Abstract
Purpose: LHON is characterized by profound vision loss and dyschromatopsia. Our purpose is to verify the nature of the color vision loss in affected LHON patients from a giant Brazilian pedigree with the MtDNA 11778 mutation. Methods: Seven affected patients (35 to 67 yrs old), six of them in the range of CF or HM and one with 20/200 in each eye, were tested and their results were compared to off–pedigree (n= 21) and asymptomatic carriers (n= 46) with VA <20/30 and no known ophthalmological pathologies. Color vision was assessed monocularly, in a darkened room with the Trivector Cambridge Colour Vision Test (CRS Ltd) with a VSG 2/5 card and a Sony Trinitron monitor, calibrated with a CS1000 Minolta spectrophotometer. The patient was positioned at 1m or less from the monitor, depending on the remaining vision. Patients responded by pointing to the gap in the Landolt C target, were allowed to use peripheral vision and took all the time required to give a response. Controls and carriers used the response box and sat 3m away from the monitor. Results:It was possible to test six out of the seven affected patients (35–57 yrs old, median=40.5). The oldest patient did not report the presence of color at all. All others showed an equal pattern of losses: total deutan loss, high protan loss, small or absent tritan loss. Normal tritan thresholds were found in 3/6 patients. The average tritan score was 207 ± 120 u’v’ units. Conversely, all patients had unmeasurable deutan thresholds, reaching the ceiling value of 1100 u’v’ units. The protan threshold was very high but measurable in 3 of 6 patients, the same that had normal tritan thresholds. Their protan average was 465 ± 31 u’v’units. Average thresholds in off–pedigree subjects were 72 ± 32, 76 ± 39, 112 ± 46 respectively for the protan, deutan and tritan axes, and for the carriers with abnormal protan thresholds, they averaged respectively 211 ± 113, 215 ± 182; 152 ± 66. Conclusions:To our knowledge, this is the first report of detailed measurement of color vision loss in severely affected LHON patients. A profound color vision loss affects their red–green system. Despite this loss and the dramatic visual acuity impairment, their discrimination performance in the tritan axis remains almost intact or much less affected, following the trend observed in the LHON carriers. These findings show that the mechanisms responsible for LHON are highly specific within the visual system.
