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D. G. Birch, D. C. Hood, K. L. Locke; Rod Photoresponse Kinetics in Retinal Degenerative Diseases (RDDs). Invest. Ophthalmol. Vis. Sci. 2007;48(13):3743.
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Delays in b-wave implicit time are characteristic of RDDs such as retinitis pigmentosa (RP) and cone-rod dystrophy (CRD). Cone b-waves are typically delayed at an early age and remain fairly constant with progression. Rod b-waves, on the other hand, often show normal timing initially but become slower with progression of disease. Part of the delay is due to slower activation kinetics (reduced gain). Here we determine whether inactivation kinetics are also involved.
The full time course of the rod photoresponse was derived from 30 patients with RDDs. These included 8 patients with RHO mutations, 9 patients with RDS mutations, 8 patients with CRD and 5 patients with recessive or isolate RP. Twelve normal subjects were tested for comparison. The leading edge of the photoresponse was derived as described by Hood and Birch, Documenta Ophthalmol, 1997. The inactivation phase was derived using a double-flash paradigm, with a test flash of 2.4 log scot td-sec followed at varying intervals by a 4.2 log scot td-sec probe flash.
The time course of the derived rod photoresponse to the just-saturating test flash in normal subjects was similar to that described previously with an average critical time to the initiation of recovery (Tc) of 491 ± 103 ms. Mean values of Tc in patients were RHO: 180 ± 110 ms, RDS: 322 ± 113 ms, rec/iso RP: 347 ± 169 ms, and CRD: 525 ± 170 ms. All 8 patients with RHO mutations had significantly (p<0.05) faster than normal recovery. Significantly faster than normal recovery was found in 4 patients with RDS mutations, 3 patients with recessive/isolate RP and 1 patient with CRD.Discussion: Although occasional patients had normal rod photoresponse time courses, none had slower than normal times to the initiation of recovery and the majority had significantly shorter than normal recovery times. Accelerated photoresponses, along with decreases in gain, lead to desensitization of the photoresponse, prolonged b-wave implicit times, and night blindness in these patients.
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