Strabismus vs. Lazy Eye (Amblyopia): What’s the Difference?

Dr Leo Seo Wei
Senior Consultant Ophthalmologist
MBBS (NUS), MRCS (Edin), MMed (Ophth) (NUS), FRCS (Edin) (Ophth), FAMS (Ophth)

Many people confuse strabismus with lazy eye, but they are distinct conditions that affect vision differently. A lazy eye, medically known as amblyopia, is characterised by reduced vision in one eye caused by poor visual development. Strabismus, on the other hand, is a misalignment of the eyes, where they point in different directions.

In Singaporean children aged 6 to 72 months, the prevalence of strabismus is approximately 0.80% to 0.84%, with exotropia being the most common type. Meanwhile, the prevalence of amblyopia (lazy eye) in children aged 30 to 72 months is about 1.19%.

What Is Strabismus (Crossed Eyes)?

Strabismus , also known as crossed eyes, wall-eyed, or squint, is a condition in which the eyes are misaligned and do not look in the same direction at the same time. This means that while one eye may look straight ahead, the other may turn inward, outward, upward, or downward, resulting in a visible misalignment of the eyes.

In normal vision, both eyes work together to focus on the same object. With strabismus, the muscles that control eye movement do not coordinate properly, causing one eye to deviate from the point of focus.

Types of Strabismus

Strabismus is a misalignment of the eyes, and its types are primarily classified by the direction of the eye turn.

Esotropia: This is when one or both eyes turn inward, commonly known as "crossed eyes."
Exotropia: This is when one or both eyes turn outward, often referred to as "wall-eyed."
Hypertropia: This occurs when one eye turns upward.
Hypotropia: This is a less common type where one eye turns downward.

Beyond the direction of the eye turn, ophthalmologists also classify strabismus based on other factors to understand the specific condition. Frequency determines if the eye turn is constant or happens only occasionally (intermittent). Laterality checks if the misalignment always affects the same eye (unilateral) or alternates between both. Consistency assesses if the eye turn is the same in all directions of gaze (comitant) or if it changes depending on where you look (incomitant).

An ophthalmologist will perform a comprehensive examination to determine the specific type of strabismus and create the most effective treatment plan.

Causes

The main causes of strabismus often relate to problems with eye muscles, nerves, or vision. They include:

Muscle Imbalance: Weakness or overactivity in the muscles that control eye movement can cause the eyes to become misaligned.
Nerve Problems: Issues with the cranial nerves that send signals to the eye muscles can disrupt proper coordination.
Genetics: A family history of strabismus significantly increases a person's risk of developing the condition.
Vision Problems: Uncorrected refractive errors, such as hyperopia (farsightedness), can contribute to the development of strabismus as the eyes strain to focus.
Associated Conditions: Strabismus can also be linked to other medical conditions, including cerebral palsy, Down syndrome, craniofacial disorders, or other neurological and ocular diseases.

Common Symptoms

The most common signs of strabismus are easily noticeable and include:

Visible misalignment where one or both eyes turn inward, outward, upward, or downward
Double vision, which is especially common if the condition develops in adulthood
Frequent squinting or blinking
Tilting or turning the head to focus on an object
Poor depth perception
Closing or covering one eye in bright light or while focusing

Vision loss from strabismus is usually not severe unless amblyopia develops. Amblyopia may cause a “lazy eye,” which reduces vision in the misaligned eye if not treated early. Double vision and poor three-dimensional vision are more likely if the condition starts later in life.

What Is Lazy Eye (Amblyopia)?

Lazy eye, medically known as amblyopia, is a condition where vision is decreased in one eye due to poor visual development early in life, even though the eye itself may appear normal.

Amblyopia usually develops in childhood, typically from birth up to around age 7, when the visual system is still developing. If untreated, the condition can lead to permanent vision loss in the affected eye. The weaker eye may also have poor alignment, such as drifting inward or outward, but the eye can appear normal in alignment.

