Measurements at the 4 axes of the optic nerve head (inferior, superior, nasal temporal) can help distinguish a normal eye from one affected by glaucoma.
Technique: photograph of the optic disc
Measurements are taken of the disc rim thickness (see Figure page 1580) along the:
(1) inferior axis (I)
(2) superior axis (S)
(3) nasal axis (N)
(4) temporal axis (T)
In the normal eye the measurements show the relationship I >= S >= N >= T.
In a glaucomatous eye this relationship is often not observed.
In patients with normal eyes the ISNT rule is met in 82% of whites and 76% of blacks.
In patients with glaucoma the ISNT rule is met in 36% of whites and 13% of blacks (Table 3, page 1581).
Additional findings suggesting glaucoma:
(1) presence of a visual field defect
(2) larger disc area
(3) increasing axial length
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