Retinal and Choroidal Detachment

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Retinal detachment is separation of the sensory retina from the retinal pigment epithelium (RPE) (Fig. 3.15). The ERG cannot be recorded when the retinal detachment is complete. Therefore in cases of rhegmatogenous retinal detachments, the reduction of the ERG amplitude corresponds roughly to the extent of retinal detachment (Fig. 3.16).

The EOG is usually quantified by the L/D ratio, which reflects not only the function of the RPE but also the photoreceptors and neurons in the middle retinal layer. The amplitude of the EOG, called the base value, can be measured noninvasively and represents the standing potential of the eye. Animal studies have shown that the standing potential of the eye originates principally from the RPE and represents the difference between the apical and basal membrane potentials of the RPE cells [1, 2]. However, the standing potential varies considerably with many, often minor, factors so the amplitude alone is of minor clinical value, except to note that it is extremely small. However, it has been proven that in normal subjects the amplitudes in the two eyes are not significantly different when tested simultaneously [3]. It is possible, therefore, to use a relative value (i.e., the percentage of the affected eye/fellow eye), as an assay for RPE disease provided the fellow eye is sound. It should be stressed that this is probably a more accurate and sensitive indicator of a deviation from normal. With rhegmatogenous retinal detachments, the L/D ratio is low but the base value remains normal [3] (Fig. 3.17).

Choroidal detachment is separation of the choroid from the sclera, which results from accumulation of clear fluid or blood within and under the choroid (Fig. 3.15). The ERG may be normal or only slightly abnormal when only a choroidal detachment is present. It may be associated with rhegmatogenous retinal detachment, either preoperatively or postoper-atively, and is sometimes seen in the uveal effusion syndrome. Choroidal detachments may also follow trauma or glaucoma surgery, often associated with hypotonic maculopathy.

Fig. 3.15. Fundus showing retinal detachment (left) and choroidal detachment (right)

Because there are only a few reports on electrophysiological changes in eyes with a choroidal detachment, it is important to review the common EOG findings in patients with choroidal detachment [3]. When compared to the normal fellow eye, the EOG base value is not significantly reduced in eyes with rhegmato-genous retinal detachment. However, once choroidal detachment is associated with retinal detachment, the base value decreases significantly (Fig. 3.17). The base value amplitudes are shown in Fig. 3.18 as a function of the rhegmatogenous retinal detachment in 28 eyes without choroidal detachment (group 1) and 11 eyes with it (group 2). Most eyes without choroidal detachment have normal or increased base values, whereas all eyes with choroidal detachment have markedly reduced base values.

There is no significant correlation of the base value amplitude with the intraocular pressure or the extent of the choroidal detachment (Fig. 3.18).

Selective reduction of the EOG base value is also seen with hypotonic retinopathy caused by blunt trauma [4]. In such eyes, the extreme low intraocular pressure may lead to hypotonic maculopathy. The choroidal detachment is often present, although it may not always be detected ophthalmoscopically. The reason the base value does not decrease in eyes with a recent retinal detachment but does so in eyes with choroidal detachment is not known. However, these results indicate that contact between the retinal photoreceptors and RPE is less important than contact between the choroid and the RPE for generating the standing potential.

Fig. 3.16. Mixed rod-cone (bright flash) ERGs (left) and fundus drawings of three patients with rhegmatogenous retinal detachment (right)

Fig. 3.17. A Electrooculogram (EOG) of a patient with unilateral rhegmatogenous retinal detachment and a normal fellow eye. B EOG of a patient with unilateral rhegmatogenous retinal detachment associated with choroidal detachment and a normal fellow eye. In each EOG, the black and white circles indicate the normal fellow eye and the affected eye, respectively

Fig. 3.17. A Electrooculogram (EOG) of a patient with unilateral rhegmatogenous retinal detachment and a normal fellow eye. B EOG of a patient with unilateral rhegmatogenous retinal detachment associated with choroidal detachment and a normal fellow eye. In each EOG, the black and white circles indicate the normal fellow eye and the affected eye, respectively

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Group 1

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r=-0.09, p>0.05

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r=-0.26, p>0.05

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Extent of Retinal Detachment (Quadrant)

Fig. 3.18. Base value and extent of retinal detachment without (group 1) and with (group 2) choroidal detachment.The base value amplitude is expressed as a percentage of the amplitude in the normal fellow eye. The number expressing the extent of retinal detachment represents the detached quadrant (4 indicating total retinal detachment). Dotted zone indicates the range (± SD) of control baseline values obtained by recording base values for the left versus the right eyes of 30 normal subjects. (From Miyake et al. [3], with permission)

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