title: Phosphenes electrically evoked with DTL electrodes: a study in patients with retinitis pigmentosa, glaucoma, and homonymous visual field loss and normal subjects.
abstract: OBJECTIVE
  To develop an improved, easy, and safe method for reliably eliciting electrical phosphenes and to compare reported perceptions and values of chronaxie and rheobase in healthy individuals and patients with distinct ophthalmic diseases.
  
  
  METHODS
  DTL electrodes were used with a four-alternative, forced-choice method to determine psychophysically the strength-duration curves by using the Weiss model of electrical tissue stimulation in 47 subjects: healthy individuals (n = 17; n = 6 under light- and dark-adapted conditions), patients with open-angle glaucoma (n = 9), retinitis pigmentosa (RP; n = 14), amblyopia (n = 3), or homonymous visual field loss (n = 4).
  
  
  RESULTS
  In all subjects, thresholds were safely determined without side effects. Subjects reported a homogenous, central, white, steady phosphene. The rheobase was higher in glaucoma and patients with RP than in healthy subjects (0.1129 +/- 0.0314 and 0.6868 +/- 0.1054 vs. 0.0383 +/- 0.0057 mA; P = 0.05 and <0.00001, respectively), and it increased with age (r = 0.51; P = 0.038). The rheobase was lower in light- than in dark-adapted conditions (difference 0.015 mA; P = 0.016). In patients with RP, no correlation was found between rheobase or chronaxie and retinal thickness (in OCT) or number of years after reading loss, but there was a correlation between rheobase and visual acuity (P = 0.014). In patients with RP or glaucoma, no inhomogeneity of phosphenes was reported, but all patients with homonymous visual field loss reported lateralization of the phosphene into the area of visual loss.
  
  
  CONCLUSIONS
  Use of DTL electrodes to elicit electrical phosphenes is safe, fast, and reliable. It bears significant advantages over corneal electrodes and provides a valuable tool to elucidate ophthalmologic disease processes and to screen candidates for retinal prostheses.
authors.affiliation: Centre for Ophthalmology, University of Tübingen, Schleichstrasse 12-16, D-72076 Tübingen, Germany. gekeler@uni-tuebingen.de
authors.forename: Florian
authors.initials: F
authors.lastname: Gekeler
authors.forename: Andre
authors.initials: A
authors.lastname: Messias
authors.forename: Max
authors.initials: M
authors.lastname: Ottinger
authors.forename: Karl Ulrich
authors.initials: KU
authors.lastname: Bartz-Schmidt
authors.forename: Eberhart
authors.initials: E
authors.lastname: Zrenner
mesh_terms: D000328:Adult; D000368:Aged; D003623:Dark Adaptation; D004558:Electric Stimulation / Q000379:methods*; D005240:Feasibility Studies; D005902:Glaucoma, Open-Angle / Q000503:physiopathology*; D006423:Hemianopsia / Q000503:physiopathology*; D006801:Humans; D008839:Microelectrodes; D008875:Middle Aged; D010719:Phosphenes / Q000502:physiology*; D012174:Retinitis Pigmentosa / Q000503:physiopathology*; D014796:Visual Perception / Q000502:physiology*