Knowing what to look for before ordering ancillary testing will allow you to recommend the appropriate test and interpret that test correctly. For example, brain imaging obtained for the evaluation of a third nerve palsy may demonstrate a Chiari malformation. In this case, the Chiari malformation should be considered an incidental finding that is asymptomatic and that does not need to be treated or investigated further.
4.1 Electrophysiologic Testing
Electrophysiologic testing can help differentiate retinal from optic nerve disease in selected cases. It is also helpful in documenting occult abnormalities of the optic nerve or retinal function.
4.1.1 Visual Evoked Responses (or Visual Evoked Potentials)
Visual evoked responses (or visual evoked potentials) are measurements of the electrical signal recorded at the scalp over the occipital cortex in response to visual stimuli. In the test, the patient is asked to look at a TV screen on which various stimuli are provided; electrodes placed on the scalp over the occipital cortex record the responses. Each eye is tested separately.
This test is not accurate if the patient does not cooperate. Abnormal responses may occur if the patient does not look at the screen, does not focus on the screen, moves the tested eye, or is tired. Appropriate refraction is necessary.
The visual evoked response reflects the integrity of the afferent visual pathway (damage anywhere from the retina to the occipital cortex may alter the signal). It is primarily a function of central visual function because such a large region of the occipital cortex near the recording electrodes is devoted to macular projections.
Two techniques are used to record visual evoked responses: pattern stimulus, which provides a quantifiable and reliable waveform but may be absent in patients with poor vision, and flash stimulus, which is useful for patients with very poor vision in whom the pattern stimulus response is absent. The recorded responses for each eye are then compared (▶ Fig. 4.1), with the focus on the amplitude and peak latency of the waveform (P100). Classically, the P100 waveform is delayed in patients with demyelinating optic neuritis.
Fig. 4.1 Record of visual evoked responses, using flash and pattern reversal stimuli. N, Negative; P, Positive.
In most clinical situations, the visual evoked responses are of limited usefulness and are not necessary to make the diagnosis of optic neuropathy.
Visual evoked responses are most useful in evaluating the integrity of the visual pathway in infants and inarticulate adults. A preserved flash or pattern response confirms intact pathways, whereas an abnormal flash response indicates gross impairment (▶ Fig. 4.2). An abnormal pattern response is less useful: it may indicate damage, or it may be falsely abnormal.
Fig. 4.2 a, b (a) Normal and symmetric flash visual evoked responses in an infant with shaken baby syndrome, indicating intact visual pathways bilaterally. (b) Normal pattern visual evoked responses in a patient claiming poor vision in both eyes after a traumatic brain injury. This test confirms that the visual pathways are intact and proves normal central vision. N, Negative; P, Positive.
Visual evoked responses are also useful in confirming intact visual pathways in patients with markedly abnormal subjective visual responses of suspected nonorganic origin. A response with an intact pattern not only confirms intact visual pathways but also provides an estimate of visual acuity when stimuli of various sizes are used. A response with an abnormal or absent pattern does not confirm organic disease because voluntary inattention or defocusing can markedly alter the pattern waveform.
4.1.2 Electroretinogram
The electroretinogram (ERG) is a measurement of electrical activity of the retina in response to light stimulus. It is measured at the corneal surface by electrodes embedded in a corneal contact lens worn for testing. The ERG is normal in optic neuropathies.
Full-Field Electroretinogram
A full-field ERG is generated by stimulating the entire retina with a flash light source under varying conditions of retinal adaptation to dark and light (▶ Fig. 4.3).
Fig. 4.3 Normal full-field electroretinogram wave form.
Major components of the electrical waveform generated and measured include the following:
A wave (negative): primarily derived from the photoreceptor layer (outer retina)
B wave (positive): derived from the inner retina (Müller and bipolar cells)
Two other waveforms that are sometimes recorded are the c-wave originating in the pigment epithelium and the d-wave indicating activity of the OFF bipolar cells.
Rod and cone photoreceptors can be separated by varying stimuli and the state of retinal adaptation during testing.
