Reactivating ‘silent

Optic nerve stimulation

Device reverses some glaucomatous visual field loss, may treat other disease. Howard Larkin reports

Visual field defects can be reversed by reactivating “silent” neurons using electronic microstimulation of the eyes and brain, Prof Bernhard A Sabel, PhD told AAO 2020 Virtual.

Prof Sabel, who heads the Institute of Medical Psychology at the Otto-von-Guericke University of Magdeburg, Germany, described a device that delivers electrical pulses through pads on the forehead above the eyes. Stimulation travels through the eye and the optic nerved to the brain’s frontal cortex, 40 minutes per day for 10 days.

In six years of clinical studies involving more than 600 patients, 84% experienced an increase in their visual functions, which confirms earlier reports of a clinical trial where visual field size improvement averaged 24%, with 60% better vision in the impaired visual sector, Prof Sabel reported. However, individual patient responses are highly variable, ranging from “no change” to dramatic improvements (Gall et al., PLoS ONE 2016).

REACTIVATING ‘SILENT’ CELLS This amount of improvement in 10 days cannot be explained by the growth of new cells, rescuing cells from death or regeneration, Prof Sabel said.

“We rather propose that there is a large number of hypometabolic, or ‘silent,’ nerve cells, which survive the disease and can, in principle, be re-activated.”

Evidence for this reactivation includes changes in the brain functional network with increased neural signaling from the occipital cortex to the frontal cortex after stimulation, a communication channel that is lost in patients suffering from optic nerve damage, Prof Sabel explained. Likewise, microcurrent stimulation increased brain oxygenation is seen in these areas after stimulation.

On a cellular level, electronic stimulation stimulates the “silent” neurons to fire more nerve signals, called “action potentials”, which leads to potassium release. This elevated potassium is sensed as tiny currents (ion flow) by nearby blood vessels and triggers upstream dilation. Disruption of this “neurovascular coupling” mechanism, as in vascular dysregulation in glaucoma, renders the cells inactive.

“However, we believe that with microcurrent stimulation we mimic this natural mechanism by stimulating artificially both the neurons and the blood flow to function. This is the basis of the reactivation of silent neurons and subsequent visual field improvement,” he suggested.

TREATING OTHER RETINAL DISEASE This technique has the potential to restore visual function not only in glaucoma, but also optic nerve damage and retinal trauma, diabetic neuropathy, vision loss after stroke or brain trauma and macular degeneration, Prof Sabel said. The treatment has no sideeffects, and its effect is stable in most patients. But about 16% of patients do not respond and duration ranges from several months to years. The causes of this variability are currently being studied.

“My ‘audacious’ proposal is visual field defects can be reversed by reactivating ‘silent’ neurons,” Prof Sabel concluded. “There is more light at the end of the tunnel of blindness.” Bernhard A Sabel: imp@med.ovgu.de www.savir-center.com