Hyperosmolarity response of ocular standing potential as a clinical test for retinal pigment epithelium activity diabetic retinopathy

Rapid resolution of retinoschisis with acetazolamide

PURPOSE

To report the results of an acetazolamide (Diamox(®)) treatment regimen in a genetically confirmed case of X-linked juvenile retinoschisis (XLRS).

METHODS

A patient with XLRS was prescribed acetazolamide (Diamox(®)) at a dose of 500 mg/day, then discontinued the treatment due to non-compliance for 4 days, and finally resumed the course of treatment. Best-corrected visual acuity, retinal structure, and function were monitored with autofluorescence, spectral domain-optical coherence tomography (SD-OCT), adaptive optics scanning laser ophthalmoscopy (AOSLO), and full-field electroretinogram (ERG). Full-field ERG was performed using DTL recording electrodes and Ganzfeld stimulation according to ISCEV standards.

RESULTS

Serial monitoring of the cysts by SD-OCT revealed a strong association between the effects of acetazolamide administration and the size of the schisis. A reduction in foveal cyst size was significant in as rapid as 6 days after acetazolamide initiation. AOSLO data revealed that the resolution of cone cell images improves as the foveal schisis decreases in size.

CONCLUSIONS

Efficacy of acetazolamide in patients with XLRS can be apparent in as rapid as a week of therapy. AOSLO can be a good method to evaluate the cone cells after acetazolamide treatment in the early stages of XLRS.

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

The fast oscillation of the electrooculogram reveals sensitivity of the human outer retina/retinal pigment epithelium to glucose level

The effect of acute blood glucose elevations on human outer retinal function was examined. Electrooculograms were recorded as the background light cycled on/off with a 2-min period, eliciting rapid changes in the corneo-retinal standing potential known as the fast-oscillation of the electrooculogram. Recordings were made while subjects fasted and after they consumed 100 g of D-glucose. In all subjects, blood glucose levels strongly affected fast oscillation amplitude, which reflects photoreceptor-driven changes in RPE cell chloride concentration. The sensitivity of RPE metabolism to glucose fluctuations may relate to changes in the blood-retinal barrier that are known to occur in diabetes (e.g. macular edema).

Electrical responses from diabetic retina

Diabetic retinopathy has long been considered to be a retinal manifestation of systemic diabetic angiopathy. Indeed, it is therapeutically true. However, the prolongation of OP peak latency in diabetic eyes without any angiographic evidence of angiopathy leads us to presume that certain neuronal disorders occur early in diabetic eyes. Even though we cannot neglect the possibility that the prolongation of the OP peak latency may derive from undetectable retinal hypoperfusion, it is still far from conventional diabetic angiopathy. Rather, the status should be properly termed "intraretinal diabetic neuropathy" in that the neurones are the disturbed cells to cause visual dysfunction. Thereafter, the OP amplitude diminishes as retinopathy advances, probably depending on the degree of retinal circulatory disturbance. Marked diminution of the OP amplitude predicts rapid progression and poor prognosis of retinopathy. Diabetic retinal pigment epitheliopathy as manifested by one of our non-photic EOG responses is another kind of early ocular involvement of diabetes. Because its mechanisms are not yet known, so far we have not succeeded in correlating it to any kind of subjective visual index. Routine fundus inspection or fluorescent fundus angiography is incapable of detecting the compromised neural retina and/or retinal pigment epithelial integrity and thus the electrophysiology of vision has the edge in ophthalmology.

Combined photic and nonphotic electro-oculographic responses in the clinical evaluation of the retinal pigment epithelium

In an attempt to simplify the recording technique in electrophysiologic evaluation of the retinal pigment epithelium, we combined the electro-oculographic light rise, hyperosmolarity and acetazolamide responses in a single recording session. Recordings were performed in six normal subjects and in seven patients with diabetic retinopathy or retinitis pigmentosa. In the patients with background diabetic retinopathy, the hyperosmolarity responses were slightly reduced, while the acetazolamide response and the light rise was normal. In the patients with proliferative diabetic retinopathy, the hyperosmolarity response and light rise were remarkably reduced, while the acetazolamide response was normal. In the patients with retinitis pigmentosa, the hyperosmolarity response and light rise were decreased, while the acetazolamide response was normal. Despite a small study population, we concluded that the clinical results from our combined recording protocol were essentially the same as those reported for each response separately. Because this recording technique simplifies electrophysiologic evaluation of the retinal pigment epithelium, it may help clarify the mechanisms or localization of retinochoroidal and pigment epithelial diseases.

Suppression of the hyperosmolarity response after cataract surgery

The hyperosmolarity response was recorded with the electrooculogram in the dark before and after cataract extraction, to investigate a possible disorder in the retinal pigment epithelium after cataract extraction. The hyperosmolarity response was suppressed one week after surgery, especially after intracapsular extraction. The response recovered up to the preoperative level in most cases one or two months after surgery. These findings indicate reversible functional disorders of the pigment epithelium after cataract extraction, particularly intracapsular. This dysfunction can be revealed by the hyperosmolarity response. This response is clinically useful for diagnosis of pigment epitheliopathy after cataract extraction.