Does Surgical Resection of Horizontal Extraocular Muscles Disrupt Ocular Proprioceptors?

Abstract

Purpose: It has been promoted that disturbance of ocular proprioception may play a role in the pathogenesis of concomitant strabismus and other types of oculomotor anomalies. The aim of the study was to obtain knowledge about how surgical foreshortening of the myotendinous region potentially affects the proprioceptors that resides in this area of the muscles and to test the hypothesis that avoiding disruption of ocular proprioceptors result in a more favorable long term postoperative result.

Patients and methods: The distal end of the lateral and medial rectus muscles from patients with manifest concomitant strabismus with a deviation of ≥15 prism diopters (PD) were collected during strabismus surgery and processed for light microscopy by standard histochemical techniques. Histological analysis served to differentiate between the tissue samples containing pure tendon, versus samples containing the myotendinous junction. Criteria for successful outcome was defined as a residual angle of deviation less than 10 PD. The binocular status of the patient was measured pre- and post-operatively at 6-months of follow-up.

Results: Tissue samples from 43 patients (median age 19 years old, range 3-58 years) were collected during surgery. Twenty-six of the samples contained pure tendon, while 17 contained muscle fibres. The evolution of the post-operative result revealed a moderate reduction in the residual angle of deviation in patient-samples containing pure tendon. In contrast, the residual angle of deviation clearly increased in patient-samples containing muscle fibres. The difference between the two groups reached statistical significance after 6 months. Successful outcome was found to be more than three times more likely in cases where surgery was performed in pure tendon, compared to muscle fibres.

Conclusion: The current study supports the hypothesis that avoiding disruption of ocular proprioceptors, located in the distal myotendinous region, results in a more favorable postoperative result.

Keywords: proprioception; strabismus; surgical outcome.

Figure 1

Micrograph showing a transverse section of tendon obtained from a temporal rectus muscle, displaying an small artery enveloped by collagen and fibrocytes. No evidence of muscle fibres. Section stained with Toluidine blue.

Figure 2

Micrograph showing a transverse section of human extraocular muscles displaying the diversity in the fibre population. The light staining muscle fibres have large diameters and well-developed sarcoplasmic reticulum. The darker staining muscle fibres have smaller diameters and scares amounts of sarcoplasmic reticulum. Section stained with Toluidine blue.

Figure 3

Micrograph showing a transverse section of human extraocular muscles displaying the preterminal distal end of a small dark staining muscle fibre where contractile elements are encapsulated by a few surrounding sheets of connective tissue (arrow). Section stained with Toluidine blue.

Figure 4

Micrograph showing a transverse section of human extraocular muscles displaying a muscle spindle with 5 intrafusal fibres encapsulated by a capsule of perineural tissue. Stained with Toluidine blue.

Figure 5

Angle of deviation evolution at 6 month follow-up.

Figure 6

Preoperative (left) and postoperative (6 month follow-up) (right) binocular vision status according to muscle morphology.

Figure 7

The associations between type of deviation (esotropia/exotropia) and surgical interference with the myotendinous region.

Figure 8

Muscle resection amount in relation to muscle morphology.