- 1.
What is the electroretinogram?
The electroretinogram (ERG) is a recording of the electrical discharges from the retina elicited by a flash of light. This response is secondary to transretinal movement of ions induced by the light stimulus.
- 2.
How is an electroretinogram performed?
Light is delivered uniformly to the entire retina. Ganzfeld or full-field stimulation is achieved with a bowl perimeter. The light-induced electrical discharges from the eye are recorded with a corneal contact lens electrode.
- 1.
The a wave is the initial negative ERG waveform arising from photoreceptor cells.
- 2.
The positive b wave following the a wave is generated by the Müller cells and bipolar cells in the outer retina.
- 3.
Oscillatory potentials are small wavelets that may be superimposed on the b wave and arise from cells in the midretinal layers ( Fig. 5-1 ).
Figure 5-1
Nocturnal scotopic (dark-adapted) and photopic ERG responses to a high-intensity (0 dB) light flash demonstrating the a wave and b wave. Oscillatory potentials are present on the ascending limb of the b wave. The implicit time is measured from the stimulus onset to the peak of the a wave (1) or b wave (2). The a-wave amplitude is measured from the baseline to the trough of the a wave, and the b-wave amplitude is measured from the trough of the a wave to the peak of the b wave.
- 4.
Under certain recording conditions, additional waveforms may be noted, such as the c wave following the b wave. This reflects electrical activity at the level of the retinal pigment epithelium and is recorded in the dark-adapted eye.
- 5.
The early receptor potential is a rapid transient waveform that occurs immediately after a light stimulus. This response originates from the bleaching of photopigments at the level of the photoreceptor outer segments.
- 3.
What parameters are measured during evaluation of an electroretinogram?
Two major ERG parameters, amplitude and implicit time, are measured. The amplitude (microvolts) of the a wave is measured from baseline to the trough of the a wave. The b-wave amplitude is measured from the trough of the a wave to peak of the b wave. The implicit time (milliseconds) is the time from the stimulus onset to peak of the response.
- 4.
How is the electroretinogram amplitude affected in retinal disorders?
The full-field light-evoked ERG is a mass response reflecting activity from the entire retina. The amplitude of the ERG is proportional to the area of functioning retina stimulated and is abnormal only when large areas of the retina are functionally impaired.
- 5.
Describe different stimulus conditions and the associated photoreceptor response.
Certain light stimuli allow the isolation of either the cone or the rod responses so that each photoreceptor type can be studied independently ( Table 5-1 and Fig. 5-2 ). After sufficient dark adaptation (known as scotopic conditions), the rod responses are optimized. Under light-adapted or photopic conditions, the rods are sufficiently dampened so that the response is primarily from the cones.
Table 5-1
Photoreceptor Response Associated with Various Stimulus Conditions
State of Adaptation
Light Stimulus
Photoreceptor Response
Scotopic
Dim white (24 dB)
Rod
Scotopic
Dim blue (10 dB)
Rod
Scotopic
Bright white (0 dB)
Mixed response: Maximal rod and cone
Scotopic
Red (0 dB)
Mixed response: early cone, late rod
Scotopic
Bright white (0 dB)
Cone oscillatory potentials
Photopic
Bright white (0 dB)
Cone
Photopic
White flicker at 30 Hz
Pure cone
dB, Decibels;
Hz, hertz.
Figure 5-2
The normal ERG cone response to a flicker light stimulus at 30 Hz.
- 6.
What five responses are evaluated during a standard full-field electroretinogram?
- •
Rod response (dark-adapted)
- •
Maximal combined rod–cone response (dark-adapted)
- •
Oscillatory potentials
- •
Single-flash cone response (light-adapted)
- •
30-Hz flicker cone response
- •
- 7.
How is the electroretinogram affected in age-related macular degeneration?
When age-related macular degeneration is characterized by small localized perimacular lesions, the full-field ERG is normal. The entire retina is stimulated by the bright flash in the Ganzfeld and, as a consequence, the full-field ERG is not affected when small areas of the retina are damaged.
- 8.
What does the electroretinogram demonstrate in retinal ganglion cell disease?
