Strabismus [Squint]
Strabismus is a condition where the eyes are misaligned and do not point in the same direction when focusing on an object. One eye may look straight ahead while the other turns inward (esotropia), outward (exotropia), upward (hypertropia), or downward (hypotropia). It can be constant or intermittent and may lead to amblyopia (lazy eye) if untreated.
Types of Strabismus
Strabismus can be classified based on various factors, such as direction of eye deviation, constancy, cause, and association with ocular movements. Understanding these classifications is crucial for accurate diagnosis and management.
1. Based on Direction of Deviation
A. Esotropia (Convergent Strabismus):
One or both eyes turn inward.
Subtypes:
- Accommodative Esotropia: Associated with hypermetropia; corrected by glasses.
- Non-accommodative Esotropia: Not corrected with glasses.
- Congenital Esotropia: Present before 6 months of age.
B. Exotropia (Divergent Strabismus):
One or both eyes turn outward.
Subtypes:
- Intermittent Exotropia: Occurs occasionally, often when the patient is tired.
- Constant Exotropia: Present all the time.
- Sensory Exotropia: Due to poor vision in one eye.
C. Hypertropia (Vertical Strabismus – Upward Deviation):
One eye is higher than the other.
D. Hypotropia (Vertical Strabismus – Downward Deviation):
One eye is lower than the other.
E. Cyclotropia:
The eye rotates around its visual axis.
- Incyclotropia: Upper pole of the eye rotates inward.
- Excyclotropia: Upper pole of the eye rotates outward.
2. Based on Constancy
A. Constant Strabismus:
The misalignment is always present.
B. Intermittent Strabismus:
The misalignment occurs occasionally, often under fatigue or illness.
3. Based on Onset
A. Congenital (Infantile) Strabismus:
Present at or shortly after birth (within first 6 months).
B. Acquired Strabismus:
Occurs after 6 months of age due to refractive errors, trauma, or neurological causes.
4. Based on Ocular Movement Involvement
A. Concomitant (Non-paralytic) Strabismus:
- Angle of deviation remains the same in all directions of gaze.
- No limitation of eye movements.
- Common in children and usually associated with refractive errors.
B. Incomitant (Paralytic/Restrictive) Strabismus:
- Angle of deviation varies with gaze direction.
- Limitation of movement in the direction of the affected muscle.
- Causes include cranial nerve palsies, trauma, thyroid eye disease.
5. Based on Cause
A. Refractive Strabismus:
Caused by uncorrected refractive errors, typically hypermetropia leading to accommodative esotropia.
B. Sensory Strabismus:
Due to visual impairment in one eye; the eye with reduced vision deviates.
C. Paralytic Strabismus:
Results from cranial nerve (III, IV, VI) palsy affecting extraocular muscles.
D. Restrictive Strabismus:
Caused by physical restriction of eye movements (e.g., thyroid eye disease, orbital fractures).
6. Based on Laterality
A. Unilateral Strabismus:
The same eye is always deviated.
B. Alternating Strabismus:
Either eye may deviate at different times.
Special Types of Strabismus
1. Latent Strabismus (Heterophoria)
A hidden eye deviation that appears only when binocular fusion is disrupted OR deviation controlled by fusion, apparent only when fusion is disrupted (e.g., during cover test).
Characteristics:
- Controlled by the brain's fusion mechanism under normal conditions.
- Becomes apparent only when one eye is covered, and fusion is broken.
- Asymptomatic in most cases, but may cause eye strain or headaches.
Types:
- Esophoria - Eye turns inward
- Exophoria - Eye turns outward
- Hyperphoria - Eye turns upward.
- Hypophoria - Eye turns downward.
- Cyclophoria - Eyes rotate abnormally around their vertical axis
- Anisophoria - Eyes have an imbalance of muscles that varies depending on which direction you look.
2. Manifest Strabismus (Heterotropia):
Constant deviation visible without covering either eye.
Characteristics:
- Always present (constant) or intermittently visible.
- Cannot be controlled by the patient.
- Detected without any special test (visible during routine observation).
Types:
- Esotropia,
- Exotropia,
- Hypertropia,
- Hypotropia,
- Cyclotropia,
- Anisotropia, etc.
3. Pseudo-strabismus
Apparent misalignment without true deviation (common in children with a broad nasal bridge).
4. Duane Retraction Syndrome
Congenital condition where horizontal eye movement is limited, and the eye retracts into the orbit on attempted adduction.
