What is Lateral Rectus Muscle?
The lateral rectus muscle is one of the six extraocular muscles that regulates eye movement. The lateral rectus muscle’s function is abducting the eye. Abduction is the term for the eye’s movement away from the nose, or towards the ear. When the eye’s lateral rectus muscle is contracting, the eye moves outward towards the ear.
Damage of the nerve that controls the lateral rectus muscle can result in a palsy (weakness) of the muscle. A lateral rectus palsy, also known as a sixth nerve palsy, can cause symptoms such as double vision and an eye turn. A lateral rectus palsy may indicate neurological abnormalities of the brain. Possible causes of a lateral rectus palsy are diabetes, hypertension, trauma, a brain tumor, or elevated intracranial pressure. A lateral rectus muscle palsy is a serious concern that an eye doctor should examine right away.
- The lateral rectus muscle is the eye muscle that abducts the eye, moving it away from the nose or toward the ear.
- If the lateral rectus muscle is damaged, the eyes may turn inward, resulting in double vision.
- A lateral rectus muscle weakness can be caused by diabetes, hypertension, intracranial tumor, or other conditions. It should be examined promptly by an eye doctor.
Structure And Function Of The Lateral Rectus Muscle
When we look to the left or to the right, we are using the lateral rectus muscle. Each eye has its own lateral rectus muscle. The lateral muscle function is to abduct the eye, which is the action of moving the eye outward, or away from the midline. You can remember this function by imagining that the eye is ‘abducted by aliens’, so the eye is moved away from the center position.
The lateral rectus muscle originates at the common tendinous ring, or the annulus of Zinn, located at the back of the eye socket. It inserts close to the outer corner of the eyeball. The actual muscle is hidden inside of the eye socket and cannot be seen by the naked eye. The muscle is approximately 40 millimeters long.
The medial rectus muscle does the opposite action of the lateral rectus muscle. Each eye muscle has an ‘oppositely paired’ muscle that does the opposite action. The lateral rectus muscle function is to abduct the eye (away from the nose). The oppositely paired muscle, which is the medial rectus muscle, adducts the eye and moves it inwards (closer to the nose). For example, when you try to look at your nose and your eyes cross, you are working both of the eye’s medial rectus muscles. With any ‘oppositely paired’ muscles, as one muscle contracts, the other muscle relaxes. So when the lateral rectus muscle contracts, the medial rectus muscle relaxes an equal amount.
For example, when you look to the right:
- The right eye’s lateral rectus muscle contracts
- The right eye’s medial rectus muscle relaxes
- The left eye’s medial rectus muscle contracts
- The left eye’s lateral rectus muscle relaxes
Each eye muscle is connected to a cranial nerve that controls it. The lateral rectus muscle’s input and ‘control center’ is the abducens cranial nerve. The abducens nerve is also called cranial nerve six, or the sixth nerve. The abducens nerve controls the action of the lateral rectus muscle. The abducens cranial nerve is located in the pons of the brain.
It is important to remember that the lateral rectus muscle input is controlled by a cranial nerve located in the brain, because if the lateral rectus muscle stops working properly, the brain may also be affected. For example, if there is a brain tumor pressing on the right abducens nerve, it can decrease the input of the right eye’s lateral rectus muscle.
The lateral rectus muscle and other eye muscles are distinct from skeletal muscles in the rest of the human body. Unlike the muscles in your arms or legs, the eye muscles can move very quickly and they stay strong even after you use them for many hours (like when reading a book). In fact, for an hour of reading a book, the eyes make approximately ten thousand coordinated eye movements.
Like every extraocular muscle, the lateral rectus muscle receives its main blood supply from the muscular branches of the ophthalmic artery, as well as a small amount of blood supply from the lacrimal artery. Blood from the lateral rectus muscle drains into the superior and inferior ophthalmic veins, which eventually drains into the cavernous sinus.
The lateral rectus muscle can become paralysed, weak, and stop moving if the abducens nerve is damaged. The name of this condition is Abducens Nerve Palsy, or sixth nerve palsy. An abducens nerve palsy affecting adults over 50 years old is commonly due to microvascular disease, such as diabetes or hypertension (high blood pressure). However, a lateral rectus muscle palsy can also have a serious underlying cause such as a brain tumor or elevated pressure in the brain. Any new symptoms of double vision should always be examined by an eye doctor promptly.
Symptoms of a Lateral Rectus Muscle Palsy:
- Horizontal double vision, worse when looking far away
- Eyes appear ‘turned in’
- Head turn toward the affected eye’s side
- Inability to move the eyes towards the ears
Potential Causes of a Lateral Rectus Muscle Palsy:
- Hypertension (high blood pressure)
- Internal carotid artery aneurysm
- Space-occupying brain tumor
- Elevated intracranial pressure
- Myasthenia gravis
- Giant cell arteritis
- Trauma to the head or eyes