Qualitative and quantitative analyses of Müller's muscle in dysthyroid ophthalmopathy

A portion of the Müller's muscle of the superior eyelid was obtained from 3 men and 6 women with dysthyroid ophthalmopathy, aged from 19 to 58 years, at the time of surgery to treat excessive lid retraction. Disturbance of ocular motility was seen in 7 out of the 9 cases. Electron microscopy of the excised portion of Müller's muscle showed that the muscles were classifiable into three types, 1) cells with normal appearance, 2) dark cells and 3) clear cells: the percentages of these cells averaged 35.7% (30.9-42.1%), 39.4% (24.8-56.3%) and 24.9% (12.8-40.0%), respectively. A quantitative analysis of the cell shape revealed that the dark cells were more irregular and the clear cells more rounded than the normal cells. The dark cells had a high electron density with a dense distribution of actin-like filaments, and the clear cells showed a low electron density with irregular cytoplasmic distribution. It was thought that the dark cells represented contractile changes and the clear cells represented degenerative changes. The dark cells appeared to decrease and the clear cells appeared to increase, as the exophthalmos increased. In 2 cases with no ocular motility disturbance, the percentage of the dark cells was higher and that of the clear cells lower than in the other cases with ocular motility disturbance. It is suggested that the contraction of the Müller's muscle contributes to lid retraction in dysthyroid ophthalmopathy and the muscles undergo degenerative changes as the ophthalmopathy progresses.