A fundus examination may reveal media opacities or fundus abnormalities that explain a patient’s decreased vision. The clarity of the view with the direct ophthalmoscope can suggest how much visual impairment is caused by these media opacities. Using either direct ophthalmoscopy or slit-lamp biomicroscopy, the clinician can assess the appearance of the optic nerve head (ONH) and macula. The ONH is examined for evidence of pallor, edema, excavation, or other abnormalities; the macula is examined for evidence of pigmentary disturbance, edema, scarring, or other disruption of structural integrity.
ONH pallor indicates optic atrophy, which is the hallmark of damage to the retinal ganglion cells. Although the examiner is only able to observe pallor of the neuroretinal rim, ONH pallor can occur from damage to any portion of the ganglion cells, from the ganglion cell bodies to their synapses at the lateral geniculate nucleus. ONH pallor does not occur immediately after injury but takes at least 4–6 weeks from the time of axonal damage. Severe damage causes the ONH to appear chalky white (Fig 3-2). Mild forms of pallor remain more difficult to detect. Evaluation of the capillary net, the network of fine blood vessels on the ONH, may be helpful; the net becomes thin or absent in early atrophy even when pallor is still very mild. In addition, dropout of the retinal nerve fiber layer (RNFL) may precede atrophy. When viewed with a red-free filter, fine defects appear as dark bands among normal striations. These defects, called rake defects owing to their similarity to rake marks in soil (Fig 3-3), initially affect the thickest portion of the RNFL, the superior and inferior arcades. However, true temporal pallor must be carefully distinguished from the pallor of the ONH in a pseudophakic eye. If only 1 eye is pseudophakic, the difficulty of comparing ONH pallor in the 2 eyes makes this task particularly challenging.
Figure 3-2 Optic nerve head (ONH) pallor. A, Diffuse optic atrophy. B, Normal ONH appearance.
(Courtesy of Steven A. Newman, MD.)
ONH edema results from impaired axoplasmic flow from any cause, including increased intracranial pressure, local mechanical compression, ischemia, and inflammation. ONH and retinal vascular changes can also accompany ONH edema (Fig 3-4). Regardless of cause, the following clinical features of ONH edema may be observed:
elevation of the ONH with variable filling in of the physiologic cup; retinal vessels may appear to drape over the elevated ONH margin
blurring of the ONH margins
peripapillary RNFL opacification; the RNFL becomes grayish white and opalescent with feathered margins, obscuring portions of the retinal vessels that course within this level of the retina
hyperemia and dilation of the ONH surface capillary net
retinal venous dilation and tortuosity
peripapillary hemorrhages, exudates, or cotton-wool spots
retinal or choroidal folds or macular edema
Figure 3-3 Rake defects. ONH shows temporal pallor with a broad region of nerve fiber layer dropout (left), contrasted with glistening intact nerve fiber layer (right).
(Courtesy of Anthony C. Arnold, MD.)
Figure 3-4 Papilledema. A, Right eye. B, Left eye. The ONH margins are blurred, with grayish-white, opalescent thickening of the peripapillary nerve fiber layer (arrows), cotton-wool spots, and flame hemorrhages. The retinal vessels are partially obscured at the ONH margin and within the peripapillary retina.
(Courtesy of Sophia M. Chung, MD.)
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.