Causes

The primary causes of amblyopia, or lazy eye, are related to how the eyes and brain work together. These include:

Muscle Imbalance (Strabismus): When the eyes are misaligned due to a muscle imbalance, they cannot work together properly. The brain then ignores the image from the misaligned eye, leading to amblyopia.
Refractive Errors: A significant difference in vision between the two eyes (due to nearsightedness, farsightedness, or astigmatism) can cause the brain to favour the stronger eye and neglect the weaker one.
Vision Obstruction (Deprivation): Any condition that blocks clear vision in one eye during childhood—such as a cataract, a drooping eyelid (ptosis), or a corneal scar—can prevent proper visual development and lead to amblyopia.
Genetics: A family history of amblyopia or related eye conditions increases the risk of a child developing the condition.
Associated Conditions: Amblyopia can also be linked to other medical issues, including premature birth, low birth weight, developmental disabilities, and other neurological or ocular diseases.

Common Symptoms

The most common signs of a lazy eye, or amblyopia, are often related to its underlying causes, such as strabismus. They can include:

Visible misalignment , where one eye may turn inward, outward, upward, or downward
Poor depth perception due to impaired binocular (two-eyed) vision
Squinting or closing one eye especially in bright light or to focus
Tilting or turning the head to improve focus or reduce double vision
Vision that does not improve with glasses alone
Sometimes, symptoms are so subtle that they are only detected during a routine eye exam

Vision loss from a lazy eye occurs when the brain favours one eye over the other, suppressing visual input from the weaker eye. Over time, this suppression prevents the neural pathways between the brain and the affected eye from developing properly, leading to reduced visual acuity in that eye.

Can You Have Both?

You can have both strabismus and amblyopia simultaneously. When strabismus causes the brain to ignore the input from one eye, that eye's vision can deteriorate, causing amblyopia. This combination is commonly referred to as "strabismic amblyopia" and is a frequent scenario in paediatric vision problems.

Diagnosis

Diagnosing strabismus and amblyopia requires a comprehensive eye exam and specialised tests to assess eye alignment and vision. Your ophthalmologist will check overall eye health and visual acuity, with a focus on how well your eyes align and work together. Common tests include:

Cover Test: This test detects misalignment by covering one eye and observing movement in the other.
Alternate Cover Test: This test identifies subtle misalignments by alternating the cover between eyes.
Hirschberg Test: This test estimates eye misalignment by observing the light reflection on the cornea.
Krimsky Test: This test uses prisms to measure the precise amount of eye turn.
Photoscreening: This test quickly screens for risk factors in children who cannot use standard eye charts.

Treatment for Strabismus

Treatment for strabismus is tailored to the individual and may include a combination of the following methods:

Glasses or Contact Lenses: Correcting underlying refractive errors, such as farsightedness, can improve eye alignment, especially in accommodative esotropia.
Patching: This is often used to strengthen the weaker eye when amblyopia coexists with strabismus.
Vision Therapy: Eye exercises are used to improve coordination, focusing, and binocular vision.
Prism Lenses: Special lenses are used to modify light paths, which helps reduce double vision and allows the eyes to work together more effectively.
Botulinum Toxin (Botox) Injections: Injections can be used to temporarily weaken specific eye muscles to improve alignment.
Surgery: This involves adjusting the length or position of the eye muscles to permanently realign the eyes when other treatments are insufficient.

Regular follow-up exams are crucial to track progress and adjust the treatment plan as needed.

Treatment for Amblyopia

The primary goal of amblyopia treatment is to strengthen the weaker eye and encourage the brain to use it. Treatment options include:

Patching: This involves covering the stronger eye for prescribed periods to force the brain to use and develop the weaker eye.
Atropine Eye Drops: Used as an alternative to patching, these drops temporarily blur the vision in the stronger eye to stimulate the use of the weaker eye.
Corrective Lenses: Glasses or contacts are used to correct refractive errors (like nearsightedness, farsightedness, or astigmatism) that contribute to amblyopia.
Vision Therapy: This involves a programme of specialised eye exercises designed to improve visual acuity and coordination in the affected eye.

Consult Dr Leo Seo Wei for Your Eye Care

Dr Leo Seo Wei is a senior consultant ophthalmologist specialising in the diagnosis and treatment of both strabismus and amblyopia. With over 20 years of clinical practice and training at top institutions like the University of Michigan, she brings expertise in managing these complex eye conditions for both children and adults.

Dr Leo uses modern diagnostic technology and treatment methods, including vision therapy, patching, atropine eye drops, and innovative surgical techniques, ensuring personalised care tailored to each patient’s unique needs.

If you think your child may have strabismus or amblyopia, schedule a comprehensive eye exam with an eye care professional. Book an assessment with Dr Leo Seo Wei today for expert care.