The double-bellied inferior oblique muscle: clinical correlates

We previously reported an 8% incidence of double-bellied inferior oblique (IO) muscles at the surgical capture site (10-12 mm from insertion) in cadaveric specimens. This companion study sought to determine how often this anomaly is encountered at surgery for clinically overacting IO muscles and whether clinical findings or surgical outcomes in cases with double-bellied muscles differ from those with single-bellied muscles.

For 7 years we collected preoperative, intraoperative, and postoperative data on all patients for whom one surgeon performed primary IO weakening operations for overactions. We compared eyes with double-bellied IO muscles to those with single-bellied muscles on 4 variables--gradings of preoperative IO and superior oblique (SO) actions, presence of fundus excyclotropia, differences between horizontal deviations in upgaze and downgaze, and presence and sizes of primary position hypertropias--to determine whether one or more of them could predict the presence of a double-bellied muscle. Finally, we assessed postoperative IO actions to determine whether the presence of a double-bellied muscle influenced the effectiveness of IO weakening surgery in reducing overaction.

Among 162 patients (247 eyes) who underwent this surgery, 77 (77 eyes) had unilateral surgery and 85 (170 eyes) bilateral. Twenty-seven (10.9%) of the 247 muscles had double bellies. Among all variables compared, only the incidence of fundus excyclotropia differed significantly between groups, occurring more often in eyes with double-bellied IO muscles (48% vs 27%; P =.041). The efficacy of weakening surgery in reducing overactions was similar in both groups.

The finding that eyes with double-bellied IO muscles showed a higher incidence of fundus excyclotropia suggests that the presence of a second belly may alter the physiologic action of the IO muscle.