Ординатура / Офтальмология / Английские материалы / Ophthalmic Ultrasound A Diagnostic Atlas 2nd edition_ DiBernardo, Greenberg_2006

Aside from the clinical evidence of optic nerve disease based on findings ranging from afferent pupillary defect, decreased color vision, pale discs or disc swelling, the retrobulbar optic nerve is inaccessible for clinical evaluation. With the development of computed tomography (CT) and magnetic resonance imaging (MRI), visualization of this portion of the optic nerve is possible. But however sensitive these tests may be, there is still a wide variance in the differentiation of specific lesions involving the optic nerve. In particular, CT and MRI are less sensitive in distinguishing fluid beneath the nerve sheaths versus a solid mass lesion.

Using contact B-scan, the echographer has the capability of evaluating the optic nerve from the disc to about two-thirds into the orbit. Performing axial scans is an excellent way to evaluate the intraocular portion of the nerve for disc elevation; however, it is not the best way to rule out optic disc drusen because there is usually significant sound attenuation from the lens, which may mask the appearance of drusen, especially if they are small. Both drusen and elevation can be imaged using a longitudinal approach, or with cross-sectional views. If drusen or disc elevation is present, it can be easily identified.

In cross section, the retrobulbar nerve appears as a distinct, round, relatively low reflective structure. When there is fluid within the sheaths, an echolucent crescent shape can be imaged to the right of the round nerve proper. If solid thickening of the nerve is present, the borders may not be as distinct as the normal nerve, and if calcium is present within the sheaths, it can easily be detected.

To image the nerve with the B-scan, the patient should be fixating in primary gaze. The probe is placed temporally and the back of the probe is rotated forward with the gain set at a medium level. Obtaining

measurements of the retrobulbar optic nerve using the B-scan can be difficult, and it is not recommended, but the information obtained may be adequate for evaluation of children and non-cooperative patients.

Standardized A-scan evaluation of the retrobulbar optic nerve has been shown to be an effective, accurate method for determining the diameter of the optic nerve and for detecting the presence of increased subarachnoid fluid (ISAF) or solid lesions involving the nerve. The A-scan probe is also placed temporally with the patient fixating in primary gaze. The gain should be set at the standard tissue sensitivity setting for the probe/machine combination. Because the optic nerve is a homogeneous structure, it will produce a low reflective defect within the normally high reflective echo pattern of the orbital tissue. Measurements should be taken from both the anterior and posterior portions of the nerve, as it is possible for a patient to have both a fluid and solid component to optic nerve thickening. Failure to evaluate the nerve completely could lead to misdiagnosis.

The normal nerve measures from 2.3 to 3.3 mm in diameter from the inner aspects of the arachnoid sheaths. If the nerve measures larger than normal in primary gaze, a 30-degree test should be performed to determine if the increased thickness is due to fluid within the arachnoid sheaths or if it is secondary to a solid mass lesion. To perform this test, the patient should be instructed to look 30 degrees toward the probe that is placed on the globe temporally. Measurements should then be repeated both anteriorly and posteriorly and compared with the measurements obtained in primary gaze. As the eye moves 30 degrees, if fluid is present, it will flow back along the nerve and the measurements obtained will be smaller (at least 10%) than the primary gaze measurements. If no fluid is present,

the measurements in 30-degree gaze will remain similar to those obtained in primary gaze. In this case, the possibility of solid optic nerve thickening exists.

Optic Nerve Head Drusen

Past studies suggest that many optic disc drusen are not detected or appreciated during routine clinical examination. Patients may present with no visual complaints, but abnormal appearing discs (blurred margins or elevation) may be appreciated during funduscopic examination. Symptoms may be present, including visual field loss or rarely, an acute decrease in vision.

Optic disc drusen can be located superficially or covered by a thin layer of tissue. They can also be buried deep within the plane of the posterior sclera. They can

be noted unilaterally, but in most cases they are noted to be bilateral. Fluorescein angiography has been used to confirm the suspicion of optic disc drusen, and sometimes the diagnosis can be made using CT. Over the last decade, B-scan ultrasound has proven to be an accurate, rapid, non-invasive and inexpensive way of diagnosing and documenting the presence of drusen in both clear and opaque media.

Using basic echographic examination techniques and a decreased gain setting for better resolution, drusen appear as very highly reflective, foreign body–like lesions at the optic nerve head. Measurement of the diameter of the retrobulbar optic nerves using standardized A-scan is always recommended to rule out enlargement of the nerve secondary to other optic nerve pathology.

Suggested Readings

Boldt HC, Byrne SF, DiBernardo C. Echographic evaluation of optic disc drusen. J Clin Neuroophthalmol 1991;11:85–91

Darnley-Fisch DA, Byrne SF, Hughes JR, Parrish RK II, Feuer WJ. Contact B-scan echography in the assessment

of optic nerve cupping. Am J Ophthalmol 1990;109: 55–61

Gans MS, Byrne SF, Glaser JS. Standardized A-scan echography in optic nerve disease. Arch Ophthalmol 1987;105:1232–1236