Sunday, November 13, 2011

Optic Nerve Head Drusen

Signs and Symptoms
A condition involving retained hyaline bodies in the anterior optic nerve, optic nerve head drusen (ONHD) has been referred to in the literature by many diverse and confusing names. Among the descriptive terms are congenitally elevated or anomalous discs, pseudopapilledema, pseudoneuritis, buried disc drusen, and disc hyaline bodies. ONHD is encountered in approximately 1 percent of the general population, and is bilateral in 70 percent of cases. The condition occurs primarily in Caucasians and is believed to demonstrate an autosomal dominant inheritance pattern with incomplete penetrance. Typically, patients with ONHD remain asymptomatic, and the finding is discovered only on routine ocular evaluation. In some instances, however, the condition can present with mildly decreased visual acuity and visual field defects. An afferent pupillary defect may be noted if the condition is both significant and asymmetric. Reports of recurrent, transient visual obscurations associated with disc drusen have also been documented.

The classic appearance of ONHD involves bilaterally elevated optic discs with irregular or "scalloped" margins, a small or nonexistent cup, and unusual vascular branching patterns that arise from a central vessel core. Often there are small, refractile hyaline deposits visible on the surface of the disc and/or in the peripapillary area. ONHD most often manifests on the nasal disc margin, but can be found within any part of the nerve head. In younger patients, the disc elevation tends to be more pronounced and the drusen less calcific, making them less visible ophthalmoscopically, and hence offering a more challenging diagnostic dilemma. Unlike true disc edema, ONHD very rarely presents with juxtapapillary nerve fiber edema, exudate, or cotton-wool spots.

There is no histopathological correlation between drusen of the optic nerve head and retinal drusen; the former represent acellular laminated concretions, often partially calcified, possibly related to accumulation of axoplasmic derivatives of degenerating retinal nerve fibers. As mentioned, ONHD tend to remain "buried" in children, but slowly become visible as they enlarge toward the disc surface and as the overlying retinal nerve fiber layer progressively thins. They are usually ophthalmoscopically detectable by the early to mid-teens, although these authors have seen patients in their mid-20s who continue to display "buried drusen". Within the optic nerve, the hyaline bodies are confined anterior to the lamina cribosa and thus can compress and compromise the nerve fibers and vascular supply, leading to visual field defects and disc hemorrhages. Along with slowly developing optic atrophy in extreme cases, disruption of the juxtapapillary tissue can result in choroidal neovascular membrane formation leading to subretinal hemorrhage and disciform retinal scarring.

Although ONHD is typically classified as a benign condition, it can lead to modest visual compromise. First and foremost, ONHD must be clearly differentiated from acquired disc edema, which warrants immediate neurologic evaluation and treatment. Management of ONHD includes prompt and proper diagnosis, which is aided greatly by the use of ocular ultrasonography. The high reflectivity of the calcified hyaline bodies is dramatically evident on B-scan ultrasonography, even at particularly low gain levels. Careful evaluation of optic nerve function is imperative, and should include standard Snellen acuity, contrast sensitivity, color vision testing, and threshold visual fields. Photodocumentation should be obtained for future monitoring. It is also important for patients to self-monitor their vision periodically, particularly because of the risk of choroidal neovascular membrane formation. Should these membranes be noted or suspected, intravenous fluorescein angiography is recommended, followed by laser photocoagulation as indicated. More routine cases should be monitored every six to 12 months.

Clinical Pearls

Some suggest that the vast majority of congenitally anomalous, elevated optic discs are likely associated with nerve head drusen. Recognize the clinical features associated with ONHD in comparison to those features indicative of true optic disc edema. In ONHD, expect a typically "normal" pink to pinkish-yellow color, rather than a pale waxy disc or a hyperemic disc. In addition, a spontaneous venous pulsation is present in about 80 percent of patients with ONHD, but is absent in cases of true disc edema. Most importantly, recognize that while the disc margins may be irregular in ONHD, rarely are they blurred or obscured.

Ultrasonography is probably the single most important ancillary test to perform in adults with ONHD. This procedure should be performed on all adult patients presenting with elevated optic discs that are not definitively identifiable as true optic disc edema based upon ophthalmoscopic observation or history. Keep in mind, however, that optic disc drusen are generally not calcified in children and adolescents. Hence, ultrasonography may prove to be of little help in diagnosing these cases.

While visual fields are an important method of documenting and monitoring optic nerve compromise secondary to ONHD, they are neither uniform nor diagnostic. The more common patterns encountered in ONHD include nasal step defects, enlargement of the physiologic blindspot, arcuate scotomas, sectoral field loss, and altitu

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