[Age-dependent changes in transverse sections of muscle fibres from the exterior eye muscles in man]

Muscle fibers of extraocular muscles (M. rectus internus, M. rectus externus and M. rectus inferior) from 27 patients ranging in age from birth to 83 years were measured planimetrically in cross sections between the origin-, insertion-, and middle-thirds of the muscle length. In all the muscles the middle third of the muscle possessed fibers with an average thickness greater than in the third of the muscle at either end (origin or insertion). This observation was, however, dependent on the age of the patients. This difference in muscle fiber thickness was most constant in patients in the age groups 20 to 30 and 40 to 50 years. On patients 60 years and older, the muscle fibers in all portions of the muscle were thicker; however, the M. rectus externus and especially the M. rectus internus also possessed a larger number of thinner fibers. The percent of thin fibers in the M. rectus internus was even greater than that found in one year-old patients. These observations are most likely explained by a decrease in the accomodation capacity in older patients and a correlative reduced convergence activity of the muscle fibers that are only active during convergence, resulting in decreased demand on the muscle fibers and their consequent atrophy.