Strabismus and Amblyopia

Introduction

Strabismus and Amblyopia are critical topics in pediatric ophthalmology and are high-yield areas for the MCCQE1. As a future Canadian physician, you must be proficient in screening, early detection, and appropriate referral of these conditions to prevent permanent vision loss.

Strabismus: A misalignment of the eyes where the visual axes do not meet at the point of fixation.
Amblyopia: A functional reduction in visual acuity of one or both eyes caused by disuse during the critical period of visual development, without apparent structural abnormality of the eye itself.

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Critical Concept: Amblyopia is the leading cause of monocular vision loss in adults. It is preventable and reversible if detected and treated during the critical period of visual development (typically before age 7-8).

MCCQE1 Objectives and CanMEDS Framework

For the MCCQE1, candidates are expected to demonstrate competence in the following areas regarding ocular misalignment and visual development:

Medical Expert: Differentiate between normal variants (pseudostrabismus) and pathological misalignment. Understand the causes of amblyopia.
Health Advocate: Implement vision screening guidelines recommended by the Canadian Paediatric Society (CPS).
Collaborator: Recognize when to refer to an ophthalmologist or optometrist within the Canadian healthcare system.

Pathophysiology and Classification

Understanding the mechanism is essential for answering MCCQE1 logic-based questions.

Strabismus

Strabismus disrupts binocular vision and depth perception (stereopsis). In children, the brain suppresses the image from the deviating eye to avoid diplopia (double vision), leading to amblyopia.

Nomenclature of Deviations

Prefix: Describes direction (Eso- = in, Exo- = out, Hyper- = up, Hypo- = down).
Suffix: Describes stability.
- -tropia: Manifest deviation (always present or present under normal binocular conditions).
- -phoria: Latent deviation (only appears when binocular fusion is broken, e.g., covering one eye).

Amblyopia

Amblyopia is a cortical problem, not an eye problem. It results from abnormal visual stimulation during development.

Strabismic Amblyopia

Caused by misalignment. The brain suppresses the image from the deviating eye to prevent double vision.

Clinical Presentation and History

History Taking (Key Questions)

When a parent presents with a child for “wandering eyes,” ask:

Onset: Constant or intermittent? (Intermittent exotropia is common when tired).
Duration: Since birth or acquired?
Family History: Strong genetic component for strabismus and refractive errors.
Birth History: Prematurity and low birth weight are risk factors for Retinopathy of Prematurity (ROP) and strabismus.
General Health: Developmental delays, cerebral palsy (high association with strabismus).

Physical Examination: The Screening Toolkit

For the MCCQE1, you must know how to screen.

1. Visual Acuity Assessment

Pre-verbal (0-2 years): Fix and Follow behavior.
Toddlers (2-5 years): Picture charts (e.g., Lea symbols, Allen figures).
School age (>5 years): Snellen chart or Tumbling E.

2. Corneal Light Reflex (Hirschberg Test)

Shine a light at the patient’s eyes from arm’s length.

Normal: Reflex is central and symmetrical in both pupils.
Esotropia: Reflex is displaced temporally on the deviating eye.
Exotropia: Reflex is displaced nasally on the deviating eye.

3. Cover Tests (The Gold Standard)

Step 1: Cover-Uncover Test (Detects Tropias)

Have the child fixate on a target (toy or light). Cover the “good” eye.

If the uncovered eye moves to take up fixation, a manifest strabismus (tropia) is present.
Example: If the eye moves out to fixate, it was turned in (Esotropia).

Step 2: Alternate Cover Test (Detects Phorias)

Quickly move the occluder from one eye to the other, not allowing the child to fuse images.

Observe the eye just as it is uncovered.
If it moves to fixate, a latent strabismus (phoria) is present.

4. Red Reflex Test (Bruckner Test)

Essential to rule out life-threatening or sight-threatening pathology.

Leukocoria (White Pupil): Immediate referral required. Differential includes Retinoblastoma, Congenital Cataract, or Retinopathy of Prematurity.

Differential Diagnosis

Condition	Key Features	Management
Pseudostrabismus	Broad, flat nasal bridge (epicanthal folds) makes eyes appear crossed. Hirschberg test is normal (symmetrical).	Reassurance. Resolves with facial growth.
Infantile Esotropia	Large angle deviation presenting <6 months. Cross-fixation is common.	Surgery usually required.
Accommodative Esotropia	Onset 2-5 years. Caused by high hyperopia (farsightedness). Eyes over-converge while trying to accommodate.	Corrective glasses (full hyperopic correction).
Intermittent Exotropia	Eye drifts out when tired, ill, or daydreaming.	Observation, patching, or surgery if progressive.
Cranial Nerve Palsies	CN III: Down & out, ptosis, mydriasis. CN IV: Up & in deviation, head tilt. CN VI: Eye cannot abduct (turn out).	Investigate underlying cause (trauma, intracranial pressure).

