Squint (strabismus) Flashcards
What is a squint (strabismus)?
A squint (strabismus) is misalignment of the visual axis. This means that the eyes are not directed at an object at the same time: while one eye is directed at an object, the other eye may turn in (esotropia), out (exotropia), up (hypertropia), or down (hypotropia). This misalignment may present occassionally or constantly, commonly with onset from childhood, or less commonly, from adulthood
What is a pseudosquint?
A pseudosquint is the impression of misaligned eyes when no squint is present. For example, prominent epicanthic folds may partially cover the nasal sclera and give the appearance of a squint
Other causes include having a broad, flat nose; ocular hypo-/hypertelorism (abnormally reduced or increased distance between the eyes)
A squint can be classified in different ways based on a number of features, what are they?
The age of onset:
- Congenital/ Infantile - onset is within the first 6 months of life (usually idiopathic and the child is otherwise healthy)
- Acquired - onset is after the first 6 months of life
When it is present:
- Constant - present all the time
- Intermittent - occurs at certain times or under certain conditions. Can be worse at the end of the day or with fatigue
The direction of the deviation: (see image)
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Horizontal deviation
- Esotropia - where one or both eyes turn inwards (convergent squint)
- Exotropia - where one or both eyes turn outwards (divergent squint)
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Vertical deviation
- Hypertropia - where one or both eyes turn upwards (upward squint)
- Hypotropia - where one or both eyes turn downwards (downward squint)
- Cyclotorsional - rotation of an eye around its visual axis
The fusional status:
The term ‘fusion’ implies ‘binocular vision’, which is defined as the state of simultaneous vision that is achieved by the coodinated use of both eyes, so that separate images arising in each eye are appreciated as one single image by the process of fusion (your brain takes information from both eyes and gives one single image rather than two separate ones)
- _*Manifest squint (-tropia)_ - seen when no fusional control (= the brain is interpreting information from the normal eye only) is present. There is a deviation in one eye whilst the other eye takes up fixation, meaning that when one eye views the object of interest, the other eye is deviated (see image)
- Note that the patient cannot see two images at a time in a squint, because we only have one brain, which cannot cope with 2 different images, so the brain blocks the information coming from the misaligned eye and receives information from only the normal eye
- _*Latent squint (heterophoria)_ - fusional control is present (= the brain is interpretating information from both eyes) and there is normal ocular alignment, but the squint occurs only when the use of the two eyes together is interrupted, for example by covering one eye (i.e. when binocular fusion is suspended)
The laterality:
- Unilateral - there is a definite preference for fixation of vision with one eye; therefore, the squint always appears to be in the other eye
- Alternating - there is an alternation of fixation of vision from one eye to the other. Therefore, the squint can alternate from one eye to the other
Variation with gaze position
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Concomitant (non-paralytic) squint
- Typical of most childhood squints
- Where the size of the deviation does NOT vary with direction of gaze
- There is no paralysis or limitation of eye movements, but the balance between the extraocular muscles in the two eyes has been lost
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Incomitant (paralytic) squint
- Where the size of the deviation is affected by the direction of gaze
- This may indicate an *acquired neurological and muscular disorders (i.e. damage to the extraocular muscles or their nerves such as CN3, 4, 6 palsies, ocular myositis, MG, and Graves’ disease), causing paralysis of one or more of the extraocular muscles, resulting in limitation of eye movements
Give two examples of a common type of squint in children?
Congenital/ infantile squint
- Convergent squint (esotropia) is much more common than divergent (exotropia)
- The angle of the squint is often large
- Usually no refractive error
- Cross-fixation is frequent - both eyes have good vision and the child tends to use the right eye to look to the left and the left eye to look to the right
Accommodative esotropia
- Generally seen in children between 6 months - 5 years old
- It may begin as an intermittent esotropia, typically when the child is tired or when concentrating on objects close by, and in time becomes constant!
- A fully accommodative esotropia is fully correctable with glasses for hypermetropia (long-sightedness) and binocular function is present with the glasses on
- A partially accommodative esotropia is only partially corrected with glasses for hypermetropia and binocular function is not present
a) . Which direction of deviation is most commonly seen in children with a squint?
b) . Is pseudosquint associated with an increased risk of developing an actual squint?
a) . Esotropias (eye turn inwards) i.e. convergent squint
b) . YESSSSS! Young children diagnosed with a pseudosquint are at increased risk of developing a squint and undergoing surgery for a squint
What’s the difference between a squint (strabismus) and lazy eye (amblyopia)?
