Accessory lateral rectus orbital geometry in normal and naturally strabismic monkeys

Area of Science:

• Ophthalmology
• Comparative Anatomy
• Primate Anatomy

Background:

• Esodeviations, or inward turning of the eyes, are common in humans.
• The role of specific orbital muscles in preventing such deviations is not fully understood.
• The accessory lateral rectus muscle is present in some primates but absent in humans.

Purpose of the Study:

• To investigate the anatomical and functional characteristics of the accessory lateral rectus muscle in macaque monkeys.
• To determine the potential role of this muscle in maintaining ocular alignment and preventing esodeviations.
• To explore the evolutionary implications of the presence or absence of this muscle in relation to esodeviation prevalence.

Main Methods:

• Anatomic dissections of macaque monkey orbits.
• Quantitative assessment of the orbital geometry and muscle attachments.
• Comparative analysis of orbits from normally aligned and esotropic monkeys.

Main Results:

• The accessory lateral rectus muscle was identified and its geometry analyzed.
• Functional assessment indicates the muscle produces globe elevation and abduction.
• The abducting action could counteract nasal drifts, supporting its role in preventing esodeviations.
• Two naturally esotropic monkeys lacked or had an underdeveloped accessory lateral rectus muscle.

Conclusions:

• The accessory lateral rectus muscle likely plays a role in preventing esodeviations in monkeys.
• The absence or underdevelopment of this muscle in esotropic monkeys supports its functional significance.
• The evolutionary loss of this muscle system in humans may contribute to the high prevalence of esodeviations.