The medial rectus is one of seven extraocular muscles. These include four rectus muscles, two oblique muscles, and the levator palpebrae superioris. The medial rectus is one of the four rectus muscles, which also include the lateral rectus, the superior rectus, and the inferior rectus. The oblique muscles are the superior and inferior obliques.
Structure and Function
The medial rectus is an adductor, and functions along with the lateral rectus which abducts the eye. These two muscles allow the eyes to move from side to side. With head facing straight and the eyes facing straight ahead, the eyes are said to be in primary gaze. From this position, an action from an extraocular muscle produces a secondary or tertiary action. Although the globe can be moved about 50 degrees from primary position, usually during normal eye movement only 15 degrees of extraocular muscle movement occurs before the head movement begins.
Each of the extraocular muscles has a functional insertion point, which is at the closest point where the muscle first contacts the globe. This point forms a tangential line from the globe to the muscle origin and is known as the arc of contact. The medial rectus has an arc of contact of 7 mm, whereas the lateral rectus has an arc of contact of 12 mm.
The mesenchyme of the head, including the orbit and its structures, arise primarily from two precursors, mesoderm, and neural crest cells. The extraocular muscles originate from mesoderm, but the satellite and connective tissue of the muscle arises from neural crest cells. Most of the remaining connective tissue of the orbit also is derived from neural crest cells.
Blood Supply and Lymphatics
The medial rectus receives blood through the inferior muscular branch of the ophthalmic artery. The primary blood supply for all of the extraocular muscles are the muscular branches of the ophthalmic artery. The two branches are the inferior muscular branch and the superior muscular branch. The larger inferior muscular branch supplies the medial rectus, inferior rectus, and inferior oblique. The superior muscular branch supplies the lateral rectus, superior rectus, superior oblique and levator palpebrae superioris. Additionally, the lateral rectus muscle receives some blood supply from the lacrimal artery.
Venous drainage is similar to the arterial system and empties into the superior and inferior orbital veins. Usually, there are a total of four vortex veins, and these are found at the lateral and medial sides of the superior and inferior rectus muscles.
The medial rectus is innervated by the lower division of cranial nerve III (oculomotor). Cranial nerve III is divided into upper and lower divisions, with the upper division innervating the superior rectus, and levator palpebrae superioris, and the lower division to the medial rectus, inferior rectus, and inferior oblique. The lateral rectus is innervated by cranial nerve IV (trochlear), and the superior oblique is innervated by cranial nerve VI (abducens).
The medial rectus and the lateral rectus make up the horizontal rectus muscles. The superior and inferior rectus muscles form the vertical rectus muscles. Each of the rectus muscles originates posteriorly at the Annulus of Zinn and courses anteriorly. The medial rectus courses along the medial orbital wall. Each of the rectus muscles inserts on the globe at varying distances from the limbus, and the curved line drawn along the insertion points makes a spiral that is known as the Spiral of Tillaux. Starting at the medial aspect of the globe, the medial rectus inserts at 5.5 mm from the limbus, the inferior rectus inserts at 6.5 mm from the limbus, the lateral rectus inserts at 6.9 mm from the limbus, and the superior rectus at 7.7 mm from the limbus.
The medial rectus is 10.3 mm wide at its insertion on the globe. The tendon is 3.7 mm, measured from the origin. The entire length of the muscle is 40.8 mm.
These muscles and their insertions have anatomical and surgical considerations which will be discussed further.
Extraocular muscles have a large ratio of nerve fibers to skeletal muscle fibers. The ratio is 1:3 to 1:5, compared to other skeletal muscles which is 1:50 to 1:125. Extraocular muscles are a specialized form of skeletal muscle with a variety of fiber types, including both slow tonic types which resist fatigue and also saccadic (rapid) type muscle fibers.
The size of the medial rectus muscle, as well as its insertion point on the globe from the limbus and other anatomical measurements, may vary widely from one individual to the next. The numbers described in this article reflect average distances.
Congenital differences in extraocular muscles can cause ocular misalignment. See the Clinical Significance section for more details regarding strabismus.