A squint or stabismus simply means misalignment of the eye.
A lazy eye or amblyopia is a functional disability in which the brain is unable to process inputs from one eye and over time favours the other eye, causing structural changes in the visual pathway and cortex. This ultimately results in decreased or complete loss of vision in an eye that otherwise appears normal.
(The visual cortex and the eye pathways continue to develop in a child as he grows. This is called neuroplasticity or visual plasticity, which is defined as the ability of the brain to reorganise the structure and function of its connections in response to changes in the environment. It is considered that the brain is plastic and neural networks are initially shaped by experience during the sensitive period and subsequently stabilised during normal development. So in order for the visual cortex to develop normally, it has to receive information from the eye as a stimulus. In lazy eye, the visual cortex is not receiving any inputs from one eye, as a result, the visual cortex remodels and adapts to only receive inputs from the other eye. This remodelling process continues until the child is about 7 years old. If the lazy eye is not corrected by this age, the brain will lose its plasticity and the child will not be able to see with the affected eye permanently. This is why it’s important to treat lazy eye before 7 by encouraging the child to see with their affected eye more (this is done by covering their healthy eye with an eye-patch, forcing the child to use the lazy eye to see things)
_*A squint can cause lazy eye but a lazy eye cannot cause a squint_
What are the causes of a squint in children?
A squint develops because there is a lack of coordination that prevents the gaze of both eyes being focused on the same point in space - either due to an imbalance between the extraocular muscles in both eyes (Concomitant) or a paralysis in one or more of the extraocular muscles (non-concomitant/ paralytic)
Causes include:
- Most squints are idiopathic!
- Refractive errors, esp hypermetropia (long-sightedness), which are strongly associated with accommodative esotropia
- Other causes of poor visual acuity or blindness in one or both eyes (so-called sensory squint), including:
- Retinoblastoma
- Cataract
- Ocular malformations
- Optic neuropathy e.g. optic neuritis
- _*Amblyopia (can be a cause and a complication of squint)_
*Lower visual experience (sensation, cognition, processing and perception) in an eye can cause the eye to drift out of correct alignment!
- Neurodevelopmental conditions such as cerebral palsy and Down syndrome
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Congenital abnormalities of the extraocular muscles or their innervation, such as Brown’s syndorme and Duane’s syndrome
- Brown’s syndrome is a rare form of strabismus characterised by limited elevation of the affected eye (particularly during adduction) due to malfunction of the superior oblique muscle
- Duane’s syndrome is a rare form of strabismus characterised by inability of the eye to move outward due to abnormal development of the CN6 (abducens nerve)
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Diseases that affect extraocular muscles or their innervation
- Brain tumours
- Brain abscess, meningitis
- Head injury, orbital fracture
- Hydrocephalus
- Myasthenia gravis
- Cranial nerve palsies (esp CN3, 4, 6 as they innervate the extraocular muscles)
- Diabetes mellitus (–> peripheral neuropathy), Graves’ disease (causes CN6 palsy)
Is amblyopia a cause or a complication seen in a squint?
Amblyopia can be both a cause of squint and a complication of squint!
Give 5 risk factors for developing a childhood squint
Prematurity and low birth weight
Maternal smoking during pregnancy
Anisometropia (when the eyes have unequal refractive power, causing them to focus unevenly)
Hypermetropia (long-sightedness)
FHx of strabismus
Pseudosquint
Give 3 complications of a squint
Amblyopia (lazy eye)
- The visual cortex may suppress the image from a squinting eye to prevent double vision. Consequently, development of the visual pathways and areas in the brain responsible for vision pertaining to that eye is impaired, and visual acuity is reduced. This is not immediately correctable by refraction
- Squint is a major cause of amblyopia in children!
- The visual loss will become permanent and irreversible if not treated early in childhood whilst the visual pathways are still developing. As a broad rule, _amblyopia needs to be treated by *age 7 years_, and the earlier the treatment the better the outcome
Loss of/ failure to develop binocular vision
- This results in poor stereopsis (perception of depth), which requires binocular vision to develop fully!
Compensatory head postures
Poor eye contact
Social and psychological problems - children with a squint maybe stigmatised in school and suffer from a loss of self-esteem
Does squint carry a good prognosis after treatment?
YESSSS, with timely and appropriate treatment, esp if treatment is initiated soon after a squint develops, the outlook is good! Treatment can prevent amblyopia and correct misalignment of the eyes
An untreated childhood squint that is present after 3 months of age does not usually resolve without treatment
When are the eyes assessed in newborn and children?
