Dominic Stone
Variations of extraocular muscles are rare, but a double-bellied superior rectus muscle has been observed in an adult male cadaver aged 70 years. The superior rectus muscle had two equal-sized bellies, which took separate origins from the common tendinous ring and united to form a common belly 1 cm before the insertion.
| Characteristics | Values |
|---|---|
| Name | Double-bellied superior rectus muscle |
| Muscle type | Extraocular |
| Number of bellies | Two |
| Belly size | Equal |
| Origin | Common tendinous ring |
| Insertion | 1 cm before insertion, the two bellies united to form a common belly |
| Nerve supply | Oculomotor nerve |
| Age | Observed in a 70-year-old adult male cadaver |
| Sex | Male |
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What You'll Learn
Double-bellied superior rectus muscle
Congenital variations of extraocular muscles are rare. One such variation is the double-bellied superior rectus muscle, which has been observed in an adult male cadaver aged 70 years. The superior rectus muscle had two equal-sized bellies, which took separate origins from the common tendinous ring and united to form a common belly 1 cm before the insertion. Due to the duplication, the muscle extended laterally beyond the levator palpebrae superioris. Both its bellies were supplied by the oculomotor nerve. To the best of the researchers' knowledge, this is the first report on the doubling of the belly of the superior rectus muscle.
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Multipennate muscles
The deltoid muscle of the shoulder is a common example of a multipennate muscle. It covers the shoulder but has a single tendon that inserts on the deltoid tuberosity of the humerus.
Congenital variations of extraocular muscles are rare, but one example of a double-bellied muscle is the superior rectus muscle, observed in an adult male cadaver aged 70 years.
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Congenital variations of extraocular muscles
Congenital fibrosis of the extraocular muscles (CFEOM) is a group of genetically defined eye movement disorders. The syndrome is clinically characterised by congenital non-progressive ophthalmoplegia caused by dysinervation of the cranial nerves with or without ptosis. As a main sign of CFEOM, extraocular muscles get shrunken and fibrotic, which makes surgery more technically demanding and the result more unpredictable, which makes the treatment challenging and highly customised. CFEOM1 is the most common form in the group, affecting 1 in 230,000 people. Inheritance is usually in an autosomal dominant manner, although de novo mutation and the disease resulting from germline mosaicism of one of the parents are also possible. The disorder affects some or all of the oculomotor nuclei and nerve itself, as well as the trochlear nucleus and its innervated muscle.
CFEOM is a rare congenital syndrome characterised by non-progressive unilateral or bilateral restrictive strabismus with or without ptosis. Various phenotypes of CFEOM have been described, which all result from fibrosis of the extraocular muscles causing restricted ocular motility, especially restricted ocular elevation. Patients classically present with a chin-up head position to compensate for poor ocular elevation and ptosis, although presentations may vary. At least seven causative genes and one locus are responsible for the five subtypes, named CFEOM-1 to CFEOM-5. The following genes have been implicated in CFEOM: COL25A1, KIF21A, PHOX2A, TUBA1A, TUBB2B, TUBB3. These mutations are most commonly inherited in an autosomal dominant or autosomal recessive fashion, although sporadic mutations can also occur.
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Muscles with curved fibre bundles
The passive elastic modulus of muscle fibre is size-dependent. In a study, muscle fibres and fibre bundles were extracted from lumbar spine multifidus and longissimus of three cohorts of rats and humans.
A skeletal muscle contains multiple fascicles, which are bundles of muscle fibres. Each individual fibre and each muscle is surrounded by a type of connective tissue layer of fascia. Muscle fibres are formed from the fusion of developmental myoblasts in a process known as myogenesis, resulting in long multinucleated cells.
Congenital variations of extraocular muscles are rare. One example of a double-bellied superior rectus muscle was observed in an adult male cadaver aged 70 years. The superior rectus muscle had two equal-sized bellies, which took separate origins from the common tendinous ring and united to form a common belly 1 cm before the insertion.
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The belly of the superior rectus muscle
The superior rectus muscle is a muscle in the orbit. It is one of the extraocular muscles, which are rare. It originates from the annulus of Zinn and inserts into the anterosuperior surface of the eye. The muscle's primary function is elevation, although it also contributes to intorsion and adduction. It is associated with a number of medical conditions, and may be weak, paralysed, overreactive, or even congenitally absent in some people.
The superior rectus muscle has been observed to have two bellies in some cases. In one instance, an adult male cadaver aged 70 years was found to have a superior rectus muscle with two equal-sized bellies, which took separate origins from the common tendinous ring and united to form a common belly 1 cm before the insertion. Due to the duplication, the muscle extended laterally beyond the levator palpebrae superioris. Both its bellies were supplied by the oculomotor nerve. This is the first known report of the doubling of the belly of the superior rectus muscle.
The originating fibres of the superior rectus are located inferolaterally to the superior oblique and inferomedially to the levator palpebrae superioris muscle. The levator palpebrae superioris eventually takes a more medial course, covering the majority of the superior rectus’ superior surface. The most distal part of the muscle runs superficially to the tendon of the superior oblique muscle. The superior branch of the oculomotor nerve (CN III) courses over the proximal third of the muscle’s inferior surface before it pierces the muscle belly to provide innervation. The lateral and medial sides of the muscle are cushioned within the periorbital fat tissue.
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Frequently asked questions
A double-bellied superior rectus muscle is a rare congenital variation of an extraocular muscle.
Only one case has been reported, in an adult male cadaver aged 70 years.
The superior rectus muscle has two equal-sized bellies, which take separate origins from the common tendinous ring and unite to form a common belly 1 cm before the insertion.
Due to the duplication, the muscle extends laterally beyond the levator palpebrae superioris.