The ganglion cells do not play a role in generation of the full-field ERG. Thus, disorders primarily affecting ganglion cells, such as glaucoma, do not alter the full-field ERG. On occasion, the b wave may be reduced in optic atrophy or central retinal artery occlusion, This is postulated to result from transsynaptic degeneration from ganglion to the bipolar cell layer.
- 9.
Describe the clinical situations in which an electroretinogram is utilized.
- •
To diagnose a generalized degeneration of the retina
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To evaluate family members for a known hereditary retinal degeneration
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To assess decreased vision and nystagmus present at birth
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To assess retinal function in the presence of opaque ocular media or vascular occlusion
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To evaluate functional visual loss
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- 10.
List the retinal degenerations in which an electroretinogram can help clarify the diagnosis.
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Retinitis pigmentosa and related hereditary retinal degenerations
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Retinitis pigmentosa sine pigmento
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Retinitis punctata albescens
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Leber’s congenital amaurosis
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Choroideremia
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Gyrate atrophy of the retina and choroid
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Goldman-Favre syndrome
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Congenital stationary night blindness
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X-linked juvenile retinoschisis
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Achromatopsia
- •
Cone dystrophies
- •
Disorders mimicking retinitis pigmentosa
- •
- 11.
What are the clinical and electroretinogram features of retinitis pigmentosa?
Retinitis pigmentosa (RP) is an inherited retinal disorder of the photoreceptors and other retina cell layers. Inheritance may be autosomal dominant, autosomal recessive, or X-linked. Both the rods and, to a lesser extent, the cones are abnormal in retinitis pigmentosa. Clinical features include decreased night vision (nyctalopia), visual field loss, and abnormal ERG ( Fig. 5-3 ). Ocular features include waxy pallor of the optic nerve, attenuated retinal vessels, mottled retinal pigment epithelium with bone-spicule pigmentation, cellophane maculopathy, cystic macular edema, pigment cells in the vitreous, and cataracts.
Figure 5-3
The ERG in retinitis pigmentosa reveals an extinguished response to scotopic blue and scotopic red light stimuli.
The ERG shows reduced amplitude (usually b wave) and prolonged photopic implicit time in early RP. Over time, the ERG becomes extinguished with no detectable rod or cone responses to bright white light.
- 12.
What does the electroretinogram demonstrate in female carriers of X-linked retinitis pigmentosa?
ERG abnormalities are noted in the majority of female carriers, including prolonged photopic b-wave implicit time and/or a reduction in the amplitude of the scotopic b wave in the dark-adapted eye. Retinal examination in this group may be normal or demonstrate milder retinal findings without subjective complaints.
- 13.
What does the electroretinogram reveal in congenital rubella syndrome?
Diffuse pigmentary retinal changes in congenital rubella syndrome may be confused with retinitis pigmentosa. However, the ERG is normal in congenital rubella. Other ocular signs of rubella include deafness and congenital cataracts.
- 14.
Describe the electroretinogram in X-linked retinoschisis.
ERG reveals reduced scotopic and photopic b-wave amplitude, reflecting widespread midretinal anatomic changes induced by the schisis or splitting of the retina. Clinical findings include peripheral retinoschisis cavities in 50% of cases and foveal cystic changes in almost all cases.
- 15.
What does the electroretinogram demonstrate in progressive cone dystrophy?
The ERG shows markedly reduced photopic flicker response and a normal rod scotopic response. This disorder initially affects peripheral cones, progressing to involve central cones. When the central cones are intact, the visual acuity and color vision are preserved; however, the acuity eventually decreases to the 20/200 range.
- 16.
Why is the electroretinogram useful in patients with congenitally decreased vision?
Three disorders characterized by nystagmus, congenitally reduced vision, and normal retinal examination can be diagnosed with an ERG:
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Achromatopsia (also known as rod monochromatism) is a nonprogressive autosomal-recessive near absence of cones. The ERG reveals absent cone function and normal rod function.
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Leber’s congenital amaurosis is a congenital autosomal recessive form of retinitis pigmentosa. The ERG is markedly reduced or extinguished with profound visual impairment.
