Ординатура / Офтальмология / Английские материалы / Electrodiagnosis of Retinal Disease_Miyake_2005

It is extremely important to monitor the location of the stimulus when recording the focal macular ERGs in patients with an idiopathic macular hole to determine the relation between the stimulus spot and the macular hole. This relation can be studied precisely with our system because focal macular ERGs are recorded under the fundus monitor. As shown in Fig. 4.19, when the spot is smaller than or equal to the macular hole, a response cannot be recorded (Fig. 4.19A). When the spot is slightly larger than the macular hole, the stimulus light stimulates the parafoveal cuff and a reduced response with delayed implicit time is recorded (Fig. 4.19B). When the spot is much larger than the macular hole, a large response is elicited from the normal perifoveal retina. Although the amplitude is still smaller than that of the

normal fellow eye, the implicit time is not delayed because the implicit time of the b-wave reflects the function of the surrounding normal retina.

Another interesting phenomenon is seen in the focal macular ERGs elicited by a large spot in patients with an idiopathic macular hole. When the stimulus used to stimulate the affected eye and the normal fellow eye are balanced to elicit nearly equal-amplitude a-waves and b-waves, the OPs of the affected eyes are always larger than those of the fellow eyes (Fig. 4.20). This is caused by the unique distribution of OPs around the macular region, as described earlier [1]. Namely, the distribution of OPs is larger in parafovea than fovea and eyes with macular hole are stimulated more in parafovea than fovea (see Section 1.2.3, Fig. 1.28).

Recent advances of vitreoretinal surgery have made it possible to close the macular hole and recover the normal configuration of the macula, which can be confirmed by OCT (Fig. 4.21). The surgical technique to close the macular hole consists of standard three-port pars plana vitrectomy and separation of the posterior hyaloid from the retina after removing the cortical vitreous. Residual vitreous cortex is removed when it is present around the macular hole. After vitrectomy, fluid-gas exchange is performed [2].

The changes in the focal macular ERGs before and after macular hole surgery [3] are shown in Fig. 4.22. A 65-year-old man had noted decreased vision in his left eye for 6 months. Examination disclosed a stage 3 fullthickness macular hole, and his visual acuity was 0.08 (OD) and 1.0 (OS). Focal macular ERGs were recorded with 2°, 3°, and 4° stimu-

lus spots from both eyes before surgery. The different stimulus spot sizes are photographically superimposed on the fovea to compare the sizes of the stimulus spot and the macular hole. In the affected eye, responses were not recorded when the stimulus size was equal to or smaller than the size of the macular hole (2° and 3°). When the stimulus spot was larger than the macular hole (4°), a response suggestive of a focal macular ERG was recorded (A).

The patient underwent surgery, and the macular hole was closed. Six months later his visual acuity improved to 0.4, and focal macular ERGs were successfully recorded even with the smallest 2° spot. These results confirmed that the area of retinal deficit, the macular hole, was closed by the surgery, and function had returned (B).

The focal macular ERGs elicited by 4° and 5° stimulus spots before and after surgery in four

patients with a full-thickness macular hole are shown in Fig. 4.23. The amplitudes of the b- wave of the focal macular ERGs were increased in all eyes, and the implicit times were decreased after the surgery. In our study of a large series of patients [3], the preoperative b-wave implicit time measured with a 5° spot significantly correlated with the postoperative corrected visual acuity (Fig. 4.24). In addition, the postoperative b-wave amplitude and implicit time of the focal macular ERGs elicited by a 5° spot significantly correlated

A

with the postoperative visual acuity. These results strongly suggest that parafoveal function in the area surrounding the macular hole is closely related to the visual prognosis after surgery in patients with an idiopathic macular hole.

To increase the closure rate of macular holes, removing the internal limiting membrane (ILM) in addition to routine vitrectomy has recently been advocated [4]. The exact role of the ILM in retinal physiology has not been determined, although the ILM is the basal

B

lamina of Mueller cells and is connected to the end-feet of the Mueller cells [5]; and Mueller cells are involved in generation of the ERG b- wave [6]. In addition, the anionic sites on the ILM may act as a charge barrier between the retina and the vitreous cavity [7].

The changes in the focal macular ERG after conventional vitrectomy without ILM removal (ILM-on) and with ILM removal (ILM-off) are compared [8] in Fig. 4.25. Six weeks after vitrectomy without ILM peeling, the amplitudes of the b-waves and OPs of the focal macular ERGs were larger, and an additional increase was observed 6 months after surgery. On the other hand, the patient with ILM-off showed a

marked decrease in the amplitudes of the b- waves and OPs 6 weeks after surgery, which then recovered to the preoperative level at 6 months.

