Extraocular Muscle Enlargement in Thyroid Eye Disease Using Volumetric Analysis

Area of Science:

• Ophthalmology
• Endocrinology
• Medical Imaging

Background:

• Thyroid eye disease (TED) is often associated with extraocular muscle (EOM) enlargement.
• Traditional teaching suggests a specific descending order of EOM enlargement in TED: IR, MR, SR, LR, SO, IO.
• Recent literature questions this established order of EOM involvement.

Purpose of the Study:

• To compare EOM volumes in a cohort of TED patients.
• To investigate the actual order of EOM enlargement in TED.
• To challenge or confirm the commonly taught sequence of muscle involvement.

Main Methods:

• Retrospective, non-randomized case-control study design.
• Comparison of 28 TED orbits with 31 normal orbits using high-resolution CT scans.
• Manual segmentation of orbital soft tissues and statistical comparison of volumes.

Main Results:

• Superior rectus/levator palpebrae superioris complex (SRC) and inferior rectus (IR) volumes were significantly enlarged in TED patients compared to controls.
• Mean SRC volume was 2.3 times larger, followed by IR (2.1 times), SO (1.8 times), MR (1.7 times), LR (1.6 times), IO (1.6 times), and orbital fat (1.4 times).
• Over 50% of TED orbits showed SRC or IR volumes exceeding two standard deviations above the normal average.

Conclusions:

• The findings suggest the SRC may be the most severely affected EOM in thyroid eye disease.
• This contradicts the previously accepted descending order of EOM enlargement in TED.
• The study highlights the need to re-evaluate the typical understanding of EOM involvement in TED.