Ординатура / Офтальмология / Английские материалы / Electrodiagnosis of Retinal Disease_Miyake_2005

When massive vitreous hemorrhage prevents ophthalmoscopic examination of the fundus in patients with proliferative diabetic retinopathy, it makes it difficult to predict the surgical and visual outcome after vitrectomy. In these eyes, the amplitudes of the ERGs may be markedly reduced by various factors: pathological changes induced by the diabetic retinopathy; earlier PRP; vitreous hemorrhage. As mentioned, the PRP reduces the ERG amplitude, but the b/a ratios are unchanged (Fig. 3.3). Because most patients with vitreous hemorrhage have undergone PRP, it is difficult to arrive at a prognosis of the outcome after vitrectomy using only the a-wave and b-wave amplitudes. The b/a ratios provide more useful information about the prognosis after vitrectomy [4].

The preoperative full-field ERGs elicited by a single bright flash can be used to classify patients with diabetic retinopathy into three groups (Fig. 3.6): Group A includes those with a b/a ratio ≥ 1.0, and the OPs are clearly recordable. Group B includes those with a b/a ratio ≥ 1.0, but the OPs are absent. Group C comprises those with a b/a ratio < 1.0, with no recordable OPs. The distribution of the postoperative visual acuity for each group is shown in Fig. 3.7. The postoperative visual acuity for group C was significantly worse than that for group A or group B. The low b/a ratios may indicate a more severe ischemic retina, which in turn may account for the relatively good correlation with visual acuity. However, among the

patients in group C, there were some whose postoperative visual acuity was good, indicating that a b/a ratio < 1.0 is not necessarily a contraindication for surgery.

The light-filtering effect of a dense vitreous hemorrhage should also be considered when evaluating the preoperative ERG in diabetic patients. Severe vitreous hemorrhage reduces the intensity of the stimulus light reaching the retina, which can increase the b/a ratio (see Section 1.1.1). Therefore, a lower b/a ratio in a patient with an opaque vitreous indicates a greater decrease in the b/a ratio in clear media, suggesting more severe ischemia of the retina.

Another interesting observation is that most patients who have distinct OPs preoperatively have good visual acuity (>0.5) after surgery. This observation is important when we discuss the visual prognosis with patients before surgery.

Thick proliferative tissues are found at the disk (Fig. 3.8) intraoperatively in 36% of the patients in group A, 67% in group B, and 90% in group C. Hirose [5] suggested that the fibrous proliferation at the disk may restrict retinal circulation by compressing the central retinal artery.

In conclusion, it should be emphasized that a considerable amount of useful information can be obtained from a simple preoperative ERG on diabetic patients with massive vitreous hemorrhage.

Diabetic maculopathy is one of the leading causes of blindness in diabetic retinopathy. The functional and morphological analysis of diabetic maculopathy is thus important for prognostic evaluation and treatment indications.

The OCT images and the focal macular ERGs obtained from representative patients with diabetic maculopathy are shown in Fig. 3.9. Normal control (A), mild maculopathy (B), or focal macular edema (C) often show an

abnormality of only the macular OPs, particularly a delay in the implicit times. The OCT images of these patients may show a macula slightly thicker than normal. Patients with diffuse macular edema may have more severe alteration of the OPs in regard to both amplitude and implicit time as well as a decrease in the b-waves. In patients with diffuse macular edema, the OCT always shows a thicker macula, sometimes associated with cystic spaces (D). Patients with ischemic maculopathy with an avascular zone in the macula (E) usually have undetectable OPs, reduced a-wave amplitude, and more reduced b-wave amplitude. OCT images show that the macula can be either thicker or thinner than normal. A patient with pigment epitheliopathy of the macula (F) can have undetectable OPs with markedly reduced a-waves and b-waves. The OCT may show a rather thin macula.

We studied the focal macular ERGs recorded from 73 eyes at various stages of diabetic maculopathy and compared them to 62 age-

matched controls (Fig. 3.10). The amplitudes of the a-waves, b-waves, and OPs in eyes with diabetic maculopathy, and with ophthalmoscopically normal macula and macular microaneurysms, were not significantly different from normal, but they were reduced in eyes with focal and diffuse macular edema. However, the implicit times of the OPs were significant delayed even when the macula was ophthalmoscopically normal. Thus, as seen in full-field ERGs, the macular OPs are the most sensitive indicator of the functional changes during early diabetic maculopathy.

Diabetic macular edema is the most common cause of decreased visual acuity in patients with diabetic maculopathy. At present, vitrectomy is an investigational technique used for treating diabetic macular edema, sometimes resulting in recovery of the anatomic configuration of the macula without photocoagulation [6].

The question then arises as to whether recovery of macular configuration is accompanied by recovery of the macular ERGs. We

found [7] that anatomical resolution of the macular edema as detected by OCT preceded functional recovery of the macular ERG 6 months after vitrectomy (Fig. 3.11). Many patients did not show significant recovery of the macular ERGs despite good recovery of macular configuration. However, 12 months after surgery, visual acuity was significantly improved and the b-wave amplitude of macular ERG had increased significantly (Fig. 3.12). Although there was a wide range of changes in the b-wave amplitude at 12 months,the increase in the b-wave correlated with the decrease in the foveal thickness (Fig. 3.12).