Analysis of a large series of patients showed that visual acuity increased significantly after surgery in both groups, and no significant difference was found between the two groups. However, recovery of the b-wave in focal macular ERGs in the ILM-off group was significantly delayed until 6 months after surgery compared with that of the ILM-on group, suggesting slower recovery of retinal physiology in the macular region in the ILMoff group.

References

1.Miyake Y, Shiroyama N, Ota I, Horiguchi M (1988) Oscillatory potentials in electroretinograms of the human macular region. Invest Ophthalmol Vis Sci 29:1631–1635

2.Johnson RN, Gass JD (1988) Idiopathic macular holes: observations, stages of formation, and implications for surgical intervention. Ophthalmology 95:917–924

3.Terasaki H, Miyake Y, Tanikawa A, Kondo M, Ito Y, Horiguchi M (1998) Focal macular electroretinograms before and after successful macular hole surgery. Am J Ophthalmol 125:204–213

4.Park DW, Sipperley JO, Sneed SR, Dugel PU, Jacobson J (1999) Macular hole surgery with internallimiting membrane peeling and intra-vitreous air. Ophthalmology 106:1392–1397

5.Nishihara H (1989) Studies on the ultrastructure of the inner limiting membrane of the retina. I. Surface replication study on the inner limiting membrane of the retina. Acta Soc Ophthalmol Jpn 93:429–438

6.Terasaki H, Miyake Y, Piao CH, Hori K, Niwa T, Kondo M (2001) Focal macular ERGs in eyes after removal of macular ILM during macular hole surgery. Invest Ophthalmol Vis Sci 42:229–234

A macular pseudohole resembles a fullthickness macular hole in terms of its shape, but the pathophysiology is quite different. The pseudohole is created by spontaneous contraction of an epiretinal membrane surrounding the fovea. The pseudohole is not a retinal hole but a round defect of the epimacular membrane over the fovea. The natural course of a macular pseudohole is generally good, with preservation of near-normal visual acuity [1]. In some cases, however, further contraction of the epiretinal membrane distorts the fovea, resulting in decreased visual acuity.

A fundus photograph and an OCT image of an eye with a macular pseudohole are shown in Fig. 4.26. When the focal macular ERG elicited by a 15° spot was analyzed [2], we found a significant correlation between the reduction of the a-waves and b-waves and the parafoveal thickness (Fig. 4.27). There was a significant reduction in the mean amplitudes of the a-

waves, b-waves, and OPs in the affected eyes compared with fellow eyes. The relative amplitudes of the a-wave, b-wave, and OPs were 84.0%, 74.8%, and 62.4%, respectively. Thus, the decrease in the amplitude was greatest for the OPs followed by the b-waves and then the a- waves. The implicit times of a-waves, b-waves, and OPs were delayed significantly. These results were similar to those obtained from eyes with an idiopathic epimacular membrane without a macular pseudohole (see Section 4.3).

A significant correlation was found between the relative amplitude (affected eye/normal fellow eye) of the b-wave elicited by the 5° stimulus and the visual acuity (Fig. 4.28). This correlation probably resulted from using the 5° spot, which is small and reflects the function of the small central area where the epiretinal membrane is absent.

References

1.Creven CM, Slusher MM, Czyz CN (1998) The natural history of macular pseudoholes. Am J Ophthalmol 125:360–366

2.Suzuki T, Terasaki H, Niwa T, Mori M, Kondo M, Miyake Y (2003) Optical coherence tomography and focal macular electroretinogram in eyes with epiretinal membrane and macular pseudohole. Am J Ophthalmol 136:62–67

Age-related macular degeneration (AMD) is a chronic degenerative disease that affects primarily the choriocapillaris, Bruch’s membrane, and the RPE. It is the most common cause of legal blindness in the United States and Europe, and the incidence of AMD in Japan has been increasing during the past 20 years. AMD is classified into two types: dry and exudative. The

exudative form, also called disciform macular degeneration, is characterized by the development of choroidal neovascularization, which causes serous and hemorrhagic detachment of the retina, resulting in poor central vision. The ophthalmoscopic appearance of AMD is shown in Fig. 4.29. Some AMD is treatable with laser photocoagulation, photodynamic therapy, or

vitreoretinal surgery. Because the condition tends to be self-limiting, retinal function in the periphery is relatively well preserved and complete loss of vision is rare.

With both types of AMD, full-field ERGs are usually nearly normal, and the focal macular ERGs and multifocal ERGs demonstrate only abnormal macular function (Fig. 4.30).