Developmental adaptations in the extraocular muscles ofMacaca nemestrinamay reflect a predisposition to strabismus

Incomitant strabismus: does extraocular muscle form denote function?

The paradigm that an underacting extraocular muscle (EOM) is always atrophic or hypoplastic, and an overacting EOM should always be enlarged, leads to inconsistencies with clinical observations. It is inconsistent with the findings of normal extraocular muscle diameters in patients with apparent superior oblique muscle palsy, "overacting" inferior oblique muscles, and the superior rectus muscle overaction / contracture syndrome, among other clinical entities. These inconsistencies can be reconciled if one accepts the possibility that EOM contractile activity may reflect a change in neural input to an anatomically normal muscle, and / or that muscle contractile activity may be altered by a shift in fiber type and distribution within a normal-sized muscle. This remodeling may occur as a result of vergence adaptation or any change in neural stimulus to the muscle. There is substantial evidence to suggest that both these theoretical possibilities may likely occur.

Biological organization of the extraocular muscles

Extraocular muscle is fundamentally distinct from other skeletal muscles. Here, we review the biological organization of the extraocular muscles with the intent of understanding this novel muscle group in the context of oculomotor system function. The specific objectives of this review are threefold. The first objective is to understand the anatomic arrangement of the extraocular muscles and their compartmental or layered organization in the context of a new concept of orbital mechanics, the active pulley hypothesis. The second objective is to present an integrated view of the morphologic, cellular, and molecular differences between extraocular and the more traditional skeletal muscles. The third objective is to relate recent data from functional and molecular biology studies to the established extraocular muscle fiber types. Developmental mechanisms that may be responsible for the divergence of the eye muscles from a skeletal muscle prototype also are considered. Taken together, a multidisciplinary understanding of extraocular muscle biology in health and disease provides insights into oculomotor system function and malfunction. Moreover, because the eye muscles are selectively involved or spared in a variety of neuromuscular diseases, knowledge of their biology may improve current pathogenic models of and treatments for devastating systemic diseases.

The orbital pulley system: a revolution in concepts of orbital anatomy

Magnetic resonance imaging (MRI) now enables precise visualization of the mechanical state of the living human orbit. Resulting insights have motivated histological re-examination of human and simian orbits, providing abundant consistent evidence for the active pulley hypothesis, a re-formulation of ocular motor physiology. Each extraocular muscle (EOM) consists of a global layer (GL) contiguous with the tendon and inserting on the eyeball, and a similar-sized orbital layer (OL) inserting on a connective tissue ring forming the EOM pulley. The pulley controls the EOM path and serves as the EOM's functional origin. Activity of the OL positions the pulley along each rectus EOM to assure that its pulling direction shifts by half the change in ocular orientation, the half-angle behavior characteristic of a linear ocular motor plant. Half-angle behavior is equivalent to Listing's law of ocular torsion, and makes 3-D ocular rotations effectively commutative. Pulleys are configured to maintain oblique EOM paths orthogonal to half-angle behavior, and violate Listing's law during the vestibulo-ocular reflex. Rectus pulley positions shift during convergence, facilitating stereopsis. Innervations, fiber types, and metabolism of the OL and GL differ, consistent with the elastic loading of the former, and viscous loading of the latter. Disorders of the location and stability of rectus pulleys are associated with predictable patterns of incomitant strabismus that may mimic cranial nerve palsies. Surgical interventions improve defective pulley function. Understanding of ocular motor control requires characterization of the behavior of the EOM pulleys as well as knowledge of angular eye orientation.

Visual system maldevelopment disrupts extraocular muscle-specific myosin expression

The genetic and epigenetic influences that are responsible for the establishment and maintenance of the unique phenotype of the extraocular muscles (EOMs) are poorly understood. A role for visual cues in shaping EOM maturation was assessed in rats by using two visual deprivation paradigms, dark rearing and monocular deprivation. Isoforms of the contractile protein myosin heavy chain (MHC) were used as an index of phenotypic change in developing and adult EOMs after these visual insults. In rats that were dark reared during the visual critical period, the proportion of EOM fibers expressing either fast or slow MHCs was decreased significantly. EOM-specific myosin was also sensitive to dark rearing during the critical period, as evidenced by a significant decrease in its mRNA in EOMs from these rats. EOM-specific MHC did not change in either dark-reared rats returned to normally illuminated conditions or in adult rats denied visual experience for a similar time period. These data suggest that there may be a critical period during development when alterations in visual activity have significant consequences for the eye muscle phenotype. In contrast to dark rearing, monocular deprivation had a minimal effect on expression of the typical myosin isoforms and no effect on EOM-specific myosin expression. Collectively, these data confirm the hypothesis that visual input to the oculomotor system during development modulates EOM-specific MHC expression.

The oculomotor periphery: the clinician's focus is no longer a basic science stepchild

The study of the oculomotor periphery, the extraocular muscles and their orbital attachments, is undergoing a rapid expansion. This is an important progression for both basic and clinical communities as, for too long, the ophthalmologist has worked primarily in the periphery and the basic researcher has been occupied with study of the central components of the oculomotor system. From recent studies, it is clear that the morphology, cell and molecular biology, and genetics of the eye muscles and their corresponding motoneuron pools, and muscle attachments within the orbit are more complex than has heretofore been appreciated.

Extraocular muscles: basic and clinical aspects of structure and function

Although extraocular muscle is perhaps the least understood component of the oculomotor system, these muscles represent the most common site of surgical intervention in the treatment of strabismus and other ocular motility disorders. This review synthesizes information derived from both basic and clinical studies in order to develop a better understanding of how these muscles may respond to surgical or pharmacological interventions and in disease states. In addition, a detailed knowledge of the structural and functional properties of extraocular muscle, that would allow some degree of prediction of the adaptive responses of these muscles, is vital as a basis to guide the development of new treatments for eye movement disorders.