Full-field ERG is useful in detecting diffuse retinal disease in the setting of generalized or peripheral vision loss. Disorders such as retinitis pigmentosa, cone–rod dystrophy, toxic retinopathies, and retinal paraneoplastic syndromes may present with variably severe visual loss and minimally visible retinal abnormalities. The ERG is invariably severely depressed by the time patients complain of visual loss, making full-field ERG testing very useful. Although poorly cooperative patients can make interpretation of the full-field ERG more difficult (there can be background noise due to frequent blinking and squeezing), the responses cannot be substantially altered voluntarily (unlike visual evoked responses).
Because the full-field ERG measures only a mass response of the entire retina, it may be normal in minor or localized retinal disease, particularly maculopathies, even with severe visual acuity loss.
Multifocal Electroretinogram
Multifocal ERG simultaneously records ERG signals from up to 250 focal retinal locations within the central 30 degrees. The individual responses are mapped topographically (▶ Fig. 4.4).
Fig. 4.4 a–d (a,b) Normal multifocal electroretinogram (ERG) showing the normal foveal peak. (c,d) Abnormal multifocal ERG showing a decreased superior foveal peak in a patient with a superior branch retinal artery occlusion. (Courtesy of Dr. M. S. Lee.)
This technique is extremely helpful in detecting occult focal retinal abnormalities within the macula. Unlike full-field ERG, however, uncooperative patients can alter the responses on a multifocal ERG by not fixating accurately.
4.2 Fundus Autofluorescence Imaging
Fundus autofluorescence (FAF) imaging is helpful in diagnosing retinal conditions at an early stage by showing abnormalities that are often invisible to standard fundus photography and ophthalmoscopy (▶ Fig. 4.5). Autofluorescence is caused by the presence of lipofuscin, an aging pigment fluorophore produced by the outer segments of the photoreceptors and stored at the level of the retinal pigment epithelium. Two abnormal states of lipofuscin exist, hyperautofluorescence and hypoautofluorescence (▶ Fig. 4.5b); both are associated with various retinal disorders. Optic nerve head drusen are also usually hyperautofluorescent (▶ Fig. 4.6).
Fig. 4.5 a, b Autofluorescence imaging of the macula showing extensive bilateral maculopathies. (a) On the color fundus photo, the macula has an abnormal mottled appearance with pigmentary changes. (b) The autofluorescence imaging demonstrates large areas of hypofluorescence (dark) involving both maculae.
Fig. 4.6 a, b Autofluorescence imaging of the optic nerve in optic nerve head drusen. (a) Superficial optic nerve drusen appearing as multiple small round calcifications in the optic nerve head on a color fundus photo. (b) The same optic nerve head in autofluorescence imaging showing the drusen as hyperfluorescent.
4.3 Retinal Fluorescein Angiography
Intravenous (IV) fluorescein angiography is a photographic method of angiography that does not rely on radiation. After IV injection of fluorescein solution (in an arm vein), rapid-sequence retinal photography is performed by using a camera with spectrally appropriate excitation and filters (the fluorescein absorbs blue light and becomes fluorescent, which can be captured on photographs).
Fluorescein angiography is helpful in studying the vascular filling patterns of the choroidal, retinal, and optic nerve head arteries and veins. It can also help differentiate macular from optic nerve–related visual loss by showing macular changes that are not always easily visible on funduscopic examination. However, imaging of the macula with optical coherence tomography has now replaced fluorescein angiography for most of these cases.
The test requires the cooperation of the patient, who needs to be able to sit up and fixate. It also requires relatively clear ocular media.
Fluorescein angiography is usually well tolerated, although side effects include nausea, vomiting, and vasovagal responses. True allergic reactions are rare. The fluorescein is excreted in the urine (which becomes yellow and fluorescent) within 24 to 36 hours.
The technique for fluorescein angiography is as follows. Color photos and red-free photos are taken prior to the injection of fluorescein. Once the fluorescein is injected, multiple retinal photos are taken on the same eye (chosen by the clinician) to study the choroidal and retinal vascular filling dynamic. A time (since the injection) is noted on each photo (▶ Fig. 4.7). Delay in the choroidal filling, as well as delay or asymmetry in the retinal vascular filling, is indicative of vascular disease. Photos are then taken of both eyes depending on the reason for the test. Abnormal fluorescence of the retina (indicating staining, pooling, or leakage of the fluorescein or blockage of the fluorescence) is indicative of retinal or choroidal disorders. Late photos may show leakage of the vessels (as in vasculitis) or of the optic nerve (as in optic nerve edema) (▶ Fig. 4.8 and ▶ Fig. 4.9).
