ELECTRO-OCULOGRAPHY

The electro-oculogram (EOG) is an electrophysiologic test that measures the function of the retinal pigment epithelium (RPE). The EOG measures the resting potential of the RPE, which depends on metabolic activity of the RPE. Clinically, information is gained from the EOG by comparing the amplitude of the resting potential in the light-adapted vs dark-adapted state. As a result, the EOG is a change in the amplitude of the resting potential under lightand dark-adapted states and is expressed as a ratio of the maximum light-adapted to minimum darkadapted potentials (the Arden ratio). The normal range for this ratio is between 1.9 and 2.8. A ratio of 1.7 to 1.8 is marginally subnormal, while a ratio of less than

1.7 is considered subnormal. The EOG, like the electroretinogram (ERG), is a mass response. The EOG findings are usually consistent with the ERG with a few notable exceptions. An abnormal EOG associated with a normal ERG is demonstrated in Best’s disease and possibly dominant drusen and pattern dystrophy. A normal EOG associated with an abnormal ERG may be seen in patients with X-linked juvenile retinoschisis.

Clinical Features

The most important clinical use of the EOG is to help diagnose patients with Best’s disease. The Arden ratio is appreciably reduced in this disease, while the ERG is typically normal to even supranormal. The reduced Arden ratio not only may be reduced in patients with obvious disease, but may also be reduced in patients with the gene for the disease but no clinically evident fundus findings. In cases of Best’s disease, in which only one eye manifests fundus changes at an early stage, both eyes will typically show reduced EOG ratios. The EOG may also be used to monitor patients taking chloroquine or hydroxychloroquine, as these drugs are potentially toxic to the RPE.

Technique

The EOG, unlike the ERG, does not use a corneal contact electrode. To perform the procedure, the patient is exposed to ambient room illumination and the eyes are dilated. Electrodes are attached just lateral to the inner and outer canthi of each eye. The subject then alternately fixates between two blinking lights 30º apart in a ganzfeld dome. This procedure is first done in the dark for 12 minutes and then in an illuminated ganzfeld dome for another 12 minutes. Care should be taken to increase the illumination slowly to prevent tearing, which can alter electrode resistance.

Side Effects

As the EOG does not use a corneal contact lens, the possibilities of surface irritation and corneal abrasion are minimized. The EOG may be useful in pediatric patients and in some adult patients unable to tolerate the ERG.

A patient’s view of the ganzfeld dome during electrooculography. Note the two dark fixation lights 30° apart.

Demonstration of a young child with Best’s disease participating in an electro-oculographic study.

WAVEFORM 1 (L)

CURSOR 1: 19 Min 358 uV

CURSOR 2: 31 Min 1503 uV

ARDEN RATIO: 4.20

1

Time Base: 1 minute/div

A normal electro-oculogram is demonstrated in this patient with Stargardt disease. The Arden ratio (light peak to dark trough) is 4.2.

The most common indication for an electro-oculogram (EOG) is Best’s disease. The EOG findings may be abnormal in individuals with pattern dystrophy and macular drusen.

uV/div

2

1

Time Base: 1 minute/div

Abnormal electro-oculogram is demonstrated in this patient with Best’s disease. The Arden ratio for each eye is less than 1.7.

ECHOGRAPHY

Ultrasound was developed in the early 1900s as a method to detect underwater objects. It was not until the 1960s that ultrasound was developed for ophthalmic applications. Today, ultrasound is a common and often indispensable diagnostic test.

The principle of ultrasonography is based on the piezoelectric effect of certain crystals and ceramic materials. The piezoelectric effect is characterized by the ability of the crystal or ceramic material to convert an electrical pulse, generated by a transmitter, into sound waves and then to receive echoes of the same wavelength and convert them back into electricity. The electrical potentials are then presented on an oscilloscope screen.

Diagnostic ultrasonography projects high frequency sound through soft tissues. The sound waves are partially reflected at various tissue interfaces. The reflections are recorded as either echo spikes (A-scan) or brightness dots (B-scan). Both of these interpretations allow for evaluation of posterior segment structures such as vitreous, choroid, and retina through opaque media.

Clinical Features

Ophthalmic ultrasonography is used for a variety of purposes including measurement of axial length (A-scan), corneal pachymetry (A-scan), diagnostic B-scan, and standardized echography. Only diagnostic B-scan ultrasonography is discussed here. B-scan ultrasonography is useful when opaque media are present, precluding a direct examination of posterior segment structures. In the presence of corneal scars, dense cataracts, and vitreous opacities, B-scan ultrasonography is helpful in identifying and characterizing retinal detachment, choroidal detachment, vitreous detachment, intraocular foreign bodies, and intraocular tumors.

On B-scan ultrasonography, retinal detachment appears as a smooth, mobile, highly reflective membrane. When fully detached, the retina can usually be noted to remain attached at the optic disc and ora serrata. Chronic retinal detachments become less mobile and may often appear thickened.

Choroidal detachment appears as a smooth, convex, and immobile reflective membrane. In contrast to retinal detachment, choroidal detachment can be noted to extend beyond the ora serrata. Choroidal detachment does not extend to the optic disc.

Almost all intraocular foreign bodies are detectable with B-scan ultrasonography. Metallic, stone, glass, and plastic objects are highly reflective and produce highamplitude echoes. These echoes may be accompanied by acoustic shadowing.

Technique

Ophthalmic ultrasound is a diagnostic technique providing real-time images of tissues. Analysis is facilitated when the clinician partakes in the actual examination itself. Both A-scan ultrasonography and B-scan ultrasonography are performed by a contact or water immersion technique in which fluid coupling is present between the globe and the ultrasound probe. The examination should be performed in a systematic manner obtaining transverse, oblique, longitudinal,

and axial scans.

Side Effects

Diagnostic ultrasonography, as described here, uses frequencies in the megahertz range. Significant heat levels are therefore not generated, making this a safe and effective imaging modality.

A B-scan image of a patient with a choroidal melanoma. Note the mushroom-shaped outline of the lesion, which results from growth of the lesion through Bruch’s membrane.

A B-scan image of a patient with a circumscribed choroidal hemangioma. Note the solid convex appearance to this lesion. Often, this is difficult to distinguish from a choroidal melanoma on B-scan ultrasonography alone.

A standardized A-scan of the same lesion demonstrates low to medium internal reflectivity, suggestive of a choroidal melanoma.

Standardized A-scan of the same patient with choroidal hemangioma, demonstrating medium to high internal reflectivity. This pattern is suggestive of a choroidal hemangioma.

A longitudinal B-scan image of a patient with a funnel retinal detachment. Note the attachment to the

optic disc.

Transverse B-scan image of the same patient with a funnel retinal detachment. As a result of the position from which the scan was obtained, the retinal detachment appears as a circle.