Within 72 hrs as part of the Newborn Physical Examination and then at 6-8 weeks as part of Infant physical examination
At school entry (around 4-5 years old), vision screening is offered to all children to identify common causes of reduced vision, including refractive error (long- or short-sightedness) and squint.
What clinical features would you see in a child with a squint?
Presentations:
- _*Red flags_ - recent trauma, symptoms of raised ICP (e.g. morning headaches, N&V), sensory/ motor symptoms, new strabismus in a school-aged child, limited abduction, diplopia, nystagmus
- Take note of the age of onset of the squint
- Reduced VA
- Diplopia
- Asthenopia (i.e. eye strain, fatigue or pain), particularly in the afternoon or at the end of the day
- Poor academic/ work performance
- Behavioural problems
- Social withdrawal / difficulty blending in with friends
- Walking difficulties i.e. bumping into objects, tripping over
What examinations/ tests would you do to confirm the presence of a squint?
General inspection of the eyes
- Any asymmetry in eye position and any ocular abnormalities e.g. pupil asymmetry and ptosis
- Examine eye movements, particulary abduction in cases of esotropia
- Assess for nystagmus
_*Corneal light reflex test (Hirschberg test)_
- Have the child fixate on a light (a pen torch) held about 50 cm in front of their eyes
- Observe the light that reflects back from their corneas, which is normally in the same place (centre of pupil) on both corneas
- Any asymmetry of the reflected light –> squint
_*Cover test to determine if a manifest squint is present (see image)_
- Get the child to view an object such as a toy, at 33 cm away from their eyes
- Whilst fixation is maintained, cover one eye with a piece of card
- As the cover is introduced over one eye, watch the uncovered eye for any movement. Then repeat, covering the other eye
- In a manifest squint when the straight eye (i.e. normal eye) is covered, the squinting eye will move to align with the fixation object
- If no manifest squint, look for latent squint by doing the cover/ uncover test
_*Cover/ uncover test (AKA ‘alternate cover test’) to determine if a latent squint is present_
- As the child fixates on the toy, cover one eye for 3 seconds, then quickly remove the cover and watch for any movement of that eye
- In latent squint, the eye will drift under the cover. On removing the cover, the eye will straighten to regain binocular vision
- Instead of removing the cover, it maybe easier to elicit a latent squint by moving the cover slowly back and forth between the eyes until the test is finished (the ‘alternate cover test’)
- _*Each time the cover is moved, you are looking at the eye that has just been uncovered to see if there is any movement_
_*If a squint is found, check the range of eye movements to determine if it’s a concomitant squint or an incomitant squint (which is more likely to be related to a more serious cause)_
- Aim to get the child to follow a toy or light, to each side and up and down to assess if the eyes can move fully in all directions
- For children with esotropia, it maybe necessary to test the movements uni-ocularly by covering one eye and encouraging the child to follow the toy with the uncovered eye
- This is because with both eyes open, the child may cross-fixate to the sides (usuaing the convergent right eye to look to the left and/or the convergent left eye to look to the right)
_*Bruckner test_
- It’s used to identify the presence of small-angle strabismus
- Stand 50 cm away from the patient and use an ophthalmoscope to assess both of the patient’s red reflexes simultaneously, with the patient fixating at a target next to the ophthalmoscope
- Look for any asymmetry of red reflexes - in size, shape, colour, and brightness - may suggest an ocular disorder, including strabismus, anisometropia, or media opacity that is obstructing the visual axis (e.g. cataract)
- At the same time, exclude leukocoria
Prism cover test
- A much more objective and accurate way to measure the angle of deviation of a small-angle tropia!
Alternate prism cover test
Assess for any abnormal head posture - for example, hypertropia in the affected eye may be associated with a head tilt to the opposite side and depression of the chin
Assess for features that may suggest a neurological cause of squint e.g. cerebral palsy
Test VA and consider fundoscopy
What are the types of squint that may give rise to a particular concern about its underlying cause?
- Constant unilateral exotropia
- Any acquired incomitant squint
- Sudden, late-onset (over 3-4 yrs old) esotropia, particularly if there is no FHx and no significant hypermetropia (long-sightedness)
Cover test
See image
Watch: https://www.youtube.com/watch?v=dsH7eK5o-y4&ab_channel=MinaaAshfaq
Cover-uncover test
See image
Bruckner’s test
See image
Corneal light reflex test (Hirschberg test)
See image
