What is Strabismus?

Overview

Strabismus (also known as a squint) is misalignment of the eyes that occurs when looking at an object, meaning that the eyes point in different directions. This leads to vision problems. It is most common in children, although it can develop throughout life.

Strabismus results in someone seeing either the same object in two different places or, more rarely, two different objects in the same place. If someone suffers with strabismus for a long period, the brain might suppress one image to reduce the abnormality of vision. However, this is called a lazy eye and it is very important to treat quickly to prevent permanent visual impairment.¹

Because our two eyes are not in exactly the same location on our face, the image that each eye sees is slightly different. Luckily, the brain overlaps both images so we see a single non-blurry image. 

Using our eyes in this way is a form of binocular (both eyes) vision which allows us to perceive the depth of the image (i.e. how far away things are). It is important to coordinate the eyes so they are correctly aligned to look at the image in the way the brain expects them to.

The eyes are coordinated (i.e. moved) by the intraocular muscles, which are controlled by nerves running down from the brain. Therefore, issues with this part of the brain, these nerves, or these muscles can mean that the eyes are not coordinated as they should be. This is one reason for strabismus. 

A second reason for strabismus is that the part of the brain that interprets images is faulty.² A third reason is that the eyes are accommodating for a different vision problem, commonly a refractive issue (e.g. long-sightedness), which causes the eyes to misalign.

Types of strabismus

Strabismus is generally classified based on which way the eye turns:

• Esotropia (inward)
• Exotropia (outward)
• Hypotropia (downward)
• Hypertropia (upward)

Strabismus can be further classified as:

• Constant (classification by frequency)
• Intermittent (classification by frequency)
• Unilateral (only one eye displays strabismus)
• Alternating (both eyes may display strabismus at different times)
• Congenital (classification by cause)
• Accommodative (classification by cause)

Certain types of strabismus are more common than others. In children, the most common forms are accommodative esotropia, congenital esotropia, and intermittent exotropia.³

Pseudostrabismus is seen in infants younger than three months. It is common for infants this age to have misaligned eyes but this is not strabismus, rather pseudostrabismus, which is due to the facial features not having fully developed at this stage. For infants younger than three months, misaligned eyes are not usually a cause for concern.

Causes of strabismus

There are a variety of causes of strabismus; some result in a child being born with strabismus, other times it develops later. Strabismus sometimes runs in families. Sometimes the specific cause is unknown. 

Children often develop strabismus through compensating for other vision issues such as short-sightedness or long-sightedness. This is called an accommodative strabismus and usually presents in children aged 2-5. 50% of all childhood esotropias are fully or partially accommodative.⁴

Rarer causes of strabismus include:

• Certain infections
• Genetic disorders
• Developmental delays
• Cerebral palsy
• Problems with the central nervous system
• Retinoblastoma (a type of eye cancer) 

Signs and symptoms of strabismus

The misalignment of the eyes is usually obvious to observe. However, in rare instances, it is not.

Other signs to look out for are squinting, closing one eye, or tilting the head when looking at an object.

Management and treatment for strabismus

It is important to get treatment for strabismus. Strabismus patients have a good prognosis (outcome) if they are diagnosed and treated early.⁴ It is important to rule out retinoblastoma or a lazy eye (amblyopia) because these are conditions that must first be treated.¹

Benefits of treatment of strabismus:

• Restores binocular vision and depth perception
• No more double vision
• Normal head posture
• Normal aesthetic appearance
• Increases visual field 

Use of glasses, eye exercises, eye muscle injections, and surgery may be used to manage and treat strabismus.

Glasses

Using glasses may help to correctly align the eyes.

Eye exercises

Physiotherapy helps to improve the coordination of the eye muscles. It is carried out with an orthoptist, a specialist in eye movement.

Eye exercises may be used in combination with surgery. Eye exercises have been shown to reduce misalignment remaining after surgery.⁵

Eye muscle injections

Eye muscle injections help align the eyes by weakening the muscles controlling the eye. The effect generally lasts three months. In some cases, eye muscle injections have been shown to be more successful at aligning the eyes than strabismus surgery.⁶

Surgery

Surgery may be recommended if other forms of treatment are unsatisfactory. There are long-term improvements in quality of life for strabismus patients post-surgery.⁷ Additional surgery may be required soon after the first or after many years if the strabismus reoccurs.

Surgery may:

• Weaken the eye muscles
• Strengthen the eye muscles
• Adjust the angle at which the eye muscle pulls the eye

It is important to be aware of the risks of strabismus surgery. Most commonly, a good clinical outcome is achieved. However, a poor clinical outcome occurs in 1 in 2400 cases and adverse events are reported in 1 in 400 cases.⁸ Despite these figures, 80% of patients do not regret having surgery.⁹

Diagnosis of strabismus

When a patient is being assessed for strabismus, the doctor will assess the misalignment of the eyes, the range of movement of the eyes, and long-sightedness. 

To assess misalignment, the doctor may shine a penlight at the eyes and see whether the light appears in the pupils of each eye. Additionally, they may ask a patient to focus on an object and then cover each eye in turn to assess whether the eye readjusts to see the object. 

To assess long-sightedness, the doctor will do this via cycloplegia, where the ciliary muscle of the eye (responsible for focusing on objects) is temporarily paralysed.

Risk factors

Factors that increase the risk of strabismus include: 

• Family history
• Undiagnosed vision problems may lead to accommodative strabismus
• Certain genetic disorders

Complications

It is important to treat strabismus because it may result in a lazy eye. Over time, the brain will compensate for the blurred image by blocking out one image sent from one of the eyes. Therefore, normal vision does not develop properly. Because this issue lies in the brain rather than the functionality of the eye, if this is not treated at a young age, the vision problem may become permanent.¹

Children who move their head to help align their eyes may develop abnormal growth of bones in the head and neck.¹

FAQs

How can I prevent strabismus?

Strabismus cannot be prevented if it is related to genetics. However, complications can be reduced if any form of strabismus is diagnosed and treated early.

How common is strabismus?

2-5% of the general population have strabismus.⁴

When should I see a doctor?

You should see a doctor if your child is displaying any signs of strabismus past the age of three months. These include squinting, closing one eye, and tilting of the head to look at something. 

If the child’s parents wear thick glasses, and if strabismus has not already developed by age 3.5 years, then consult a doctor.³

Summary

Strabismus is when the eyes are misaligned, resulting in abnormal vision. There are a large number of possible causes and it may develop any time in life or you can be born with it. 

It is very important to detect strabismus early because it can be caused by a rare childhood cancer or result in a lazy eye, which can cause permanent vision damage if not treated at a young age. 

Glasses, eye exercises, eye muscle injections, and eye surgery are all examples of management and treatment methods for strabismus. Strabismus patients have a good prognosis if they receive early diagnosis and treatment.