What is the Medial Rectus Muscle?
The medial rectus is one of seven extraocular muscles of the eye. The extraocular skeletal muscles of the eye include four rectus muscles, two oblique muscles, and the levator palpebrae superioris. The medial rectus is one of the four rectus muscles. The medial rectus muscle is the largest of the extraocular muscles and its size is likely from frequency of use in convergence.
The medial rectus is an adductor and works together with the lateral rectus which abducts the eye. The medial rectus pulls the eye towards the nose while the lateral rectus moves the eye toward the ear. These two muscles enable the eyes to move side to side. When the head is facing straight and the eyes are facing straight ahead, the eyes are in primary gaze. From this position, an extraocular muscle’s action produces a secondary or tertiary action.
- The medial rectus is one of seven extraocular muscles.
- The medial rectus is an adductor and pulls the eye towards the nose.
- The medial rectus can be affected in strabismus and in thyroid eye disease.
Understanding the Medial Rectus Muscle
The primary blood supply for all the extraocular muscles is the ophthalmic artery. The medial rectus is innervated by the lower division of cranial nerve III (oculomotor). Each of the rectus muscles originate from behind at the Annulus of Zinn and courses towards the front. The medial rectus runs along the medial orbital wall. Each rectus muscle inserts on the globe at varying distances from the limbus; the medial rectus inserts at 5.5 mm from the limbus and is 10.3 mm wide at its insertion on the globe.
Conditions Affecting the Medial Rectus Muscle
Weakness, injury, or paralysis that involves the medial rectus muscle can lead to strabismus. 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.
Changes in the medial rectus muscle can also be used to detect progressive thyroid disease. In fact, imaging of the medial rectus muscle can be used to predict the development of optic neuropathy in thyroid eye disease.
Medial Rectus Muscle Diagnosis
The function of the medial rectus muscle is evaluated by an eye care provider during an eye exam. Extraocular muscle movement can be assessed by having the patient look in nine directions. Assessment begins in 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 tests these positions by having the patient follow the clinician’s finger as it traces a wide letter “H” in the air. Further testing of the extraocular muscles includes cover tests, corneal light reflex, dissimilar image tests, and dissimilar target tests.
Medial Rectus Muscle Surgery
Surgery for strabismus may involve the medial rectus muscle. Recession of the medial rectus is a measured retroplacement of the muscle from its original insertion site. It is the easiest and most effective way to weaken the medial rectus. Resection is removing a part of the medial rectus and reattaching it at its original insertion site, which effectively strengthens the action of the muscle. Unsatisfactory muscle alignment is common and may require additional surgeries. Refractive changes may occur when two rectus muscles of one eye are operated on.
Damage to the medial rectus muscle is possible during surgery, but unlikely. However, blood vessels may be compromised during surgery of the medial rectus. The vessels that supply blood to the medial rectus also supply almost all of the temporal half of the anterior segment of the eye. Care must be taken by the surgeon to avoid disrupting the blood supply.