Barrett's index is used to assess for dysthyroid optic neuropathy, a severe complication of thyroid-associated orbitopathy that can lead to permanent blindness 1.
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Measurement
Measurement is calculated on coronal CT or MRI imaging of the orbits at a point halfway between the posterior globe and the orbital apex. Two measurements are taken for each eye:
-
horizontal index
(width of medial rectus + width of lateral rectus) / orbital width through optic nerve x 100%
-
vertical index
(height of superior rectus + height of inferior rectus) / orbital height through optic nerve x 100%
The larger of these two indices represents the more significant impingement and is the value taken as Barrett's index 2.
Interpretation
A Barrett's index of ≥60% is highly sensitive and specific for dysthyroid optic neuropathy 3. A Barrett's index of <50% almost always excludes dysthyroid optic neuropathy 1.
History and etymology
This method was first described by American radiologist Lynn Barrett in 1988 2.
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