A disparity in the time course and degree of recovery of the foveal thickness and macular function was found in eyes with diabetic macular edema after vitrectomy. It is conceivable that diabetic macular edema, an indication for vitrectomy, is often associated with additional macular pathology, such as ischemia, which cannot be improved by vitrectomy. Such pathology may influence the focal macular ERGs more than the changes induced by the macular edema.

References

1.Yonemura D, Aoki T, Tsuzuki K (1962) Electroretinogram in diabetic retinopathy. Arch Ophthalmol 68:19–24

2.Kondo T, Miyake Y (1985) Effects of panretinal photocoagulation on posterior fundus in diabetic retinopathy: analysis of local macular electroretinogram and visual evoked response. Acta Soc Ophthalmol Jpn 89:535–543

3.Miyake Y (1988) Study on local ERG. Acta Soc Ophthalmol Jpn 92:1419–1449

4.Hiraiwa T, Horio N, Terasaki H, Suzuki T, Yamamoto E, Horiguchi M, et al (2003) Preoperative electroretinogram and postoperative visual outcome in patients with diabetic vitreous hemorrhage. Jpn J Ophthalmol 47:307–311

5.Hirose T (1977) Evaluation of retinal function in the presence of vitreous opacities. In: Vitreous surgery and advances in fundus diagnosis and treatment. Appleton-Century-Crofts, New York, pp 79–97

6.Lewis H, Abrams GW, Blumenktanz MS, Campo RV (1992) Vitrectomy for diabetic traction and edema associated posterior hyaloidal traction. Ophthalmology 99:753–759

7.Terasaki H, Kojima T, Niwa H, Piao CH, Ueno S, Kondo M, et al (2003) Changes in focal macular electroretinograms and foveal thickness after vitrectomy for diabetic macular edema. Invest Ophthalmol Vis Sci 44:4465–4472

There are two important questions to consider about eyes with a central retinal vein occlusion (CRVO): Can the waveform of the ERG be used to classify the CRVO as ischemic and nonischemic types? Can ERGs be used to predict the development of neovascular glaucoma?

It has been reported that the b/a ratios of the ERGs of the affected eyes of patients with a CRVO who developed neovascularization of the iris (NVI) were significantly lower than the

ratios of the fellow eyes and lower than the ratios in the affected eyes of individuals who did not develop NVI [1, 2]. However, there are also cases where the ERG of a patient with an initially normal b/a ratio changes so the ratio becomes <1.0 or vice versa (Fig. 3.13). This suggests that the initial findings may change during the course of the disease. Even though such case variance is present, the b/a ratio is an important index for evaluating the prognosis of CRVO.

In cases of complete central retinal artery occlusion (CRAO), the ERG consists of a normal or supernormal a-wave and a markedly reduced b-wave [2]. However, this is not the case in most patients [3]. Platelet-fibrin and cholesterol are the major types of emboli, and both are soft and consequently quickly become fragmented and cast into the distal radicles of the retinal circulation. As a result, by the time the patient is examined fluorescein angiography often fails to detect complete obstruction of the central retinal artery.

The ERGs elicited by a single bright flash (mixed rod and cone ERG),the visual fields,and visual acuities of five patients with CRAO are shown in Fig. 3.14. Despite the extremely constricted visual fields and poor visual acuity, the ERGs in the affected eyes are well preserved. Although the b/a ratio in the affected eyes is lower than that of fellow eye, none of the ERGs from the affected eyes has a negative configuration. These results suggest that the function of the retinal layer related to the ERG can recover to some degree following recovery of the retinal

circulation, as demonstrated by fluorescein angiography. However, because the ganglion cells are more vulnerable to ischemia, they suffer irreversible damage soon after the occlusion. Thus, the marked deterioration of the subjective visual function in patients with CRAO may result largely from the damage to ganglion cells.

The fundus appearance of ophthalmic arterial occlusion is similar to that of CRAO in the acute stage. However, when the ophthalmic artery is occluded, not only central retinal

References

1.Karpe G, Uchermann A (1955) The clinical electroretinogram. IV. The electroretinogram in circulatory disturbances of the retina. Acta Ophthalmol 33:493–516

2.Sabates R, Hirose T, McMeel JW (1983) Electroretinography in the prognosis and classification of central retinal vein occlusion. Arch Ophthalmol 101:232–235

artery circulation but also choroidal circulation is disturbed, resulting in marked deterioration of the a-wave [3]. This finding is important for differentiating CRAO from ophthalmic arterial occlusion.

Electrooculography (EOG) of CRAO shows a depressed light rise with a low base value. This finding indicates that the mid-retinal cells necessary for detecting the EOG light rise are compromised, producing an abnormal light peak/dark trough (L/D) ratio [3, 4].

3.Miyake Y (1974) Electro-oculographic change in retinal arterial occlusion and its analysis. Acta Soc Ophthalmol Jpn 78:311–323

4.Gouras P, Carr RE (1965) Light-induced DC responses of monkey retina before and after central retinal arterial interruption. Invest Ophthalmol 4:310–317

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 postoperatively, and is sometimes seen in the uveal effusion syndrome. Choroidal detachments may also follow trauma or glaucoma surgery, often associated with hypotonic maculopathy.