Imaging of the optic nerve with standardised echography.

Standardised echography is a unique method of ophthalmic ultrasound that requires special instrumentation, well prescribed methods of examination and a high level of operator skill. Because of its anatomical features and position in the orbit, the optic nerve is well suited for imaging by this technique, particularly for the detection of subtle lesions in its anterior portion and nerve head. Standardised A-scan can measure precisely the width of the nerve, differentiate between fluid and solid lesions as a cause of nerve swelling (the 30 degree test) and, by examining reflectively and internal structure, provide "ecographic tissue diagnosis" in a high proportion of cases. Contact B-scan is suitable for detecting lesions of the optic disc (drüsen, swelling and large cupping), calcification in the nerve (meningioma, deep drüsen), and for the topographic display of lesions. CT-scan is superior in displaying the apical and intra-cranial portion of the nerve, and the simultaneous presentation of both orbits, peri-orbital structures and bony walls.