Extraocular Muscles Tension, Tonus, and Proprioception in Infantile Strabismus: Role of the Oculomotor System in the Pathogenesis of Infantile Strabismus-Review of the Literature

Comitant strabismus etiology: extraocular muscle integrity and central nervous system involvement-a narrative review

Strabismus is not a condition in itself but the consequence of an underlying problem. Eye misalignment can be caused by disease, injury, and/or abnormalities in any of the structures and processes involved in visual perception and oculomotor control, from the extraocular muscles and their innervations to the oculomotor and visual processing areas in the brain. A small percentage of all strabismus cases are the consequence of well-described genetic syndromes, acquired insult, or disease affecting the extraocular muscles (EOMs) or their innervations. We will refer to them as strabismus of peripheral origin since their etiology lies in the peripheral nervous system. However, in most strabismus cases, that is comitant, non-restrictive, non-paralytic strabismus, the EOMs and their innervations function properly. These cases are not related to specific syndromes and their precise causes remain poorly understood. They are generally believed to be caused by deficits in the central neural pathways involved in visual perception and oculomotor control. Therefore, we will refer to them as central strabismus. The goal of this narrative review is to discuss the possible causes behind this particular type of eye misalignment and to raise awareness among eyecare professionals about the important role the central nervous system plays in strabismus etiology, and the subsequent implications regarding its treatment. A non-systematic search was conducted using PubMed, Medline, Cochrane, and Google Scholar databases with the keywords "origins," "causes," and "etiology" combined with "strabismus." A snowball approach was also used to find relevant references. In the following article, we will first describe EOM integrity in central strabismus; next, we will address numerous reasons that support the idea of central nervous system (CNS) involvement in the origin of the deviation, followed by listing several possible central causes of the ocular misalignment. Finally, we will discuss the implications CNS etiology has on strabismus treatment.

Proprioceptive contribution to oculomotor control in humans

Stretch receptors in the extraocular muscles (EOMs) inform the central nervous system about the rotation of one's own eyes in the orbits. Whereas fine control of the skeletal muscles hinges critically on proprioceptive feedback, the role of proprioception in oculomotor control remains unclear. Human behavioural studies provide evidence for EOM proprioception in oculomotor control, however, behavioural and electrophysiological studies in the macaque do not. Unlike macaques, humans possess numerous muscle spindles in their EOMs. To find out whether the human oculomotor nuclei respond to proprioceptive feedback we used functional magnetic resonance imaging (fMRI). With their eyes closed, participants placed their right index finger on the eyelid at the outer corner of the right eye. When prompted by a sound, they pushed the eyeball gently and briefly towards the nose. Control conditions separated out motor and tactile task components. The stretch of the right lateral rectus muscle was associated with activation of the left oculomotor nucleus and subthreshold activation of the left abducens nucleus. Because these nuclei control the horizontal movements of the left eye, we hypothesized that proprioceptive stimulation of the right EOM triggered left eye movement. To test this, we followed up with an eye-tracking experiment in complete darkness using the same behavioural task as in the fMRI study. The left eye moved actively in the direction of the passive displacement of the right eye, albeit with a smaller amplitude. Eye tracking corroborated neuroimaging findings to suggest a proprioceptive contribution to ocular alignment.

Clinical Manifestations, Diagnosis, and Surgery of Inferior Oblique Muscle Ectopia

Objectives

To summarize the clinical manifestations, diagnosis, treatment, and prognosis for inferior oblique muscle ectopia (IO-E). Subjects and Methods. Patients diagnosed with IO-E during strabismus surgery from March 2017 to September 2018 were included in this retrospective, cross-sectional study. All patients received preoperative Krimsky test, synoptophore, cycloplegia refraction, fundus torsion, and other strabismus-related specific tests. The anatomic variations of IO-E were always discovered during surgical procedure. Postoperative eye position and binocular visual function (BVF) were all reviewed in early days after operation.

Results

A total of 7 patients were enrolled in this study with an average age of 6.4 ± 3.8 yrs. They all presented with significant exotropia and unilateral (or bilateral) overelevation in adduction (OEA). No compensatory head position was detected. Some of them had vertical deviation, V pattern, or excyclotropia, which were indicated by fundus torsion. Monocular or binocular IO-E was distinguished during the surgery, and it could be classified into two types according to its anatomic features. In surgery, the ectopic IO muscle bundle was restored, and different IO weakening methods were employed. Meanwhile, the horizontal deviation was also corrected according to the preoperative examination. Eyes of all patients were properly aligned in the primary position after surgery. Varying degrees of BVF appeared in 3 cases.

Conclusions

IO-E is a rare congenital dysplasia variation of the extraocular muscle, which could appear as inferior oblique overaction. It is difficult to diagnose before surgery, and weakening the overactive ectopic inferior oblique was required for better prognosis if this condition was confirmed during surgery.