Eye Muscles: How Convergence Works

Convergence is the ability to turn both eyes inward toward each other to look at a close object. This is a crucial visual skill for near-work activities such as reading, desk work, or using a smartphone. It is also important in sports, such as when catching a ball. Convergence requires the simultaneous stimulation of some extraocular muscles and relaxation of others. This process is known as the near triad and involves convergence, accommodation, and miosis. On the other hand, divergence is the opposite of convergence, where the eyes turn outwards to view distant objects. While convergence insufficiency is a common condition affecting the ability to converge the eyes, convergence exercises can help improve overall eye health and treat this condition.

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Convergence and divergence

Convergence is the ability to turn both eyes inward toward each other to look at an object as it moves closer. This requires the coordinated stimulation of some extraocular muscles while others are relaxed. Specifically, convergence occurs by stimulating the medial rectus muscle of both eyes while simultaneously relaxing the lateral recti. When the eyes converge, the focusing (accommodative) system is engaged, and the pupils get slightly smaller. This set of three processes—technically termed convergence, accommodation, and miosis—is known as the near triad.

Convergence is necessary for near-work activities such as reading a book, working on a smartphone, or even in sports when catching a ball. To read, the eyes must rotate inward, the focusing system must engage, and a series of tracking movements are used to move the eyes across a page.

Convergence insufficiency (CI) is a common vision disorder that affects how the eyes work together when looking at nearby objects. It can cause blurry or double vision when looking at things up close. People with CI may experience several symptoms when reading or looking at objects close-up, such as tired or sore eyes, difficulty concentrating, and squinting. CI is one of the most common disorders of binocular vision, affecting approximately 5% of children in the United States. It can also occur in people of all ages after a brain injury, like a concussion.

CI can be treated with eye movement exercises, also known as convergence exercises, that retrain the nerves to aim the eyes properly. These exercises are often performed with a specialist and involve focusing on objects at different distances. In some cases, prism glasses or surgery may be required to strengthen the eye muscles.

Divergence is the opposite of convergence and is the ability to turn the two eyes outwards to look at distant objects.

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Convergence insufficiency

Convergence is the ability to turn both eyes inward toward each other to look at a close object. This requires the coordinated stimulation of some extraocular muscles at the same time as others are relaxed. The medial rectus muscle of both eyes is stimulated, while the lateral recti are relaxed. This is known as the near triad.

Treatment for convergence insufficiency involves eye movement exercises, also known as convergence exercises, which retrain the nerves to aim the eyes properly. Prism glasses can also be used, and in rare cases, surgery may be an option to strengthen the eye muscles.

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Eye movement exercises

One of the most common ocular motor deficiencies is poor gaze stability, which can be treated with eye movement exercises. Saccades, for example, are fast eye movements that allow a person to shift their gaze from one object to another. They are initiated by the frontal lobes and are essential for reading and retaining information. Pursuits, on the other hand, are slow and steady eye movements that allow a person to track a moving object. Good pursuits are essential for everyday functions, such as reading and playing sports.

The Brock string exercise is another example of an eye movement exercise that helps improve eye coordination. To do this exercise, a person needs a long string and some coloured beads. They hold one end of the string just below their nose and place a bead on the string. They should then look straight at the bead with both eyes open and, if their eyes are working correctly, they should see the bead and two strings in the shape of an X.

Convergence exercises can be used to treat convergence insufficiency, a condition that affects how the eyes work together when looking at nearby objects. In this condition, the nerves that control the eye muscles do not work correctly, causing blurry or double vision when looking at things up close. To treat this condition, a person can hold a pencil at arm's length, between their eyes, and slowly move it towards their nose, focusing on keeping a single image of the pencil. This exercise helps to improve eye coordination and convergence.

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Near point of convergence test

Convergence is the ability to turn both eyes inward toward each other to look at a close object. It requires the coordinated stimulation of some extraocular muscles while others are relaxed. The medial rectus muscles of both eyes are stimulated, while the lateral recti are relaxed. This process is known as the near triad.

The near point of convergence (NPC) test measures the ability of a patient to keep a target single as it moves closer to the nose. This is the distance from the eyes to the point where both eyes are just unable to maintain focus and start to experience double vision. To perform this test, a clinician will use a small penlight or other fixation targets, such as a pencil, and slowly move the light towards the patient's nose. The patient will then report when the light breaks into two lights (double vision). A good convergence ability should result in both tests showing similar results.

The NPC test is often used to assess patients with convergence insufficiency (CI), a common disorder of binocular vision that affects how the eyes work together. CI can cause double or blurry vision when looking at things up close. It is important to note that the NPC test should be performed more than once to ensure accurate results.

There are various tools and methods to measure NPC. The Dot Card or Brock String can be used, where the patient is asked to fixate on the farthest point on the card and then move to the closer point until they see two parallel lines. Another method is the Vestibular Ocular Motor Screening (VOMS) assessment, which combines oculomotor and vestibular tests with a concussion symptom scale. The Bernell VergelTM is a battery-operated device that measures convergence using a 9-point font size target.

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Extraocular muscles

The extraocular muscles are the seven extrinsic muscles of the eye in humans and other animals. They are located within the orbit but are separate from the eyeball itself. They control the movements of the eyeball and the superior eyelid. The extraocular muscles are innervated by three cranial nerves. Damage to one of these nerves will cause paralysis of its respective muscles, altering the resting gaze of the affected eye.

The seven extraocular muscles are:

• Levator palpebrae superioris: This muscle is responsible for raising the upper eyelid. This can be a voluntary or involuntary action.
• Superior rectus
• Inferior rectus
• Medial rectus: Convergence occurs by stimulation of the medial rectus muscle of both eyes while relaxing the lateral recti.
• Lateral rectus: A lesion of the abducens nerve will paralyse the lateral rectus muscle.
• Superior oblique: A lesion of the trochlear nerve will paralyse the superior oblique muscle.
• Inferior oblique: This muscle pulls the eye upward and laterally.

The first six muscles listed above are involved in eye movement. The seventh muscle, the levator palpebrae superioris, is responsible for superior eyelid movement.

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