Optic nerve drusen refer to a collection of protein and calcium that deposits on the optic nerve. The optic nerve is the physical connection that transfers information received from the eyes to the brain. Optic nerve drusen were described initially by Müller who derived the name from geodes, an old German word meaning rock cavities lined with crystals.
Also Known As
Optic nerve head drusen (ONHD)
There are two types of ONHD:
- Buried ONHD - For this type, the drusen are buried, and the optic disc and cup’s margin are obscured. It is observed in patients of a younger age. The optic disc appears swollen and resembles optic disc edema.
- Visible ONHD - As the patient gets older, drusen become visible and can be observed on the surface of the optic disc.
Causes & Risk Factors
The primary risk factor for developing ONHD is inherited optic disc dysplasia with an autosomal dominant inheritance pattern. Caucasians have a higher risk of developing ONHD compared to other races. The higher prevalence in Caucasians can be linked to optic disc diameter. Whites have smaller optic discs compared to other races. The small optic disc is associated with ONHD since the small scleral canal is a pathogenic factor.
In other cases, drusen are believed to result from axonal deterioration of the optic nerve. It could also be attributed to an inactive state of axoplasmic flow, consequent upon crowded optic nerve heads that are inborn.
The disease can be associated with other conditions such as angioid streaks, usher syndrome, retinitis pigmentosa, Alagille's syndrome and Noonan syndrome.
Signs & Symptoms
ONHD patients are often asymptomatic. However, some patients experience vision problems like loss of peripheral vision and greying out or temporary flickering of vision. In rare occasions, patients may have transient visual obscurations.
When clinically observed, the signs are:
- A round yellowish/ white deposit on or under the optic nerve
- Distorted edges of the optic disc
- Loss of optic disc or cup borders
- Presence of a nasal margin
Choroidal neovascularization may develop and cause a decrease in central vision.
Since optic nerve drusen lack symptoms in most cases, diagnosis is clinical. Visible or apparent drusen are easily identifiable on the optic nerve's surface. Buried drusen, on the other hand, are challenging to diagnose and can easily be confused with other conditions. The doctor needs to distinguish optic nerve drusen from other conditions that show elevation of the optic disc.
The doctor will use certain techniques for diagnosis, such as:
- Fundus exam which will show the appearance of optic disc drusen as lumpy- bumpy
- A and B scan ultrasonography - The optic disc will appear elevated and highly reflective when ONHD is present.
- Fundus autofluorescence – Drusen are highly auto- fluorescent. If they are present, they appear as bright areas on the optic disc.
- CT scans, although the scans only show calcified ONHD and this, therefore, makes its use limited. CT scans are superior to MRI scans since MRI can only detect disc oedema but not ONHD.
- OCT ( Optical Coherence Tomography) - It helps predict visual field loss and detect early thinning of the optic nerve fiber.
Optic nerve drusen has no treatment, but the patient can have follow up sessions involving visual field exams, intraocular pressure checks and optic nerve fiber analysis. Due to visual field loss, the doctor may prescribe eye drops to relieve stress on the optic disc fibers and lower intraocular pressure. However, this treatment is not yet widely accepted.
In cases where visual acuity is threatened due to subretinal choroidal neovascularization, the doctor may perform laser treatment or photodynamic therapy to treat bleeding.
Prognosis & Long- Term Outlook
Over time optic nerve drusen enlarge and can increase the risk of developing other complications. One such complication is peripheral field defects. Approximately 75% of children with ONHD develop defects in the peripheral field. The defects include enlarged blind spots, nasal step defects, arcuate scotomas, altitudinal defects and sectorial field defects. Visual field exams should be done regularly to monitor decreased peripheral vision.
Another complication that may arise is Anterior Ischemic Optic Neuropathy, especially in patients around the mid-20s or younger, without indicators of circulatory system diseases. ONHD patients with migraine headaches, systemic hypertension, oral contraceptive use, atrial septal or high altitude have reported cases of central retinal artery occlusion.
Prevention & Follow Up
Since optic nerve drusen occur as an inherited trait, it is essential for the patient diagnosed with ONHD to encourage their relatives to go for screening. The screening can help in diagnosing the condition early and ensuring it is monitored before it gets worse. No prevention measure is available for the occurrence of optic nerve drusen.