5. Brown Syndrome:
Brown syndrome is a rare eye disorder that limits upward movement of the eye. Limitation of elevation in adduction due to a tight superior oblique tendon.
Differentiate Phoria from Tropia
Cover-Uncover Test (for Tropia):
- Patient focuses on a target.
- One eye is covered while observing the uncovered eye.
- If the uncovered eye moves to fixate, tropia is present.
Alternate Cover Test (for Phoria):
- Cover alternately switched between the two eyes.
- If movement occurs as the cover is moved, indicating re-fixation, phoria is present.
Causes of strabismus
Congenital (Infantile) Strabismus:
- Idiopathic (most common)
- Genetic predisposition
- Congenital cranial nerve palsies (III, IV, VI)
- Refractive errors (especially hypermetropia causing accommodative esotropia)
Acquired Strabismus:
- Uncorrected refractive errors
- Trauma (orbital fractures, muscle entrapment)
- Neurological conditions (cranial nerve palsies, intracranial tumors)
- Systemic diseases (e.g., thyroid eye disease, myasthenia gravis)
- Sensory deprivation (cataract, corneal opacity leading to sensory strabismus)
- Paralyzing conditions (e.g., diabetic neuropathy affecting ocular muscles)
Signs and Symptoms
Symptoms:
- Eye misalignment (noticed by patient or caregivers)
- Diplopia (double vision) in acquired cases
- Eye strain or headaches
- Difficulty in reading or focusing
- Abnormal head posture (to compensate for diplopia)
- Poor depth perception
Signs:
- Visible deviation of one or both eyes
- Absent or reduced binocular vision
- Positive Hirschberg test (corneal light reflex test shows asymmetry)
- Abnormal cover-uncover and alternate cover test results
- Limited ocular movements (in paralytic strabismus)
Investigations for Strabismus
Clinical Evaluation:
- Visual acuity testing (to check for amblyopia)
- Refraction testing (cycloplegic refraction for children)
- Ocular motility assessment (to evaluate extraocular muscle function)
- Hirschberg corneal light reflex test (A healthcare professional shines a light into the patient's eyes and observes where the light reflects on the cornea; in normal alignment, the reflection will be centered in the pupil of each eye.)
- Cover tests (cover-uncover and alternate cover tests) to detect latent or manifest squint
- Bruckner test (to detect asymmetry in red reflex)
Special Tests:
- Prism bar cover test (to measure angle of deviation)
- Fundoscopy (to exclude retinal or optic nerve pathology)
- Forced duction test (to differentiate between restrictive and paralytic strabismus)
Neurological Imaging:
CT/MRI of brain and orbit if neurological causes or trauma suspected
Management of Strabismus
Non-Surgical Management:
- Refractive correction: Glasses for refractive errors (especially hypermetropia in accommodative esotropia)
- Amblyopia treatment: Occlusion therapy (patching the better eye), Penalization with atropine drops
- Orthoptic exercises: Useful for convergence insufficiency
- Botulinum toxin injections: Temporary alignment in selected cases of paralytic strabismus
Surgical Management:
Indicated when non-surgical methods fail or for cosmetic purposes
- Muscle recession: Weakening an overacting muscle
- Muscle resection: Strengthening an underacting muscle
- Transposition procedures: For specific paralytic strabismus
Management of Underlying Causes:
- Treat associated conditions (e.g., thyroid dysfunction, myasthenia gravis)
- Neurological consultation for cranial nerve palsies
Post-Operative Care:
- Regular follow-up for amblyopia management
- Assessing binocular vision recovery
- Possible repeat surgeries for residual deviation
Key Points for Exam
- Tropia = True strabismus (visible misalignment)
- Phoria = Hidden strabismus (revealed by cover tests)
- In children, manifest strabismus (tropia) requires early correction to prevent amblyopia.
- Latent strabismus (phoria) may need orthoptic exercises if symptomatic.
- Accommodative Esotropia is the most common form of childhood esotropia.
- Sixth nerve palsy causes horizontal diplopia and esotropia.
- Third nerve palsy leads to ptosis, mydriasis, and "down and out" eye position.
- Always consider serious neurological causes for sudden-onset strabismus in adults (e.g., intracranial aneurysm in III nerve palsy).
- Differentiate between concomitant (angle of deviation remains constant in all gaze directions) and incomitant strabismus (angle varies with gaze direction).
- Early detection and treatment of amblyopia are crucial in pediatric cases.
- Always rule out serious neurological causes in sudden-onset strabismus in adults.