Extraocular Muscles in Congenital Strabismus: Muscle Fiber and Nerve Ending Ultrastructure according to Different Regions

Abstract

The ultrastructure of the extraocular muscles of patients affected by congenital strabismus is not completely known, and the structures responsible of the pathogenesis of this condition are still to be determined. Specimens obtained from patients suffering from congenital strabismus were studied and compared with specimens obtained from patients enucleated for various pathologies and not affected by any disorder in the oculomotor system. The scleral myotendinous junction, where the occurrence of an altered proprioceptive innervation was already reported, was examined, and findings obtained were compared with those observed in the muscle body (venter), where motor innervation is prominent and usually described as normal. Only a small number of damaged muscle fibers was found everywhere. The damage consisted in alterations of both contractile structures and mitochondria and resulted in severer lesions in the scleral myotendinous junction rather than in the muscle body. The normal muscle fibers were innervated by motor nerve endings with normal features and by few altered proprioceptors. The less damaged muscle fibers were innervated by normal motor nerve endings and severely damaged proprioceptors. The most severely damaged muscle fibers did not receive any type of innervation. These data seem to imply that the most important functional alteration in strabismus regards the scleral myotendinous junction. It is the authors’ opinion that these findings might have a clinical importance in choosing the treatment to be pursued in patients with a squint.

Innervated Myotendinous Cylinders Alterations in Human Extraocular Muscles in Patients With Strabismus

“…In contrast, ultrastructural data on the distal myotendinous junction of the EOMs in congenital strabismus indicates the presence of an altered proprioceptive innervation 7,20. This morphologic data supports the hypothesis that a disturbance of ocular proprioception, in the myotendinous junction, may play a role in the pathogenesis of oculomotor disorder.…”

Altered Protein Composition and Gene Expression in Strabismic Human Extraocular Muscles and Tendons

“…8,9,14–20 Unless specific molecules were probed, most abnormalities were subtle and to some extent also present in normal control tissues, or they could not be identified at all. However, failure to demonstrate alterations at the level of histology and even ultrastructure does not rule out the possibility of differences in molecular composition.…”

Congenital Nystagmus: Fine Structure of Human Extraocular Muscles

“…Since nystagmus associated with strabismus also shows alterations similar to those of strabismic EOM, we can pre sume that a deficient sensory innervation has been added to the muscle damage, similarly to that observed in congenital strabismus [4,5,24,25]. For this reason, it can be suggested that this disease might be dependent on both a myopathy and a neuropathy.…”

“…All other observed mitochondrial modifications (zig-zag and concentric cristae, swelling) are also severe and, there fore, lead us to believe, as in the case o f strabismus [5,7], that they could be the expression o f a generalized metabolic disease of the EOM fibers. Indeed, similar modifications have been described for hypermetabolic skeletal muscles [18,19] and for the so-called mitochondrial myopathies [20][21][22],…”

“…The functional significance of these abnormal structures, however, is not known, but it must not be underestimated. The rods, for example, which are constantly present in strabismic EOM, have been con sidered as a sign of muscle damage [5]. Rods [for a review, see 5] and subplasmalemmal or intramyofibrillar zebra bod ies also characterize skeletal muscle myopathies [23],…”