Electroretinography in patients with winter seasonal affective disorder
Introduction
Winter seasonal affective disorder (SAD) is a subtype of depressive disorder characterized by recurrent major depressive episodes that occur in autumn/winter, with full remission of symptoms in spring/summer (Rosenthal et al., 1984). The pathophysiology of winter SAD is unknown, but ocular light sensitivity hypotheses have been the focus of several investigations in seasonal affective disorder (SAD) with mixed results. The link between SAD and the retina arose from the fact that the effect of light therapy appears to be mediated through the eyes (Wehr et al., 1987). In 1990, two contrasting hypotheses were proposed to account for the role of the retina in SAD (Beersma, 1990, Remé et al., 1990). Beersma (1990) proposed SAD patients have retinal hypersensitivity to light. This would lead to perception of evening room light as an extension of the photoperiod yielding a depressogenic phase delay of circadian rhythms sometimes reported in SAD (Lewy et al., 1987). Therefore, the efficacy of morning light therapy results from a corrective realignment of circadian rhythms through resynchronisation of the biological clock with the natural light/dark cycle. In contrast, Remé and colleagues (1990) proposed that SAD patients have retinal hyposensitivity to light. This hypothesis is based on the proposition that in the normal population, the wintertime decrease of natural light exposure may lead to a compensatory mechanism of increased retinal light sensitivity, an ocular physiological adaptation termed photostasis that has been observed in some animal studies (Penn and Williams, 1986, Parker and Williams, 1995, Schremser and Williams, 1995a, Schremser and Williams, 1995b). They argued that if this process were absent or weakened in patients with SAD, they would be hyposensitive to light compared with normal subjects. Consequently, they would not be able to absorb enough light to maintain a euthymic state, explaining the need for light therapy.
There is support for both retinal hypersensitivity and hyposensitivity hypotheses, but the supporting data are highly dependent on the techniques used to measure light sensitivity. Studies using the dark adaptation threshold test (DAT) found no difference in the rod photoreceptor threshold obtained in patients with SAD and normal controls, either in summer or winter (Oren et al., 1993, Terman and Terman, 1999), but cone photoreceptor sensitivity was higher in both winter and summer in SAD patients than in controls (Terman and Terman, 1999). The latter result was interpreted as supportive of the hypersensitivity hypothesis in SAD, but for the cone photoreceptors only.
In contrast, using the electrooculogram (EOG) technique, EOG ratios have been found to be low in SAD patients relative to controls in winter (Lam et al., 1991, Ozaki et al., 1993), supporting the hyposensitivity hypothesis. Normal subjects were found to have increased EOG ratios in winter compared with summer, whereas no changes were observed in SAD patients (Ozaki et al., 1995). Interestingly, no change in the EOG ratio was observed after 1 week of light therapy even when light therapy was clinically effective. Also, supporting the hyposensitivity hypothesis were findings with the full flash electroretinogram (ERG) technique. Women with SAD compared with matched controls, but not men, had lower b-wave amplitudes (Lam et al., 1992). Because the light stimulus used in that study triggered a mixed rod-cone response, it was not possible to clearly specify the origin of the change. However, a small-sample, mixed-sex study using the pattern ERG (PERG), which elicits an electrical response from the retina (mostly the macular region), did not find any differences between SAD patients and controls (Oren et al., 1993). Recently, a more specific ERG technique in which various dim blue-green light intensities generate a detailed scotopic luminance response function (Hébert et al., 1996) was used to investigate seasonal change in rod sensitivity in subsyndromal affective disorder (S-SAD), that is, people who experience a non-clinical, milder form of winter depression (Kasper et al., 1989). S-SAD subjects showed a wintertime decrease in rod sensitivity, but normal subjects had no seasonal change (Hébert et al., 2002).
In the present study, we used flash ERG to assess retinal sensitivity in winter in a large group of well-diagnosed SAD patients and normal control subjects. In this study, patients and controls were tested in winter, and a short version of the rod luminance-response function was obtained with only four intensities. Retinal sensitivity was defined as the intensity necessary to reach a fixed b-wave amplitude criterion of 50 μV. We hypothesized that if the SAD patients were indeed significantly less sensitive to light than the controls, the intensity of light needed to reach the criterion during a depressive episode would be higher than in the control subjects.
Section snippets
Subjects
Twenty-seven depressed SAD patients who had been psychotropic drug-free for at least 5 weeks and neuroleptic drug-free for at least 6 months participated in this study. Inclusion criteria included a DSM-III-R diagnosis, supported by a Structured Clinical Interview for DSM-III-R (SCID), of major depressive disorder, recurrent, with a seasonal (winter) pattern (American Psychiatric Association, 1987), a score of 15 or greater on the 21-item Hamilton Rating Scale for Depression (Ham-D) (Hamilton,
Results
Table 1 presents the demographic data for the 27 SAD patients and the 23 control subjects. There was no statistical difference in age between the two groups (unpaired t-test, P>0.05). However, six (26%) of the normal controls were 50 years of age or older compared with only one (4%) in the SAD group. Because some opacification of the lens due to yellowing of the nucleus occurs with the normal aging process (Fishman, 2001), we performed a one-tailed Spearman correlation analysis to assess its
Discussion
This study provides further evidence of a relationship between SAD and retinal sensitivity. Approximately 40% of the SAD patients could be distinguished by rod sensitivity that was 1 S.D. below the mean value of the normal controls. If we assume that the wintertime hyposensitivity of 0.21 log units would ‘normalize’ in the summer, our data appear consistent with a seasonal variation of 0.18 log units reported in S-SAD with the ERG (Hébert et al., 2002). Consistent with the previous report of
Acknowledgements
This study was funded by a grant from the Medical Research Council of Canada (to RWL).
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