- •
Congenital stationary night blindness is an inherited retinal disorder (autosomal dominant, X-linked recessive, or autosomal recessive) that primarily affects rods. The ERG reveals normal photoreceptors with a normal a wave, but an abnormal bipolar cell region as demonstrated by the absent b wave.
- •
- 17.
How can the electroretinogram measure retinal function in the presence of opaque ocular media?
The full-field ERG can be used to assess the retinal function when the retina cannot be visualized, owing to cataracts or corneal or vitreous opacities. A normal ERG provides information regarding the overall retinal function, but does not indicate whether central vision is normal because macular degeneration and optic atrophy typically do not affect the ERG amplitude. A cataract or corneal opacity may act as a diffuser of light, on occasion producing a “supernormal” ERG.
- 18.
List the disorders that may demonstrate an extinguished electroretinogram.
- •
Retinitis pigmentosa and related disorders
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Ophthalmic artery occlusion
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Diffuse unilateral subacute neuroretinitis
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Metallosis
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Total retinal detachment
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Drugs such as phenothiazines or chloroquine
- •
Cancer-associated retinopathies
- •
- 19.
List the disorders that may demonstrate normal a-wave and reduced b-wave amplitude.
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Congenital stationary night blindness
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X-linked juvenile retinoschisis
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Central retinal vein or artery occlusion
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Myotonic dystrophy
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Oguchi’s disease
- •
Quinine intoxication
- •
Transsynaptic degeneration from the ganglion to the bipolar cell layer (i.e., secondary to optic atrophy or central retinal artery occlusion)
- •
- 20.
List the disorders characterized by an abnormal photopic electroretinogram and a normal scotopic electroretinogram.
- •
Achromatopsia (also known as rod monochromatism)
- •
Cone dystrophy
- •
- 21.
Name three variations of the standard electroretinogram.
- •
The focal electroretinogram is induced by a focal-directed flash of light and measures the response from the central cone photoreceptors and outer retina.
- •
The pattern electroretinogram (PERG) measures the electrical response to an alternating pattern stimulus that has a constant overall retinal luminance. The response appears to be localized to retinal ganglion cells. The PERG is extinguished after transection of the optic nerve, whereas the full-field ERG is not altered. The PERG may be used to diagnose or monitor disorders such as glaucoma, ocular hypertension, optic neuritis, optic atrophy, and amblyopia.
- •
The multifocal ERG provides an objective equivalent to the visual field by simultaneously assessing the retinal electrical response at multiple locations. The resultant local responses contain components from all levels of the retina.
- •
- 22.
What is an electro-oculogram?
The electro-oculogram (EOG) is an indirect measure of the standing potential of the eyes ( Fig. 5-4 ). This standing potential exists because of a voltage difference between the inner and the outer retina. The EOG is measured by placing electrodes near the medial and lateral canthi of each eye. The patient then moves his or her eyes back and forth over a specific distance.
Figure 5-4
Normal EOG demonstrating the dark trough and the light peak.
The clinical measurement of the EOG relies on the fact that the amplitude of the response changes when the luminance conditions are varied. After dark adaptation, the response progressively decreases, reaching a trough in 8 to 12 minutes. With light adaptation, there is a progressive rise in amplitude, reaching a peak in 6 to 9 minutes. The greatest EOG amplitude achieved in light (light peak) is divided by the lowest amplitude in the dark (dark trough). This calculated ratio is the Arden ratio . Normal subjects have an Arden ratio value of 1.80 or greater, whereas a ratio of less than 1.65 is distinctly abnormal.
- 23.
Where is the electro-oculogram response generated?
The electrical response in the EOG is generated by the retinal pigment epithelium, with the light peak being produced by a depolarization of the basal portion of the retinal pigment epithelium. To generate the EOG potential, it is necessary to have intact photoreceptors in physical contact with the retinal pigment epithelium.
- 24.
What are the clinical uses for the electro-oculogram?
