LATERAL EXTENSIONS OF THE MULLER MUSCLE

Objectives: To define the lateral extension of the Muller muscle and t o elucidate its involvement in the development and surgical treatment of Graves (thyroid) eye disease. Methods: Twelve lateral halves of orb its exenterated from patients with medial or posterior orbital neoplas ms were fixed, embedded, and step sectioned at 250-mu m intervals to p roduce histological sections. Gross anatomical dissections of human ca daver heads were also used to corroborate the histological findings. R esults: Histological analysis revealed that the Muller muscle extended laterally between the orbital and palpebral lobes of the lacrimal gla nd in all specimens. The smooth muscle fibers were found to interdigit ate with lacrimal ducts passing from the orbital to palpebral lobe and to extend close to the ductal orifices at the conjunctival surface. G ross dissections confirmed that the Muller muscle accompanied the leva tor aponeurosis lateral extension, which is known to pass between the orbital and palpebral lobes of the lacrimal gland. Conclusions: Our an atomical findings suggest that the Muller muscle may contribute to the temporal flare frequently seen in eyelid retraction associated with t hyroid eye disease. They may also explain the difficulty of treating l ateral eyelid retraction in thyroid eye disease and indicate the need for new surgical approaches for severe lateral eyelid retraction.