Binocular vision is the ability to see a single, equally formed image from 2 different retinal sources. For this to work, the retinal inputs must be simultaneous and aligned. This is called orthophoria, where the eyes are aligned symmetrically and fixate equally on objects.
Strabismus, ‘squint’, is when the eyes are misaligned. If strabismus occurs in childhood, it can lead to amblyopia.
Types of Strabismus
There are 2 broad types of strabismus (tropias and phorias):
- A tropia is when the eyes are always deviated (manifest deviation)
- A phoria is a more subtle deviation which is hidden by binocular fusion, and becomes apparent when this is broken during testing.
Strabismus is caused by abnormalities in extraocular muscle function. It can be treated with prism lenses or surgery, depending on the extent of the deviation.
Types of Tropias and Phorias
Further classified according to the direction of eyeball deviation
- Exotropia - outwardly turned (Divergent squint)
- Esotropia - inwardly turned (Convergent squint)
- Hypertropia - upwardly turned
- Hypotropia - downwardly turned
The same terminology applied to phorias
A good way to remember the direction of exotropia is the associate it with exophthalmos. Exophthalmos is the outward protrusion of the eyeball. Exotropia is the outward deviation of the eyeball
Esotropia
Esotropia is the most common form of childhood strabismus.
- It is associated with hypermetropia.
- It is classified as accommodative vs non-accommodative
Accommodative
- Fully accommodative → Esotropia resolves with correction of hypermetropia
- Partially accommodative → Esotropia partially resolves with cycloplegic hypermetropic correction and also needs treatment for amblyopia
- Convergence excess → Esotropia for only nearly vision caused by high convergence. Manage with bifocal glasses or squint surgery
Non-accommodative
- Infantile Esotropia is the commonest non-accommodative type
Exotropia
Typically associated with myopia and classified as constant vs intermittent
Intermittent
- More common
- Exotropia which worsens based on distance or near gaze.
- Near exotropia is relatively common in in the adolescent population
Constant exotropia
- Presents in infants as a result of gross abnormalities in ocular motility
Strabismus Tests
There are several special tests which are important in the characterisation of strabismus. These are tests of binocular function.
Binocular vision is also responsible for stereopsis (depth perception). You need inputs from both eyes to perceive depth.
Special Tests
- Titmus, TNO, Lang, synoptophore → Stereopsis
- worth 4-dot, Bagolini glasses and synoptophore → Sensory fusion
- Prism cover test → Motor fusion (alignment)
Hirschberg Test
Allows the clinician to identify tropias
- A light from an ophthalmoscope is shone, and the corneal light reflex is located.
- In orthophoria, the cornea light reflex should be right over the centre of the pupil.
- In tropias, the corneal light reflex is deviated.
I.e In a right exotropia:
The corneal light reflex of the right eye will be more medial to the pupil. Demonstrating outward deviation of the eyeball.
Some tropias are large enough that they can be easily seen on general inspection.
Cover Test
Involves alternating a cover between the eyes, and characteristic findings point towards tropias and phorias. You can hover a prism of known power in front of the eye to quantify the extent of the strabismus based on how many dioptres of prism are needed for correction.
I.e In esotropia of the right eye
- On covering the left eye
- Right eye moves outwards to return to the normal primary position
- On uncovering the left eye
- Right eye moves back inwards to the esotropic position
I.e In esophoria of the right eye
- On covering the left eye
- No movement of the right eye
- On moving the cover from the left eye to the right eye
- No movement of the left eye
- On moving the cover from the right eye back to the left eye
- The right eye moves out. This is to correct the esophoria which had previously occurred when the right eye was covered and binocular fusion was broken
In Phorias, there is no manifest deviation because of fusion correction. Alternating cover breaks the fusion and allows the deviation to manifest.
Restriction Syndromes
Extraocular muscle action can be mechanically restricted for various reasons. This can lead to strabismus. This section outlines 2 important restriction syndromes.
Duane Syndrome
Thought to be caused by aberrant co-innervation of both the LR and MR by CN3
- This makes the globe retract during adduction.
- Systematically associated with Goldenhar syndrome
- There are 3 types based on additional features:
1: esotropia + abduction deficit
2 : exotropia + adduction deficit
3: esotropia + abduction and adduction deficits
Duane type 1 is the commonest
Brown Syndrome
Limited elevation when adducted
- Upgaze causes a characteristic clicking sensation
- Caused by mechanical restriction of the superior oblique
- Can be found congenitally or after trauma
Strabismus Surgery
Surgery revolves around altering the action of extraocular muscles.
Technique |
Outcome |
|---|---|
Resection |
Shortens and strengthens muscle |
Recession |
Loosens and weakens muscle |
Advancement |
Strengthens previously recessed muscles |
Tucking |
Strengthens superior oblique muscle |
Amblyopia
The visual pathways develop in childhood by the age of 8. Any visual problems during this age can lead to abnormal visual development, which is then permanent throughout life. This is why strabismus and amblyopia are so important in paediatrics.
Pathology
Anything that affects vision in childhood can potentially lead to amblyopia
- Common causes include strabismus and refractive error
- Other causes include ptosis and congenital cataract.
Management
- Occlusion therapy - the good eye is occluded with an eye patch to allow the amblyopic eye to develop properly and catch up
- Atropine penalisation - atropine is topically administered to the good eye, thereby reducing its visual acuity, and allowing the amblyopic eye to develop properly
