Normal retrobulbar optic nerve measurements

The optic nerves are usually symmetric and measure the same thickness both anteriorly and posteriorly.⁴ Utilizing ultrasound 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 (Figure 13.1). Normal retrobulbar optic nerves in an adult measured at the optic nerve sheath are 2.2 to 3.3 mm in diameter.⁵ Significant variation, 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 suspicion for optic nerve pathology (Figure 13.2).⁶ There are no significant changes in optic nerve sheath diameter when measured in the Trendelenburg or reverse Trendelenburg position as compared with the supine position in healthy adults.⁷

Figure 13.1 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.

Reproduced with permission from: Lystad LD, Hayden BC, Singh AD. Optic nerve disorders. Ultrasound Clin 2008; 3:257–266.

Figure 13.2 Uveal lymphoma infiltrating the optic nerve. Fundus photograph showing diffuse uveal infiltration and normal appearing 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 = 3.2 mm).

Reproduced with permission from: Lystad LD, Hayden BC, Singh AD. Optic nerve disorders. Ultrasound Clin 2008; 3:257–266.

30° test

Increased subarachnoid fluid can be differentiated from thickening of the parenchyma or perineural sheaths with a 30° test.⁸ The patient fixates in primary gaze (straight ahead position), and the optic nerve perineural sheaths are measured anteriorly and posteriorly. The patient’s gaze is then 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.

Adults

Blaivas⁵ performed a prospective observational study on emergency department patients suspected of having elevated intracranial pressure from various intracranial disorders.⁶ 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 intracranial pressure.⁶ Such patients were predicted correctly by optic nerve sheath diameters of greater than 5 mm. The ultrasonographic measurements were correlated more closely to the intracranial pathology than the clinical examination. Mean optic nerve sheath diameter for those not meeting CT criteria for raised intracranial pressure was 4.4 mm. Several studies have also shown the efficacy of optic nerve diameter for following hospitalized patients with increased intracranial pressure, regardless of the underlying etiology.^6,9–11

Kimberly correlated optic nerve sheath diameters (measured by operators blinded to the measurements) with directly measured intracranial pressure in patients who had invasive intracranial monitors placed as part of their treatment.¹² In an adult, optic nerve sheath diameters greater than 5 mm correlated with intracranial pressure greater than 20 cmH₂O, with sensitivity of 88% and a specificity of 93%.

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 triage those who may need surgical intervention for acute increased intracranial pressure (Figure 13.3).

Figure 13.3 Papilledema. Fundus photographs (A, right eye; B, left eye) show marked elevation of the optic disc with prominent nerve fiber layer (arrows) that obscures clarity of the retinal vessels at the optic disc nerve head margin (arrowhead). Transverse B-scan shows marked elevation of the optic disc (C). Transverse B-scan shows a cross section of the retrobulbar optic nerve (arrow) and low reflective crescent-shaped echolucent area behind the nerve indicative of increased subarachnoid fluid (arrowheads) (D). Positive 30° test with diagnostic A-scan while the eye is in primary gaze (straight ahead) position with an enlarged retrobulbar optic nerve (diameter = 4.8 mm) (E). When the eye is fixated 30° laterally, note a marked decrease in the size of the retrobulbar optic nerve (diameter = 3.5 mm) (F). MRI (T1) showing increased subarachnoid fluid around optic nerves with bilateral flattening of the posterior globe and intraocular optic disc elevation (G).

Reproduced with permission from: Lystad LD, Hayden BC, Singh AD. Optic nerve disorders. Ultrasound Clin 2008; 3:257–266.

In a year-long, prospective, blinded observational study, Goel evaluated 100 consecutive head trauma patients who presented at an emergency department of a tertiary care institution.¹³ Their 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 intracranial pressure elevation. Optic nerve sheath diameter of greater than 5 mm was considered to be indicative of increased intracranial pressure. The mean optic nerve sheath diameter in those without increased intracranial pressure was 3.5 ± 0.75 mm. Of 74 patients who had an 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 intracranial pressure in this situation.

In another study, Karakitsos 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 was associated with a poor clinical prognosis. Optic nerve monitoring, had a low predictive value for brain death.¹⁴

Children

The normal range of optic nerve diameter has also been established for infants and children up to 15 years of age.^2,3,10,15,16 In a prospective study of nerve measurements in 102 children admitted to the hospital for diagnoses related to orbital or intracranial disease, but not increased intracranial pressure, range of measured optic nerve sheath diameter was 2.2–4.3 mm (mean 3.08 mm).¹ 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 the 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.^15,16 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 cmH₂O.

Malyeri performed a case-controlled study of 456 hospitalized children between the ages of 1 and 13 years.¹⁵