3.2 mm).
ultrasonographic measurements were correlated more closely to the intracranial pathology than the clinical examination. Mean sheath diameter for those not meeting CT criteria for raised ICP was 4.4 mm. Several studies have shown the efficacy of using ultrasound measures of optic nerve diameter for following hospitalized patients with increased ICP, regardless of the underlying etiol-
ogy.5,11–13
Children
The normal range for optic nerve diameter also has been established for infants and children up to 15 years of age.1,2,12,14,15 In a prospective study of nerve measurements in 102 children admitted to a hospital for diagnoses unrelated to orbital or intracranial disease, the range of measured diameter was 2.1 to 4.3 mm (mean, 3.08 mm).1 Data analysis revealed that the most rapid change in nerve sheath diameter occurred over the first 2 months of life. There was no significant difference between right and left eyes or between the
sexes. Analysis of their results found that a sheath diameter of greater than 4 mm in infants under 1 year of age and of greater than 4.5 mm in children age 1 to 15 years should be considered abnormal. Similar normative data have been observed by others.14,15 In those over 15 years of age, mean diameter of greater than 5 mm is considered abnormal and correlates with an intracranial pressure of greater than 20 cm water.8
Malayeri and colleagues14 performed a case– control study of 156 hospitalized children between the ages of 1 and 13 years. Half of the subjects had normal ICP, while the other half had a confirmed increased ICP. In the group with increased ICP, mean optic nerve diameters were 5.6 mm plus or minus 0.6 mm (range, 4.55 mm to 7.60 mm). In another study of optic nerve sheath diameters in children who had shunted hydrocephalus, those who had functioning ventriculoperitoneal shunts had a mean optic nerve sheath diameter of 2.9 mm plus or minus 0. 5 mm, compared with diameters of 5.6 mm plus or minus 0.6 mm in those who had shunt malfunction and increased ICP.15