In principle, dark-adapted sensitivity to short-wavelength stimuli may be evaluated using commercially available perimetry devices.
15,16 However, in patients with both stable and unstable fixation, gaze-contingent testing (so-called microperimetry or fundus-controlled perimetry) is preferable to ensure accurate stimulus placement during and across examinations.
17,18 Detailed measurement of the visual fields can be performed with devices such as the macular integrity assessment (MAIA) (Centervue, Padova, Italy) but standard protocols are performed under mesopic conditions. Recently, the MAIA device has been updated and is now capable of performing scotopic testing of both rods and cone, (known as the scotopic MAIA [S-MAIA]).
18 For a clinical trial setting, using an existing device approved by the US Food and Drug Administration provides advantages regarding the acceptability of the results produced. The S-MAIA uses cyan stimuli (505 nm) and red stimuli (627 nm) with the difference calculated between them. The original version had a decreased decibel range of testing under scotopic testing, but this range has since been expanded to the full 36 dB in all conditions to remove any ceiling effects and allow lower intensity lights to be presented.
18 However, the decibel scale used for scotopic tests is not directly comparable with the mesopic tests because it uses the scotopic candela per square meter scale. Both the cyan and red stimuli are thought to be rod mediated in healthy observers outside of the central retina, which features the rod-free zone. The device is calibrated according to the scotopic luminosity function. Thus, the expected cyan–red sensitivity difference would be 0 dB for loci with normative rod function.
19 Because the dark-adapted cone sensitivity is close to the rod sensitivity for the red long wavelength stimulus, isolated rod dysfunction is characterized by a predominant loss of scotopic cyan sensitivity, resulting in negative cyan–red sensitivity differences.
4,20,21 The S-MAIA has been used in the investigation of Bruch's membrane diseases, such as age-related macular degeneration and pseudoxanthoma elasticum, and was shown to allow for the identification of loci with predominant rod dysfunction.
22–24 However, the device has not been evaluated in patients with RCDs to date.