The nerves to rectus muscles and superior oblique muscles insert into the muscles at a one-third the distance from the origin to the insertion. This makes damage to these nerves during anterior segment surgery difficult, but not impossible. Additionally, instruments that are advanced 26 mm posterior to the rectus muscle insertions can cause injury to the nerve.
Blood vessels may be compromised during surgery of the medial rectus muscle. The vessels which supply blood to the extraocular muscles also supply nearly all the temporal half of the anterior segment of the eye. The majority of the nasal half of the anterior segment circulation is also derived from blood vessels that supply the extraocular muscles. Therefore, care must be taken during surgery of the medial rectus or other extraocular muscles to avoid disrupting this blood supply.
There are fascial layers to consider in the orbit that deal with the medial rectus.
Tenon's capsule is elastic connective tissue that attaches to the optic nerve posteriorly and is pierced by all of the extraocular muscles except the levator palpebrae superioris. When surgery is performed, care must be taken not to compromise Tenon's capsule 10 mm posterior to the limbus. Otherwise, adipose tissue may prolapse and cause adhesions which could limit extraocular movement.
There are other complications which may result from medial rectus surgery, which also may result from other rectus muscle surgery. Unsatisfactory alignment is the most common complication and may require additional surgery to correct this. Refractive changes may occur when two rectus muscles of one eye are operated, and this may resolve over months. Other possible surgical complications include diplopia, perforation of the sclera and postoperative infections. Although uncommon, serious infections may result after strabismus surgery, including pre-septal or orbital cellulitis and endophthalmitis.
The function of the medial rectus muscle can be assessed along with the other extraocular muscles during the clinical exam. The movement of the extraocular muscles can be assessed by having the patient look in nine directions. Starting with the primary gaze, followed by the secondary positions (up, down, left, and right) and the tertiary positions (up and right, up and left, down and right, down and left). The clinician can test these positions by having the patient follow the clinician's finger trace a wide letter "H" in the air.
Further tests of ocular alignment can be tested further by several methods, including cover tests, corneal light reflex, dissimilar image tests and dissimilar target tests. Since many patients with extraocular muscle abnormalities are young children, the clinician may need to employ various clever means such as using toys or other objects to elicit the cooperation of the child.
Strabismus, or ocular misalignment, can be caused by abnormalities in binocular vision or abnormalities of neuromuscular control. Weakness, injury or paralysis that involves the medial rectus muscle can be involved in strabismus. Though multiple conditions exist, only a few are mentioned here.
The prefix "eso" refers to a condition where the eye is rotated so the cornea deviates nasally and the fovea is rotated temporally. Esotropia refers to an esodeviation that is constant. Incomitant deviation occurs when the abnormality changes with the direction of gaze or the fixating eye. Therefore, incomitant esotropia is an eye that is deviated nasally and changes depending on gaze or fixation. Medial rectus muscle restriction or compression may cause incomitant esotropia. Several conditions may cause this such as thyroid myopathy, medial orbital wall fracture, Duane syndrome, or post-surgical excessively resected medial rectus muscle.
A-pattern and V -pattern deviation is horizontal; rectus muscles can be vertically transposed to correct A-pattern and V-pattern deviation. In A-pattern deviations, the medial rectus muscle is moved superiorly along the globe toward the location of the deviation. In V-pattern deviations, the medial rectus is moved down toward the deviation. This will weaken that muscle in the direction of gaze that the muscle is displaced. Full treatment plans for these disorders depend on the function of the other extraocular muscles, most notably the superior and inferior obliques, and assessment of over or under action is crucial.
(Click Image to Enlarge)
The extraocular muscles of the orbit
By OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons
Kono R,Ohtsuki H,Kishimoto F,Hamasaki I,Morizane Y,Shiraga F, Magnetic resonance imaging findings of age-related distance esotropia in Japanese patients with high myopia. Graefe's archive for clinical and experimental ophthalmology = Albrecht von Graefes Archiv fur klinische und experimentelle Ophthalmologie. 2019 Jan 22; [PubMed PMID: 30671656]