Double-bellied medial rectus muscle in a patient with Down syndrome and congenital esotropia

Double-bellied inferior rectus muscles with aplasia of medial rectus muscles

This report describes a rare occurrence of double-bellied inferior rectus muscle with aplasia of the medial rectus muscle in both eyes of a young woman seeking cosmetic correction for her strabismus. The patient presented with a large-angle exotropia and a -4 limitation of adduction in each eye. On attempted adduction, there was minimal downshoot and upper eyelid retraction. Magnetic resonance imaging of the orbit revealed hypoplastic medial rectus muscles; however, intraoperatively the muscle was not found at the insertion or when traced back as far as possible. The inferior rectus muscles were noted to have double bellies converging close to each other at the insertion. The patient underwent a bilateral lateral rectus recession with nasal transposition of the inferior rectus muscles and was cosmetically well aligned, with a residual exotropia of 4Δ at 3 months' follow-up. This unusual anatomical variation may be the result of deviation of muscle paths.

Supernumerary Medial Rectus Muscle of the Orbit

Supernumerary extraocular muscles (EOMs) are relatively rare in humans compared to other species. Therefore, few cases are reported on pediatric patients with strabismus. 

We describe a case of an incidental intraoperative finding of a right eye accessory medial rectus (MR) muscle in a child with normal ocular motility. This supernumerary muscle was found inserted underneath the original MR muscle and was of a similar size. In this article, we discuss the anomaly and its clinical relevance.

Examination of the Annular Tendon (Annulus of Zinn) as a Common Origin of the Extraocular Rectus Muscles: 2. Embryological Basis of Extraocular Muscles Anomalies

Purpose

Many reports have described anomalous connections of the superior rectus (SR) with other extraocular rectus muscles, in which additional heads of the other three rectus muscles likely provided the connections. We examined how these connections are established during fetal development.

Methods

We analyzed paraffin-embedded horizontal sections from 25 late-stage fetuses. Horizontal sections are best suited for understanding the mediolateral relationships of muscle origins.

Results

We confirmed a common tendinous origin of the lateral rectus (LR), inferior rectus (IR) and medial rectus (MR) muscles that was separated from the SR origin. Notably, eight fetuses (32%) had tendinous or muscular connections between the SR and other rectus muscles that had one of four morphologies: (a) a thin tendon from the SR to the common tendon of the three rectus muscles (2 fetuses), (b) a thin tendon to the LR (one fetus), (c) a thin tendon to the inferior rectus muscle origin (two fetuses), and (d) SR muscle fibers arising from an additional head of the LR (three fetuses).

Conclusions

The SR seemed to issue a thin tendon that passed along the inferior or lateral side of the oculomotor nerve. Conversely, the LR and inferior rectus muscle were likely to carry a supernumerary bundle that reached the SR. The accessory head of the medial rectus muscle showed a stable morphology in that it seemed to also provide an anomalous double head. However, the presence of an accessory head in the LR was rare. In contrast with our previously published diagram of the orbital apex, the accessory head of the medial rectus muscle passed along the lateral side of the superior oblique.