The most important clinical use for the EOG is the diagnosis of Best disease (also known as vitelliform dystrophy). Best disease is inherited autosomal dominantly with phenotypic variability. Individuals with Best disease usually have an EOG Arden ratio less than 1.5, but the ERG is normal. The EOG light rise is almost completely dependent on rod function, so it is normal in disorders of cone dysfunction. The EOG is abnormal in most other retinal disorders when the ERG is abnormal, thus, it has limited clinical utility aside from diagnosing Best disease.
- 25.
What does the electro-oculogram demonstrate in pattern dystrophies?
The EOG light-peak to dark-trough Arden ratio in pattern dystrophy is usually either normal or minimally subnormal. This finding may help distinguish pattern dystrophy from Best’s disease, in which the Arden ratio is always abnormal.
- 26.
How are the electroretinogram and electro-oculogram affected by chloroquine and hydroxychloroquine use?
Abnormal findings in the ERG and EOG have been reported in patients receiving these antimalarial drugs, which are frequently used for immune-mediated arthritides and other autoimmune disorders.
- 27.
What are the characteristics of dark adaptation?
Dark adaptometry measures the absolute threshold of cone and rod sensitivity and is tested with the Goldmann-Weekers adaptometer. Initially, the subject is adapted to a bright background light, which is then extinguished. In the dark, the patient is presented with a series of dim lights. The threshold at which the light is just perceived is plotted against time. The normal dark-adaptation curve ( Fig. 5-5 ) is biphasic. The first curve represents the cone threshold and is reached in 5 to 10 minutes. The second curve represents the rod threshold and is reached after 30 minutes. The rod–cone break is a well-defined point between these two curves. Dark adaptometry is useful to evaluate retinal disorders with night blindness and some conditions with cone dysfunction.
Figure 5-5
Normal dark adaptation curve demonstrates the rod-cone break at 7 minutes, separating the cone threshold (1) and the rod threshold (2).
- 28.
What are the indications for ophthalmic ultrasonography?
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Evaluation of the anterior or posterior segment in eyes with opaque ocular media
- •
Assessment of ocular tumor dimensions as well as their tissue characteristics, such as calcium in retinoblastoma or choroidal osteoma
- •
Evaluation of orbital disorders such as thyroid ophthalmopathy and orbital pseudotumor
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Detection and localization of intraocular foreign bodies
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Measurement of distances within the eye and orbit (also known as biometry)
- •
- 29.
What frequency is used for standard ophthalmic ultrasonography?
Ultrasound is an acoustic wave that consists of an oscillation of particles within a medium. In standard ophthalmic ultrasound, frequencies are in the range of 8 to 10 MHz. This high frequency produces short wavelengths, which allow precise resolution of small ocular structures.
- 30.
What are the principles of ultrasonography?
Ultrasound is based on physical principles of tissue–acoustic impedance mismatch and pulse–echo technology. As the acoustic wave is propagated through tissues, part of the wave may be reflected toward the source of the emitted wave (i.e., the probe). This reflected wave is referred to as an echo. Echoes are generated at adjoining tissue interfaces that have differential acoustic impedance. The greater the difference in acoustic impedance, the stronger the echo. For example, strong reflections occur at the interface between retinal tissue and vitreous fluid.
- 31.
How is the clinical ophthalmic ultrasound displayed?
The reflected echoes are received, amplified, electronically processed, and displayed in visual format as an A-scan or a B-scan ( Fig. 5-6 ):
- •
A-scan ultrasonography , or the A mode, is a one-dimensional, time–amplitude display. The horizontal baseline represents the distance and depends on the time required for the sound beam to reach a given interface and for its echo to return to the probe. In the vertical dimension, the height of the displayed spike indicates the amplitude or strength of the echo.
- •
B-scan ultrasonography , or the B mode, produces a two-dimensional, cross-sectional display of the globe and orbit. The image is displayed in variable shades of gray, and the shade depends on the echo strength. Strong echoes appear white, and weaker reflections are seen as gray.
Figure 5-6
A-scan (bottom) and B-scan (top) of the normal globe. A cross-sectional anterior–posterior view is presented in the B-scan. The lens capsule is seen toward the left of the display, and the optic nerve is seen toward the right. A vector line through the B-scan demonstrates the position of the A-scan information.
