Medial rectus anatomy in exotropia
Reviewed by Fiona Rowe

This study investigates the differences between extraocular muscles in patients with intermittent exotropia and normal controls regarding muscle fibre arrangement and diameter, and distribution and activation of satellite cells. Fifteen patients were recruited having medial rectus resection surgery. Resection sections were collected along with four medial rectus muscles from two brain-dead men without strabismus (as controls). Experiments on medial rectus muscles were at 3mm from the attachment site. Average ages were 17.33 ±13.05 years (exo) and 22 ±5.65 years (controls). Male to female ratio was 10:5 and 2:0. Exotropia angle was 36 ±16.83PD. Mean myofiber diameter on optical microscope examination was 60.21 ±1.48 (exo) and 52.27 ±0.74 (controls). The exo group was significantly thicker and diameter distribution was more irregular. Hematoxylin and eosin staining showed larger mean diameter and various distributions of diameters of medial rectus muscle fibres in exo. Controls had minimal variation in diameter. PAX7 (+) and PCNA (+) cells were identified by fluorescence microscopy. The ratio of PAX7 cells per number of muscle fibres was 0.016 ±0.014 (exo) and 0.056 ±0.015 (controls). The ratio for PCNA cells per number of muscle fibres was 0.015 ±0.017 (exo) and 0.182 ±0.102 (controls). Ratios were significantly higher in controls. There were significant differences in the medial rectus muscles of exos versus non-strabismic controls. The authors recommend further study to investigate pathogenesis of exotropia.

Increased myofiber size and reduced satellite cell numbers in medial rectus muscle of patients with intermittent exotropia.
Kim CZ, Lee SJ.
STRABISMUS
2020;29(2):201-7.