Management Principles

Management follows a stepwise approach tailored to the Canadian healthcare context.

1. Correct Refractive Errors

This is the first step. Glasses alone can cure Accommodative Esotropia.

2. Treat Amblyopia

If amblyopia is present, the “lazy” eye must be forced to work.

Occlusion (Patching): The good eye is patched for several hours a day.
Pharmacologic Penalization: Atropine drops in the good eye to blur vision (useful if compliance with patching is poor).

3. Surgical Alignment

Performed if glasses and amblyopia therapy do not fully align the eyes.

Recession: Weakening a muscle by moving its insertion backward.
Resection: Strengthening a muscle by shortening it.

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Canadian Referral Pathway: In Canada, primary care physicians (Family Doctors, Pediatricians) play a vital role in screening.

Refer to Optometrist: For initial refraction and comprehensive eye exam in cooperative children.
Refer to Ophthalmologist: For suspected strabismus, leukocoria, or failed vision screening requiring medical/surgical intervention.

Canadian Guidelines

The Canadian Paediatric Society (CPS) and the Canadian Ophthalmological Society (COS) recommend the following surveillance:

Newborns: Check for red reflex and structural abnormalities.
Infants (6-12 months): Assess fixation and alignment (Hirschberg).
Preschool (3-5 years): Visual acuity screening (subjective) and alignment.

Coverage: In most Canadian provinces, eye exams for children (typically under 18 or 19) are covered by the provincial health plan (e.g., OHIP, MSP), removing financial barriers to screening.

Key Points to Remember for MCCQE1

Leukocoria is a medical emergency (Retinoblastoma until proven otherwise).
Pseudostrabismus is distinguished from true strabismus by a normal (symmetrical) corneal light reflex.
Accommodative Esotropia is treated with glasses, not surgery initially.
The critical period for visual development ends around age 7-8; treatment efficacy drops significantly after this age.
New onset strabismus in an older child or adult requires neurological investigation (rule out tumor, elevated ICP).

Sample Question

Case Presentation

A 3-year-old girl is brought to your office by her parents who noticed that her left eye seems to turn inward, especially when she is looking at picture books. She was born at term and has no significant past medical history. On physical examination, the corneal light reflex is centered in the right pupil but displaced temporally in the left pupil. The cover-uncover test reveals an outward movement of the left eye when the right eye is covered.

Question

Which of the following is the most appropriate initial management step?

A. Reassure the parents that this is pseudostrabismus
B. Prescribe antibiotic eye drops
C. Refer for cycloplegic refraction and dilated fundus examination
D. Schedule MRI of the brain immediately
E. Recommend immediate surgical correction of the eye muscles

Click to reveal the answer and explanation

Explanation

The correct answer is:

C. Refer for cycloplegic refraction and dilated fundus examination

Detailed Analysis: The clinical presentation is classic for Esotropia (inward turning of the eye).

Diagnosis: The Hirschberg test (light reflex displaced temporally) and Cover-Uncover test (eye moves out to fixate) confirm a left esotropia.
Etiology: At age 3, Accommodative Esotropia is a leading differential diagnosis. This is caused by uncorrected hyperopia (farsightedness). The child accommodates to focus, which drives excessive convergence.
Management: The gold standard initial step is a comprehensive eye exam including cycloplegic refraction (drops to paralyze accommodation and measure true refractive error) and a dilated fundus exam (to rule out pathology like retinoblastoma or optic nerve issues). If high hyperopia is found, glasses are the first line of treatment.

Why other options are incorrect:

A: Pseudostrabismus would present with a normal, symmetrical corneal light reflex.
B: There are no signs of infection (redness, discharge).
D: Neuroimaging is indicated for non-comitant strabismus (cranial nerve palsies) or sudden onset in older children/adults with neurological signs, not for typical childhood esotropia without other findings.
E: Surgery is only considered after refractive errors are corrected and amblyopia is treated.

References

Canadian Paediatric Society. (2023). Vision screening in infants, children and youth. Retrieved from cps.ca
Canadian Ophthalmological Society. (2021). Amblyopia and Strabismus Clinical Guidelines.
Medical Council of Canada. (2024). MCCQE Part I Objectives: Ophthalmology.
Kliegman, R. M., et al. (2020). Nelson Textbook of Pediatrics (21st ed.). Elsevier.
American Academy of Ophthalmology. (2023). Basic and Clinical Science Course (BCSC): Pediatric Ophthalmology and Strabismus.