How do the extraocular muscles contribute to concomitant strabismus? Do they play an active or passive role in the deviation? ESA Lecture 2015

The role played by the extraocular muscles (EOMs) in the pathogenesis of strabismus is not yet completely clarified. The commonest theory of the pathogenesis of strabismus supposes that most strabismus results from abnormal innervation of the extraocular muscles. This may be intuitive for paralytic strabismus and for congenital cranial dysinnervation syndromes, but less obvious for concomitant infantile strabismus where it has not yet definitively clarified if the primary cause of the misalignment is sensory or motor. In concomitant infantile strabismus the muscles themselves are usually structurally normal. The abnormal eye alignment is supposed to be caused by aberrant signals from the brain that cause excessive twitch of some muscles in a certain direction of gaze, such as the excessive adduction of the eyes in patients with infantile esotropia. Such abnormal innervation of the medial rectus muscles in infantile esotropia is thought to be due to the persistence or onset of the “esotonus” after the first weeks of life. Less commonly, strabismus results from anatomic abnormalities of the orbit and/or fascial sheaths and their extensions, the so called muscle pulleys. In such conditions, the muscles are usually normal, but act abnormally due to malposition of the fibroelastic sleeves or pulleys through which the muscles travel. However, the orbital structures that surround the globe are normal in the majority of patients with concomitant strabismus. If therefore these peripheral causes can well explain some forms of strabismus like alphabetic patterns or some clinical manifestations of concomitant infantile strabismus such as elevation or depression in adduction in “V” or “A” pattern esotropia or exotropia, the main cause of concomitant infantile strabismus can only be supposed to be central and whatever it is, having as final effect an abnormal innervation of the extraocular muscles. The interaction between sensory factors and innervational factors, i.e., esotonus, accommodation, convergence, divergence, and vestibular reflexes in visually immature infants with family predisposition is suspected to create conditions that prevent binocular alignment to stabilize and strengthen. The weight of each of these sensory and innervational factors in determining the deviation, probably changes depending on the various forms of concomitant infantile strabismus. Some role in the onset of fixation instability and infantile concomitant strabismus could be played by the feedback control of eye movements and by abnormalities in eye muscle proprioception during the critical period of development of the visual sensory system. A possible role in the onset, maintenance or worsening of the concomitant deviation of abnormalities of muscle force which have their clinical equivalent in eye muscle overaction and underaction has been investigated under either isometric or isotonic conditions, and in essence no significant anomalies of muscle force have been found in concomitant strabismus