BACKGROUND

Squint, or strabismus, refers to the misalignment of the visual axes of the eyes and is thought of as a disorder of eye movement. Normal eye movement is dependent on correct functioning at three levels:

  1. 6 Extraocular muscles (EOM): work to provide a full range of eye movement.  Muscles: superior, inferior, lateral and medial rectus; superior and inferior oblique.
  2. 3 Cranial nerves: provide motor input to the EOMs, coordinating movement of both eyes. Nerves: Abducens (supplies lateral rectus), trochlear (supplies superior oblique) and oculomotor (supplies remaining 4 muscles).
  3. Higher cortical centres and other brainstem nuclei: provide input to the cranial nerves, to carry out tasks such as controlling the speed of eye movement and maintaining focus on a moving object or while the body itself is moving e.g. vestibular nuclei; gaze cortex. 

 

Normal range of movement and various terms used to describe them are as follows (click on table1/figure1 to enlarge):

    Figure 1: Cardinal positions of gaze of the eye, with corresponding EOMTable 1: Glossary of terms used to describe normal eye movements and positioning

    Coordinated eye movement and alignment allow for both eyes to be directed towards the same object. An image of said object is created on corresponding parts of each retina, and fed back to processing centres in the brain. The two retinal images are then fused and interpreted by the higher cortical centres as a single image. Because each eye has a slightly different angle and field of vision, the images formed on each retina are slightly different to each other. The brain processes these disparities to construct a 3D image of the object being viewed. This gives rise to two important concepts:

    • Binocular single vision: the perception of one single image, created from two seperate retinal images. This is important in that two eyes working together increases the total field of vision and eliminates the natural blindspot (the blindspot of one eye falls in the visual field of the other).
    • Stereopsis: the perception of a three-dimensional image, created from two seperate 2D retinal images. This is particularly important for depth perception and visual judgement.

     

    Pathological changes in squint

    Abnormalities of eye movement and misalignment of the eye, as in squint, inteferes with these two functions. Because both eyes are focused on different fields of view, the retinal images are too disparate to fuse together into a single 3D image. This can manifest in two common ways:

    • Diplopia: this tends to be the case in patients with new squint occuring after the visual pathways in the brain have fully developed. Patients describe seeing double of the same object, due to poor fusion of the two seperate retinal images created. This is often associated with eye strain, headaches and poor visual judgement.
    • Amblyopia: this tends to occur in young patients, with persistent squint, particularly under the age of 5. At this stage, visual pathways are still malleable - the brain attempts to resolve the two disparate brain images by suppressing one of them. If the image from the same eye is persistently suppressed over a period of time, the visual acuity in said eye reduces. The eye is said to be amblyopic, and the patient is effectively left with monocular vision, but no diplopia.  

    DEFINITION AND TERMINOLOGY

    Squint, or strabismus, is a misalignment of the visual axes of the eyes. A squint can be described in a number of ways - the common terminology used is discussed in table 2.          

     

    Table 2: Glossary of terms used to describe squints

    Another important system of classifying squints, based on aetiology and pathogenesis is as follows:

    • Non-paralytic squint (a.k.a concomitant squint): when the eye movements are full and equal in both eyes, but only one eye is directed towards the object being viewed. The angle of deviation between the two eyes is constant, regardless of the direction of gaze. This is the type of squint commonly associated with children.
    • Paralytic squint (a.k.a incomitant squint): when the affected/squinting eye has restricted or limited range of movement due to underaction of one or more EOMs. The angle of deviation between the two eyes is dependent on the direction of the gaze (it increases in the direction that the affected muscle normally works). Paralytic squints may occur due to (1) cranial nerve palsy; (2) EOM disease; (3) tethering of the globe in the orbit

    These two terms are illustrated in figure 2:

    Figure 2: Comparing paralytic and non-paralytic squint

    NON-PARALYTIC SQUINT

    Epidemiology:

    • Neonatal squint: common, affecting about 73% of one-month old babies. In the vast majority of cases, the misalignment of the eyes corrects itself by 4 months.
    • Pathological squints tend to occur/persist when the child is older than 6 months.
    • Most common squint = esotropia
    • Exotropias, when they occur, are more likely to be intermittent than constant.

