Barrett's index

Last revised by Joshua Yap on 18 Aug 2021

Barrett's index (BI) is used to assess for dysthyroid optic neuropathy, a severe complication of thyroid-associated orbitopathy that can lead to permanent blindness 1.

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.

A BI of ≥60% is highly sensitive and specific for dysthyroid optic neuropathy 3. A BI of <50% almost always excludes dysthyroid optic neuropathy 1.

This method was first described by American radiologist Lynn Barrett in 1988 2.

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