Previous reports have indicated that cortical cataract commences in the low
er nasal lens, possibly due to sunlight exposure because of the shallow bro
w temporally. The present study aimed to assess the lens sector distributio
n of cortical cataract in a population. The Blue Mountains Eye Study assess
ed 3654 residents aged 49-97years; 3270 phakic subjects had retroilluminati
on lens photographs graded using the Wisconsin method, which divides the le
ns into eight radial sectors with a grid overlay. Graders estimated percent
age of cortical opacity in each sector. The lower nasal area consistently h
ad the greatest opacity, a pattern present at each age and similar in both
sexes, despite the 20% greater cortical cataract in women. The mean area of
lens cortex involved by opacities in the lower nasal hemisphere was four f
old greater than in the upper temporal hemisphere at each age. The lower na
sal distribution was highly symmetrical when both eyes were affected. When
> 20% of the lower nasal lens quadrant had cortical opacity, 88% of bilater
ally affected subjects had cortical cataract in the same quadrant of the fe
llow eye. The lower nasal distribution may indicate a role for sunlight in
the aetiology of cortical cataract, which could be considered when examinin
g other cortical cataract risk factors, such as diabetes, vascular disease
and hormonal factors in women.