OPTIC DISC DRUSEN

The most common cause of pseudopapilledema is optic disc drusen. In Caucasian populations, they occur with an incidence of between 0.34% and 2%.16,17 Bilateral optic nerve drusen are observed in 75% to 86% of patients.18,19 No reliable

prevalence data are available for those of nonCaucasian heritage, where drusen are observed infrequently. The predisposing factors for drusen formation are unclear. Theories of drusen formation include mechanical obstruction to axonal transport in eyes with small sclera canals, abnormal axonal

metabolism, and leakage from abnormal vasculature at the disc head.18,19 Triggers for calcification or the length of time required for its initiation also are understood poorly.20

Adults

In the adult patients, ophthalmoscopy can usually identify optic disc drusen either by the characteristic scalloped appearance of the nerve edge or by the presence of refractile calcified particles on the nerve. These tend to predominate in the nasal portion of the nerve. Red free fundus photography highlights these refractile particles. Sometimes the optic nerve head drusen are buried, however, and cannot be seen with ophthalmoscopy. The optic discs can appear very similar to elevated discs with papilledema. In these cases, B-scan ultrasound of the nerve head is able to clearly identify calcified drusen, making CT scan an unnecessary expense (Fig. 4).

Disc drusen create a congested disc, increasing the risk for anterior ischemic optic neuropathy (AION). In the acute setting of this disease, not

only will the patient present with a typical history of vision loss, but the ultrasound findings clearly show disc edema distinguishable from the underlying calcified drusen (Fig. 5).19

Young Adults

In first two decades of life, disc drusen pose a diagnostic dilemma, because they may be buried beneath the nerve fiber layer and not visible on ophthalmoscopy. They may not be calcified and therefore, not readily detectable by ultrasonography or CT. There are certain diagnostic clues that suggest the presence of disc drusen, however. The ophthalmoscopic clues include early branching of the major retinal vessels, clear visibility of the vessels at the disc head, and presence of spontaneous venous pulsations. The absence of pulsations is nondiagnostic, as they are not visible in up to 20% of normal individuals. On OCT, eyes with buried drusen can have a decreased retinal nerve fiber layer thickness (see Fig. 5).21 It also has been noted that buried drusen are less likely to be associated with visual field