in partial fulfil]ment of the - MSpace - University of Manitoba

Classical Dark Àdaptation Test has also been shown to be useful for the
diagnoses of sub-clinical Vitamin A deficiency. The rod-cone breakpoint
is a transition point at which onset of rod-vision takes place during
dark adaptation. Norma11y, the rod-cone breakpoint is observed in about
4 minutes after the start of dark adaptation. Both the final darkadapted
threshold and rod-cone breakpoint have been shown to be related
to Vitamin A status (Vinton and Russelt, 198'1). in Vitamin À deficiency,
the time taken to observe the rod-cone breakpoint becomes prolonged
indicating delayed onset of rod-vision; the final dark adapted
threshold of retinal sensitivity is elevated. This is because, Vitamin
À is involved in the regeneration of rhodopsin. Rhodopsin is the photosensitive
chemical of the rods and is responsible for rod vision (dark
adaptation). in the presence of Vitamin À, '11-cis-retinene and alltrans
retinene are formed. Rhodopsin is then regenerated from'11-cis
retinene' and 'alL-trans' retinene. Because of this interrelationship
of Vitamin À and rhodopsin, dark adaptation ability decreases in Vitamin
À deficiency.
64
The Classical Dark Àdaptation Test
ble and valid as a functional measure
15
of
said to be reliable, reproduciearly
hypovitaminosis À (Vinton
and Russell, 1981; Carney and Russell, '1980; Van Graan et al., 1975;
Russell et al" 1973). However, this dark adaptation test is expensive,
tedious, impractical and not easily available making it unsuitable for
use, especially under field conditions (Vinton and Russell, 1981;
Thornton, 1977). The testing device and procedure may not be suited for
young children since an attentive and cooperative subject is required.
The subject is kept in Lhe dark for at least 35 minutes and has to con-