Ophthalmic Imaging in Optic Disc Disorders

Introduction:

Optic nerve head elevation is an essential physical examination used to assess individuals experiencing visual abnormalities or loss of vision. It may or may not be linked with optic disc edema (pseudedema). Neuroretinitis, optic neuritis (papillitis), elevated intracranial pressure (papilledema), uveitis, anterior ischemic optic neuropathy (AION), diabetic or hypertensive papillitis, compression, or infiltration are all common causes of optic nerve head elevation due to optic disc edema.

What Is Optic Disc Drusen?

Optic disc drusen (ODD) affects 0.3 to 2.0 percent of the population. ODD may be identified accidentally in individuals with no visual complaints during a normal ocular examination or during the headache and optic nerve head elevation assessment. ODD is more prevalent in Caucasians. They are usually idiopathic, but they might appear in families exhibiting an autosomal dominant inheritance. ODD can be associated with systemic or ocular genetic disorders.

Though the exact origin is unknown, axonopathy or defective axonal transport in congenitally dense optic discs may have a role in ODD formation by causing axonal rupture and the release of aberrant mitochondria, which act as a nidus for the production of drusen. ODD is more prevalent in small scleral canals defined by genetics or congenital optic nerve abnormalities. ODD is usually present from birth or early infancy, grows larger and more numerous during adolescence, and shifts from deep to evident as people age. Visual field loss may result from the gradual compression and impairment of nearby axons and blood supply caused by an enlarged ODD.

What Are the Characteristics of Optic Disc Drusen on Fluorescein Angiography and Color Fundus Photography?

Visible ODD can seem remarkably optically refractile under ophthalmoscopy. Eyes with ODD exhibit a lumpy-bumpy, elevated appearance with distorted disc borders. Although buried ODD are not detectable on ophthalmoscopy, they might resemble papilledema, particularly in cases involving both children (Frisén grades 1 or 2). ODD has late circumferential peripapillary staining and early or late nodular staining of the disc on fluorescein angiography but no early or late leaking.

What Are the Characteristics of Papilledema and Optic Disc Edema on Color Fundus Photography and Fluorescein Angiography?

In acute cases, the optic disc in papilledema usually shows hyperemia, dilated and convoluted veins, and splinter hemorrhages. The elevation is smooth and resembles a champagne cork in chronic cases. The appearance of optic disc edema can vary slightly depending on its source. The following conditions may be present in association with severe papilledema and optic disc edema: cotton wool spots (nerve fiber layer ischemia), exudates, Paton's lines, retinal and choroidal folds, and restricted or obscured papillary arterioles.

Spontaneous venous pulsation can be low or nonexistent in papilledema but apparent in at least one eye in optic disc edema. Papilledema is linked to early and late staining of the optic disc, increased vascularity, and normal perfusion on fluorescein angiography. Nonischemic optic disc edema is also linked with normal optic disc perfusion, whereas the majority of AION optic disc edema cases have considerably delayed optic disc filling.

What Are the Ophthalmic Imaging Methods in Optic Disc Disorders?

The purpose of ophthalmic imaging is to provide high-resolution images of the eye that are difficult to obtain through fundoscopic examination.

Optical Coherence Tomography Imaging of Optic Nerve Head Elevation:

OCT has made it possible to visualize the optic nerve head and quantify various parameters, including optic disc volume, subretinal fluid, retinal nerve fiber layer thickness, ganglion cell complex, and posterior pole anatomy. Enhanced-depth imaging OCT (EDI-OCT) was first described for visualizing the choroidal circulation using spectral-domain OCT. EDI-OCT was proposed as the new gold standard for ODD diagnosis in 2018 by the Optic Disc Drusen Studies (ODDS) Consortium. ODD appears as deposits in the anterior optic nerve of varying diameters on EDI-OCT, and horizontal hyperreflective lines and PHOMS frequently accompany them.

Autofluorescence Imaging of Optic Disc Drusen:

En-face ophthalmic imaging is widely available at point-of-care facilities. Blue or green filter sets are used in fundus autofluorescence imaging to identify ODD. Autofluorescence imaging is fast, simple to interpret, and noninvasive (no contrast is utilized). It often shows the optic disc as dark, while ODD seems bright or hyperautofluorescent.

Autofluorescence imaging is the most effective way to detect visual ODD, which appears as focal autofluorescence with uneven margins in 93 to 100 percent of eyes. The sensitivity of buried ODD ranges from 12 to 92 percent, based on the location of the drusen. Increased drusen volume and superficial anatomic position significantly affected autofluorescence imaging sensitivity. Hyperautofluorescence in ODD was identified using a new confocal scanning apparatus, allowing acquisition without needing pharmacologic pupillary dilatation.

Vascular Imaging of Optic Disc Drusen Using Optical Coherence Tomography and Angiography:

Optical nerve head swelling frequently affects the retinal vasculature. Historically, fluorescein angiography has been the most widely used procedure, even though it needs an intravenous fluorescein injection, which poses the risk of an unfavorable reaction. OCTA, or optical coherence tomography angiography, is a relatively new contrast-free vascular imaging modality.

Different layers of the retinal vasculature can be separated separately using OCTA, which scans vessels based on flow characteristics. OCTA was more effective than fluorescein angiography for imaging the deep capillary networks and the radial peripapillary.

Noisy OCTA data can be improved by image averaging, and detailed vascular measurements, such as vessel area density, vessel skeleton density, vessel diameter index, vessel perimeter index, and vessel complexity index, can be performed.

Conclusion:

Patients with optic nerve head elevation should be diagnosed using a multimodal ophthalmic imaging approach in addition to a neuro-ophthalmic examination and history. This conservative approach can assist in distinguishing between different types of ODD, papilledema, and optic disc edema, avoiding unnecessary brain MRIs and invasive lumbar puncture procedures in patients without optic disc edema. Multimodal ocular imaging and clinical suspicion are essential to identify these individuals at high risk of vision loss accurately. Individuals with ODD may also have simultaneous optic disc edema from papilledema or AION. Further research is required to confirm imaging biomarkers of optic nerve head elevation in the future and to find accurate characteristics in visual results and diagnostics.