Fig. 4.7 Normal fluorescein angiography (left eye).
Fig. 4.8 a, b Cystoid macular edema in the right eye responsible for central visual loss. (a) The macula looks relatively normal on the color photo. (b) The fluorescein angiogram demonstrates a characteristic leakage (arrow) at the level of the macula.
Fig. 4.9 a–d Central serous retinopathy in the right eye presenting with a small paracentral scotoma. There is a bullous serous detachment on the color (a) and red-free photos (b) (arrows).
(c,d) The fluorescein angiography shows dye leakage at the same level (arrows).
Demonstration of retinal small vessel vasculopathy such as vasculitis is best done with retinal fluorescein angiography. The abnormal vessels leak, and vascular abnormalities may be missed on fundus examination (▶ Fig. 4.10).
Fig. 4.10 a, b Retinal vasculitis with sheathing (arrow) of the vessels on the color photo (a) and leakage of the dye on the fluorescein angiogram (b) (arrows).
Fluorescein angiography is also very helpful for the diagnosis of giant cell arteritis. Photographs should be taken of both eyes with transit (early images after injection of IV fluorescein) on the most affected eye (▶ Fig. 4.11).
Fig. 4.11 Retinal fluorescein angiogram with transit time in the eyes of a patient with giant cell arteritis. There is an anterior ischemic optic neuropathy in the right eye with disc edema, and there are two cotton wool spots inferiorly in the left eye that are asymptomatic (color photos). The angiography shows early photos of the right eye with delayed and patchy choroidal filling. There is progressive and heterogeneous filling of the choroid, then of the retinal vessels on successive images. The late phase shows leakage of the fluorescein at the level of the right optic nerve, which is swollen (arrow).
4.4 Optical Coherence Tomography
Optical coherence tomography (OCT) is routinely performed in ophthalmology. It is a noninvasive transpupillary ophthalmic imaging technology that can image retinal and optic nerve structures in vivo with a resolution of 4 µm.
Cross-sectional images of the retina, the optic nerve, and peripapillary areas are produced using the optical backscattering of light similar to what is obtained with a B-scan ultrasound (OCT uses low-coherence near-infrared light). The anatomical layers within the retina can be differentiated, and retinal thickness can be measured (▶ Fig. 4.12). OCT is particularly useful for macular diseases (▶ Fig. 4.13); however, it requires a patient’s ability to fixate and relatively clear ocular media. The quality is often better when the pupils are pharmacologically dilated.
Fig. 4.12 Normal optical coherence tomography (OCT) of the macula. This high-resolution horizontal scan centered on the fovea allows visualization of each retinal layer.
OCT is very useful in demonstrating anatomical changes in the macular, such as edema (▶ Fig. 4.13), holes, cysts, macular traction, and an epiretinal membrane. Additionally, individual retinal layers can be analyzed and the retinal nerve fiber layer thickness and macular volume can be measured with OCT (▶ Fig. 4.14). This is important because peripapillary nerve fiber thickness and macular volume are decreased in glaucoma and other diseases of the optic nerve, such as optic neuritis. They are used to monitor disease activity in numerous optic neuropathies.
Fig. 4.13 Posterior scleritis with thickening of the posterior pole and macular and disc edema. (a) Color fundus photo showing disc edema (white arrow) and abnormal edematous macula (yellow arrow), and fluorescein angiography (image taken at 3 min, 18 s after intravenous injection of fluorescein) demonstrating leakage of dye at the optic nerve and macula. (b) Optical coherence tomographic (OCT) imaging of the macula demonstrates cystoid macular edema consistent with the leakage of dye that was noted in the fluorescein angiogram. (continued)
(continued) (c) Thickening of the choroid and posterior scleral wall (yellow arrow) and fluid in the Tenon capsule (white arrow).
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