Isolated lateral rectus muscle involvement as a presenting sign of euthyroid Graves disease

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To the Editor: We would like to share some thoughts on the case of an isolated enlarged lateral rectus muscle belly attributable to presumed euthyroid orbitopathy reported by Erdurmus and colleagues.¹ The authors made the diagnosis of thyroid associated orbitopathy (TAO) on the basis of history, physical examination, laboratory data, and radiography. However, the case described is atypical for TAO in several ways. Eyelid retraction, a finding present in most patients with TAO, was absent in this case. Rather, it appears that the patient had right blepharoptosis, as seen in the primary gaze clinical photograph of Figure 1.¹ In addition, the patient was euthyroid, a finding present in only 6% of patients with TAO.² The authors correctly mentioned that there has yet to be a published case of TAO affecting only the lateral rectus and sparing the other rectus muscles, emphasizing this rare presentation. Furthermore, the purely unilateral presentation would not be typical for TAO, which is usually a bilateral asymmetric disease. In addition, TAO more commonly affects female patients; the fact that their patient is male adds to the unusual nature of the case. Finally, TAO commonly manifests as either hypotropia or esotropia from fibrosis and scarring of the more commonly affected inferior or medial rectus muscles; the patient presented with exotropia, which would not be expected.

On the basis of the evidence against the diagnosis of TAO, we are curious about the investigative workup for other disease entities that could lead to similar clinical and radiographic presentations, such as sarcoidosis, tuberculosis, or idiopathic orbital inflammation. Of the possible diagnoses, idiopathic orbital inflammation seems most likely, given the unilateral presentation, isolated involvement of the lateral rectus muscle, and the patient’s brisk response to steroids as described in the report.

If an exhaustive investigative workup is negative and TAO is still considered to be the diagnosis, the patient should be evaluated for a concomitant diagnosis of myasthenia gravis. It is well known that patients with TAO have a greater incidence of myasthenia gravis because both entities represent autoimmune diseases. It is also well documented that patients with known TAO exhibiting exotropia and blepharoptosis, as in the described patient, may present with myasthenia gravis and should be evaluated for the disease.

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References 

1. Erdurmus M, Celebi S, Ozmen S, Bucak YY. Isolated lateral rectus muscle involvement as a presenting sign of euthyroid Graves disease. J AAPOS. 2011;15:395–397
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2. Bahn R. Graves’ Ophthalmopathy. N Eng J Med. 2010;362:726–738
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