The present study was designed to evaluate the frequency of color blindness in MS patients in contrast to normal population, moreover, the correlation between the history of ON, impaired VEPs and the color blindness was assessed in MS patients too. According to the results, the frequency of color blindness was more in multiple sclerosis patients than in normal control group. In addition, the history of ON was more common in patients with abnormal prolonged VEPs than those with the color blindness.
The Ishihara Color Test is a test for red-green color deficiencies. While the full test consists of 38 plates, testing the first 24 plates gives a more accurate diagnosis of the severity of the color vision defect.[16
] In our study, we found a relationship between the color vision deficit and an increased VEP latency.
Visual evoked potentials (VEPs) are caused by sensory stimulation of a subject's visual field. VEPs is a suitable tool for diagnosis of ON cases with equivocal clinical presentations.[17
] Visual evoked potentials are very useful in detecting the blindness in patients that cannot communicate, such as babies or animals. Other applications include the diagnosis of optic neuritis, which causes the signal to be delayed. Such a delay is also a classic finding in multiple sclerosis. Visual evoked potentials is furthermore used in the investigation of basic functions of visual perception. Sometimes, VEPs is used to determine if someone is fraudulently alleging blindness.[18
According to our study, a patient with history of deficits in color vision in addition to abnormal delayed VEPs, may be potentially a manifestation of multiple sclerosis disease, and complementary clinical and para-clinical work up is warranted.
Visual dysfunction is frequent and often irreversible. Afferent peregeniculate visual pathways (retina, optic nerves, chiasm, and tracts) are targets of inflammation, demyelination, and axonal degeneration. About 65% of the MS patients had a history of ON during their illness and it is known as the most common cause of vision loss among them.[20
] As described before, ON usually presents with monocular, acute painful vision loss.[16
] It is the heralding event in 15%-20% of patients. However, patients without a clinical history of ON also exhibit poor visual function, including worse scores on low-contrast acuity and color-sensitivity testing, when compared with age-matched controls.[7
] It is well recognized that ON can lead to color-vision deficits, although the exact nature of this loss is unclear. It is consistent with an observation of Köllner that optic nerve disease leads to a preferential loss of red-green color vision.[23
] Some reports have found this type of defect to be predominant in ON.[24
] Others suggested that, blue-yellow defects is a more common color blindness type, or both type of color deficits are equally affected.[26
] Travis and Thompson used the red-green (Rayleigh equation) and green-blue (Engelkin-Trendelenburg equation) Pickford-Nicolson anomaloscope in 18 patients with presumed ON and found mixed patterns of loss.[27
] A review of findings from the optic neuritis treatment trial also found variable loss of color vision, although selective blue-yellow loss was more common in an acute phase; red-green loss was more common after 6 months.[28
But, in our study, we found no relationship between ON and color blindness, and it is maybe due to our limited sample size or because only red-green color deficit was measured with Ishihara test in this study. In this study, no unusual or unexpected safety risks were found with VEP and Ishihara plate tests, and patients’ compliance within the intervention was good.