Optic Nerve Examination

•Small field of view but increased magnification •Significant degradation of the view from media opacities or small pupil •Lacks ability to perform stereoscopic examination •Useful when slit-lamp exam not possible •Most commonly used system •Variable magnification and field of view • Reasonable view through small pupil • Good stereoscopic view •Less degradation of view from media opacities than direct ophthalmoscope •The magnification and stereopsis obtained with a slit-lamp system is generally superior to that of a handheld or headlamp-based system. • Poor magnification, with wide field of view •Least degradation of view from media opacities •Value of stereopsis limited by poor magnification Fair to poor with small pupils •May be useful for bedside exam or in the operating room • The vertical diameter of the optic nerve can be easily estimated during a slit-lamp exam by the following method: 1. A thin slit beam is focused on the nerve through the lens of choice. 2. The vertical length of the beam is adjusted to match the height of the disc. 3. The length of the beam is read off the slit-lamp beam scale. 4. The scale reading is adjusted by the correction factor of the specific lens. •This estimation is reasonably accurate, but it tends to underestimate disc size in high levels of myopia, and overestimate in high hyperopia. • One can also use the scale projection of the direct ophthalmoscope to estimate nerve head size, with no correction factor needed. The small light cone of the direct ophthalmoscope subtends an angle of about 5 degrees, about the same size as an average optic disc, and can give a quick estimate of relative size. • Stereo-photographs are the most helpful but are more difficult to obtain in a reproducible manner. •Cameras with a prism-based fixed-angle method of taking simultaneous stereo-photos tend to produce more consistent results, but are more expensive. • While non-mydriatic cameras exist, images suffer when the pupil is smaller than 4 mm, and dilation is commonly employed to obtain the best images.