Purpose: The goal of this study was to investigate the ultrastructural changes of tendon axonal profiles of medial recti in patients with intermittent exotropia at different ages. In addition, we compared the patterns of degeneration with those of secondary exotropia over time.
Methods: Thirteen patients, with different ages, with exotropia who had undergone surgery were included in this study and divided into two groups. Eight patients had intermittent or constant exotropia; their age ranged from 6 to 45 years and they had exotropia since childhood without amblyopia, these patients were assigned to group A. The other five patients with sensory exotropia ranged in age from 15 to 52 years; they did not have exotropia until a visual insult and had poor vision in one eye, these patients were assigned to group B. All patients had the medial recti resected (3-5.5 mm) to obtain tissue samples. All specimens were examined with an electron microscope.
Results: Schwann cell degeneration was observed with increased neurofilament density, axonal vacuoles and hydropic swelling of the Schwann cells in two patients less than 10 years of age in group A. The other six patients were more than 10 years of age in group A, and it was not possible to identify the tendon axonal profiles or neural structures in the medial recti specimens of these patients. For group B, all patients had intact proprioceptor structures including Schwann cells. However, the collagen diameter decreased and density increased within the capsule according to the duration of exotropia.
Conclusion: Schwann cell degeneration of tendon proprioceptors in the medial rectus might induce the degeneration of proprioceptors in patients with intermittent exotropia over time.