Diplopia in a Patient with Thyroid Eye Disease
A 45-year-old woman with Graves' disease presents with double vision, especially when looking up and to the right. She has exophthalmos, lid retraction, and conjunctival injection. Visual acuity is 6/6 in both eyes. Pupils are equal and reactive. She has no pain. CT scan shows enlarged extraocular muscles with sparing of the tendons. She is euthyroid on carbimazole.
1. Explain which extraocular muscles are most likely involved and how the pattern of enlargement explains her diplopia.
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Most likely involved muscles: The inferior and medial recti are most commonly enlarged in thyroid eye disease (TED), followed by the superior rectus/levator complex and lateral rectus. The tendons are spared.
Explanation of diplopia:
- Enlarged, fibrotic muscles become restricted, limiting movement in the direction opposite to the muscle's action.
- Inferior rectus enlargement restricts elevation, causing diplopia on upgaze.
- Medial rectus enlargement restricts abduction, causing diplopia on lateral gaze.
- Her diplopia on upgaze and right gaze fits inferior and medial rectus involvement.
Key point: The pattern of diplopia reflects restrictive myopathy, not a cranial nerve palsy, which is why the pupils are normal and there is no pain.
2. Outline a stepwise management plan for her diplopia, distinguishing active inflammatory disease from inactive fibrotic disease.
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Assessment: Determine disease activity (e.g., CAS score, pain, inflammation) and severity (compressive optic neuropathy, exposure keratopathy).
Active inflammatory phase:
- Conservative: artificial tears, selenium, smoking cessation, control thyroid function.
- Medical: systemic steroids for moderate-to-severe active disease; consider steroid-sparing agents (e.g., mycophenolate, rituximab) or teprotumumab in selected cases.
- Orbital radiotherapy may be used for active disease.
- Prisms for symptomatic diplopia; avoid surgery during active phase.
Inactive fibrotic phase:
- Strabismus surgery to correct restrictive deviations once disease is stable for at least 6 months.
- Orbital decompression if there is compressive optic neuropathy or cosmetic disfigurement.
- Eyelid surgery for retraction after strabismus/decompression.
Order of surgery: Decompression → strabismus → eyelid, because each can affect the others.