gms | German Medical Science

We intended to develop an improved easy and safe method to reliably elicit electrical phosphenes and to compare reported perceptions and values of chronaxie and rheobase in healthy individuals and patients suffering from distinct ophthalmologic diseases.

DTL electrodes were used with an alternative forced choice (AFC) method to psychophysically determine strength-duration curves using the Weiss model of electrical tissue stimulation in 47 subjects: healthy individuals (n=17; among them 6 examined in light- and dark-adapted conditions), patients suffering from visual field loss due to glaucoma (n=9), retinitis pigmentosa (RP; n=14), amblyopia (n=3), homonymous visual field loss (n=4).

In all subjects thresholds could be safely determined without side effects. Subjects reported a homogenous central white steady phosphene. Rheobase was higher in glaucoma and RP patients than in healthy subjects; it increased with age. Rheobase was lower under light- than under dark-adapted conditions, while chronaxie was higher. There was no correlation between rheobase or chronaxie and retinal thickness (in OCT) or number of years after reading loss, but with visual acuity. In RP and glaucoma no inhomogeneity of phosphenes was reported. In contrast, all patients with homonymous visual field loss reported lateralization of the phosphene into the area of visual loss.

The use of DTL electrodes to elicit electrical phosphenes is safe, fast and reliable. It bears significant advantages over corneal electrodes and in conjunction with an AFC method provides a valuable tool to elucidate ophthalmologic disease processes. It seems to be a valid predictor for successful stimulation by retinal implants.