Muscle Fiber Hypertrophy Following Monocular Medial Rectus Recession and Lateral Rectus Resection in Rats

Lee, Kyung Moo; Park, Jong Mun; Min, Byung Moo

Abstract

Compensatory hypertrophy of muscle fibers has been known to occur when burden on a skeletal muscle is increased. However, there have not been reports concerning the response of extraocular muscles following a monocular resection and recession of horizontal muscles, which procedure is a common form of strabismus surgery. A large lateral rectus resection and medial rectus recession was performed in each right eye of 28 rats. After the horizontal muscle surgery, both medial and lateral rectus muscles were obtained from both eyes of each rat with an interval of 2 weeks from the initial operation until 8th week after operation. A mid-portion of an extirpated muscles were frozen and prepared for ATPase staining. With a differential preincubation in two buffered solutions, pH 4.6 and pH 10.3 and ATPase staining, muscle fibers were classified into 3 kinds-global singly innervated fiber(GSIF, alkali-stable), global multi-innervated fiber(GMIF, alkali-labile), and orbital surface layer fiber(OSLF). These muscle fibers were measured using micrometer to find out their diameters and compared with the diameter of extraocular muscle fibers of unoperated rats. All kinds of muscle fibers of a resected lateral rectus showed an increase in their diameteris(p<0.05, Wilcoxon rank sum test) but there was no hypertrophy in the recessed medial rectus the yolk muscle of the resected lateral rectus, medial rectus of the fellow eye also showed hypertrophy of GMIF and OSLF. The lateral rectus of the unoperated fellow eye did not reveal muscle fiber hypertrophy. The postoperative hypertrophy of muscle fibers were observed only at the 2nd and 4th postoperative week specimens. the hypertrophied muscles subsequently returned to the baseline fiber diameter.