Once patients go from early to intermediate AMD, this study found that dark adaptation seems to significantly worsen. Photo: Carolyn Majcher, OD, Elaine Petry, OD, and Sweta Das, OD. Click image to enlarge.

Classifying age-related macular degeneration (AMD) can be especially difficult in earlier stages of the disease before visual acuity becomes a reliable differentiator. Alternatively, other measures of visual function, such as rod-mediated dark adaptation, may be able to more accurately detect changes between various stages of AMD. A recent cross-sectional study found that this measurement was particularly useful in identifying patients with intermediate AMD, although it couldn’t adequately distinguish healthy patients from those with early AMD.

Using OCT, the team of researchers assessed differences in rod-mediated dark adaptation in a cohort of 459 participants with varying degrees of AMD severity (median age: 65). The eye with the worst visual acuity in each subject was observed. Beckman classification was used to stratify participants based on the stage of AMD. The cohort was also divided based on OCT grade using the following scale: OCT0: no drusen or retinal pigment epithelium (RPE) abnormalities, OCT1: drusen but no RPE abnormalities and OCT2: drusen and RPE abnormalities. The presence of subretinal drusenoid deposits was also assessed.

Rod-mediated dark adaptation for each Beckman classification group in the study is shown in the table below. As the data clearly illustrates, intermediate AMD (3) is the only disease stage demonstrating a distinct change in adaptation time compared with classification groups 0-2. This finding remained after researchers corrected for age.

Similarly, eyes in the OCT2 group had significantly worse rod-mediated dark adaptation than eyes in the OCT0 or OCT1 groups after age correction.

“Median rod-mediated dark adaptation appeared different for the three groups in the OCT classification being 5.8, 8.4 and 11.06 minutes, respectively,” the researchers pointed out in their paper on the study. “These summary statistics suggest differences in rod-mediated dark adaptation are more discernible between different grades of AMD severity when an OCT-based criterion is used as compared with the Beckman classification.”

The researchers identified subretinal drusenoid deposits on OCT in slightly less than a quarter of participants (109 of 459 eyes). They found that the presence of these deposits was associated with worse rod-mediated dark adaptation in the OCT2 group but not in the OCT1 group. “We can infer from our data that subretinal drusenoid deposit presence is associated with greater rod dysfunction in people with more severe AMD, regardless of age-effect,” the researchers noted.

In conclusion, although rod-mediated dark adaptation may not be as reliable for distinguishing early AMD or age-related changes, it may be used to classify patients with intermediate AMD. Additionally, an OCT-based criterion appeared to be more effective in distinguishing intermediate disease.

Table 1. Rod-mediated Dark Adaptation in AMD Patients by Beckman Classification Group