The A-scan is used predominantly for tissue characterization, whereas the B-scan is used to obtain architectural information. A-scans are used to determine axial lengths for intraocular lens power calculations for cataract surgery.
- •
- 32.
What lesion features are evaluated during the ultrasound examination?
- 1.
The topography (location, configuration, and extension) of a lesion is evaluated by the two-dimensional B-scan.
- 2.
The quantitative features include the reflectivity, internal structure, and sound attenuation of a lesion.
- •
The reflectivity is evaluated by observing the height of the spike on an A-scan and the signal brightness on a B-scan. The internal reflectivity refers to the amplitude of echoes within a lesion and correlates with histologic architecture.
- •
The internal structure refers to the degree of variation in histologic architecture within a lesion. Regular internal structure indicates a homogeneous architecture and is noted by minimal or no variation in the height of spikes on the A-scan and a uniform appearance of echoes on the B-scan. In contrast, an irregular internal structure is characterized by a heterogeneous architecture and variations in the echo appearance.
- •
Sound attenuation occurs when the acoustic wave is scattered, reflected, or absorbed by a tissue and is noted by a decrease in the strength of echoes either within or posterior to a lesion. It is indicated by a decrease in spike height on the A-scan or a decrease in the brightness of echoes on the B-scan. Sound attenuation may produce shadowing seen as a void posterior to the lesion. Substances such as bone, calcium, and foreign bodies typically produce sound attenuation ( Fig. 5-7 ).
Figure 5-7
B-scan showing a metallic foreign body on the retina surface. A bright echo is produced by the foreign body with shadowing of the structures posteriorly.
- •
- 1.
- 33.
How is ultrasound used in preoperative cataract evaluation?
The A-scan is used to measure the axial length of the globe, which is required in the formula to calculate the intraocular lens power. The B-scan is useful if the ocular media are opaque to assess for a retinal disorder that may affect visual outcome after cataract surgery.
- 34.
How is ultrasound used to assess intraocular tumors?
Ultrasound may be used for diagnosis, to plan treatment, and to evaluate tumor response to therapy. Specifically the tumor shape, dimensions (such as thickness and basal diameter), and tissue characteristics are evaluated, along with the presence of extraocular extension.
- 35.
What are the characteristic features of a choroidal melanoma on ultrasound?
- •
Collar button or mushroom shape on B-scan ( Fig. 5-8 )
Figure 5-8
A-scan and B-scan of choroidal melanoma. The B-scan reveals a collar-button-shaped mass with regular internal structure. A serous retinal detachment extends from the margin of the tumor. The A-scan reveals a strong initial echo from the retinal tissue overlying the tumor followed by a rapid decline in the A-scan echo amplitude (low internal reflectivity) within the tumor tissue. High reflectivity is noted again at the level of the sclera and orbital fat.
- •
Low-to-medium internal reflectivity on A-scan ( Fig. 5-8 )
- •
Regular internal structure
- •
Internal blood flow (vascularity)
- •
- 36.
Describe the ultrasound patterns in the differential diagnosis of choroidal melanoma.
Ultrasound is often used in the evaluation of choroidal melanoma, choroidal hemangioma, metastatic choroidal carcinoma, choroidal nevus, choroidal hemorrhage, and a disciform lesion. It should be combined with clinical information because there are more tumor types than differentiating ultrasound patterns ( Table 5-2 ).
Table 5-2
Ultrasound Patterns in the Differential Diagnosis of Choroidal Melanoma
Lesion
Location
Shape
Internal Reflectivity
Internal Structure
Vascularity
Melanoma
Choroid and/or ciliary body
Dome or collar button
Low to medium
Regular
Yes
Choroidal hemangioma
Choroid, posterior pole
Dome
High
Regular
No
Metastatic carcinoma
Choroid, posterior pole
Diffuse, irregular
Medium to high
Irregular
No
Choroidal nevus
Choroid
Flat or mild thickening (usually <2 mm)
High
Regular
No
Choroidal hemorrhage
Choroid
Dome
Variable
Variable
No
Disciform lesion
Macula
Dome, irregular
High
Variable
No 
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