CLINICAL

  POSTERIOR

GET A HEAD START ON ONHD

EARLY DIAGNOSIS IS KEY TO SUCCESSFUL OUTCOMES

OPTIC NERVE head drusen (ONHD) are congenital anomalies commonly detected, most often, incidentally, during routine eye examinations. ONHD can be a diagnostic challenge, mimicking papilledema; therefore, they should be considered in any case of optic disc edema. The condition also poses a threat to visual function and, as such, close monitoring is essential.

Here, I discuss the etiology, clinical signs, symptoms, diagnostic considerations and treatment and management of this condition.

ETIOLOGY

ONHD are acellular deposits of calcium, mucopolysaccharides, amino and nucleic acids and iron that accumulate within the optic nerve head. Although the pathophysiology is not completely understood, it has been postulated that abnormal axonal metabolism leads to calcium deposition that results in axonal degeneration, according to Ophthalmology.

There is continuous calcification and enlargement of the lesions as patients get older, making them more clinically visible, commonly on the nasal disc margin.

ONHD is an autosomal dominant condition primarily affecting Caucasian patients. Hence, evaluating relatives of ONHD patients may be valuable.

Bilateral presentation is common, occurring in up to 75% of cases, according to Survey of Ophthalmology.

ONHD may be associated with ocular and systemic conditions, including retinitis pigmentosa, angioid streaks, pseudoxanthoma elasticum, Alport and Alagille Syndrome, according to Survey of Ophthalmology.

CLINICAL SYMPTOMS

There are two types of ONHD, both with different presentations: (1) “visible” drusen and (2) “buried” drusen. “Visible” drusen presents as large, multilobular, yellowish-white refractile bodies. The optic nerve usually has a “lumpy-bumpy” appearance. “Buried” drusen, on the other hand, presents as a subtle elevation of the disc and obscuration of the physiologic cup, giving the appearance of a crowded disc or pseudopapilledema and is more common in younger patients (pre-teenage years). Consequently, children and young adults are particularly vulnerable to misdiagnosis.

Figure 1: HM, a 52-year-old male, presented for his routine eye examination with a complaint of irritated eyes. His medical history was positive for hypertension. Best-corrected visual acuities were 20/20 OD and OS. Pupils were unremarkable. Bilateral ONHD that was previously diagnosed was observed. Visual field testing revealed stable peripheral defects and an enlarged blind spot. OCT testing also demonstrated mild RNFL defects (See Figure 2, page 25). The patient is being monitored closely with a three-month follow-up evaluation.

Figure 2: HM’s OCT image revealed the “lumpy-bumpy” internal contour of the ONHD.

SYMPTOMS

Most ONHD patients are asymptomatic, retaining good visual acuity. As ONHD enlarge, they can impair and compress nerve fibers and vascular structures, leading to the aforementioned complications that can be visually devastating.

Patients often experience progressive visual field loss — as high as 90% — according to Archives of Ophthalmology. Other visual field defects include enlargement of the blind spot, arcuate scotoma, altitudinal defect and constriction. Progressive visual field loss may lead to blindness.

ONHD patients can also experience transient visual obscurations, which occur in about 9% of patients, according to the American Academy of Ophthalmology.

DIAGNOSTIC CONSIDERATIONS

ONHD evaluation should include the following:

• Retinal photography. Red-free photography may reveal autofluorescence of the drusen and alterations in the peripapillary retinal nerve fiber layer (RNFL).

• Visual field testing. Baseline and serial visual field testing is essential to detect progression.

• B-scan ultrasound. This is the gold standard of diagnostic evaluation of ONHD, which are seen as highly reflective, elevated lesions. There is high signal intensity of the ONHD as the gain (signal intensity) is reduced.

• Fluorescein angiography. ONHD will demonstrate focal hyperfluorescence and fluorescein staining of the peripapillary vessel wall.

• Fundus autofluorescence (FAF). FAF imaging may be used as a non-invasive, safe and rapid ancillary test for the confirmation of the diagnosis of ONHD.

• OCT. This is useful in detecting RNFL changes and OCT morphological changes associated with peripapillary neovascular membrane. It is also valuable in differentiating ONHD from optic nerve head edema, according to Archives of Ophthalmology. The most significant finding is that OCT of optic nerve edema reveals a smooth internal contour with a “lazy V” pattern, while ONHD display a “lumpy-bumpy” internal contour. Additionally, OCT enhanced-depth imaging allows evaluation of ONHD dimensions.

• CT scan. This is superior to MRI for detection of drusen, as calcium produces a bright signal at the junction of the posterior globe and optic nerve on a CT scan, making it easy to detect.

TREATMENT AND MANAGEMENT

The prognosis and clinical course of ONHD are variable, and no definitive treatment exists. Patients should be managed with baseline and serial follow-up visual field exams, OCT optic nerve fiber analysis and repeated IOP checks. If visual field loss or RNFL defects are detected, patients should be monitored closely with three-month follow-ups.

Distinction between glaucomatous damage and ONHD is not always straightforward; therefore, elevated IOP should be treated with anti-glaucoma medication. Patients with normal testing can be followed with a yearly regular examination.

Patients experiencing complications, such as peripapillary neovascular membrane threatening central vision, should be referred for laser photocoagulation or intravitreal injections with anti-VEGF.

A HEAD-ON APPROACH

ONHD is a dynamic process that poses a unique clinical challenge. Early diagnosis is not only important, but may also reassure patients and prevent unnecessary testing for intracranial disease. OM

SHERROL A. REYNOLDS, O.D., F.A.A.O., is an associate professor at the Nova Southeastern University College of Optometry and clinical preceptor/attending in the college’s diabetes and macular clinic. She is a fellow of the Optometric Retina Society and chairperson for the Florida Optometric Association Healthy Eyes Healthy People Committee. Comment at tinyurl.com/OMcomment.