     

    Aetiology:

     Non-paralytic squints may be idiopathic, developing in a child with otherwise normal eyes, or may arise secondary to another eye problem. Idiopathic cases are postulated to be related to disordered central control of eye movement), while secondary squints are related to disorders that distort retinal images produced by each eye, asymmetrically. The brain, unable to process two different images, attempts to compensate by altering eye movement and positioning. Common causes of secondary squint include:

    • Anisometropia: dissimilar refractive errors in both eyes causes the relative blurring of retinal images. 
    • Opacification of the cornea or lens (cataract): also contributes to relative blurring of images
    • Abnormalities of retina: causes incorrect transmission of images

    Other risk factors for developing squint include family history, prematurity, neonatal jaundice, neurological illness (meningitis, encephalitis, cerebral palsy) and craniofacial abnormalities (including syndromes associated with dysmorphism such as Down's).

     

    Presentation

    • Visible squint - noticed by parents/teachers
    • Functional problems - e.g. difficulty with motor skills
    • Detection at preschool/school screening clinics

     

    Evaluation

    Good history taking is important, particularly in young babies, where it is essential to differentiate pathological squint from the self-remitting squint of new borns. Key features of the latter include:

    • Intermittent squint - if pathological, may be constant or intermittent
    • Decreasing degree of deviation over time - if pathological, should worsen/stay the same
    • Gone by 4 months -  if pathological, will persist beyond this time

    Examination is aimed at

    (1) Ruling out pseudostrabismus. This refers to certain facial features that cause a person with normally aligned eyes to look like they have a squint. On examination look for features known to be associated with pseudostrabismus:

    • Prominent epicanthal folds (skin folds on either side of the nose, covering the medial canthus)
    • Broad nasal bridges 
    • Facial asymmetry

     

    (2) Confirming Squint: the following tests help identify presence of a squint

    Table 3: Clinical tests used to identify squint

    (3) Identifying the underlying problem. This includes testing eye movements, stereopsis, refraction and examination of the cornea, lens and retina

     

    Management:

    Management of a non-paralytic squint involves:

    1. Treatment of the cause, if identified: typically this involves glasses to correct refractive error
    2. Correction of amblyopia, if the child is under 8 years old (visual pathways still malleable): Amblyopia is suspected when visual acuity is not improved by glasses. Its correction involves patching the child's good eye, to allow the amblyopic eye adequate visual stimulation, thereby encouraging it to recover visual acuity. Occasionally, cycloplegic drops may also be used in the good eye. 
    3. Correction of squint: this has both functional and cosmetic benefits and is done by prisms in spectacle lenses, eye exercises (sufficient for mild exotropias) or by surgical realignment.  The latter involves manipulation of the EOMs to realign the eyes:
    • Recession of an EOM (moving its point of insertion backwards on the globe) weakens its action
    • Resection of an EOM (removing a segment of the muscle at its point of insertion) strengthens its action

    Squint surgery is required when there is no identifiable/treatable cause, or in large squints, that have not responded to correction. When indicated, it is recommended that surgery be done as early as possible, to reduce the risk of amblyopia. The surgery is variable in its results as under- or over-correction may occur. These patients may require further procedures to get satisfactory realignment. Occasionally adjustable sutures may be used, allowing for minor correction in alignment to be made in the awake patient, without having to return to theatre. A few patients go on to develop a second squint, despite initially good results after surgery.

     

    Prognosis

    Early intervention with glasses, patching and/or surgical realignment can greatly improve vision in the squinting eye and prevent amblyopia. Surgical realignment, where successful, also reduces the psychoscoial ramifications of squint. However, while these children will develop good binocular single vision, they very rarely develop good stereopsis. 

    PARALYTIC SQUINT

    Paralytic squint is caused by decreased function of one or more EOMs, due to:

    • Palsy of cranial nerve III, IV and/or VI: palsies may be isolated or multiple; unilateral or bilateral.
    • EOM disease/ocular myopathies: tend to be bilateral, and are generally associated with painful eye movement
    • Tethering of the globe: e.g. after an orbital fracture, orbital neoplasm.

     

    Presentation

    • Diplopia = main presenting complaint of patients with paralytic squint. It may be horizontal/vertical/oblique doubling of vision.
    • Other symptoms associated with underlying pathological process

    Diplopia in paralytic squint is binocular - i.e. double vision is corrected when one eye is occluded. It should be differentiated from monocular diplopia (i.e. patient still sees double when one eye is occluded), which is usually due to refractive error or cataract. 

    Notably, paralytic squint is not typically associated with amblyopia as the affected patient is usually older and has surpassed the critical period of visual development. Even in cases of congenital paralytic squint, amblyopia does not develop as there is usualy binocular vision in most directions of gaze, including primary position - squint is only manifest in one or two cardinal positions. 

     

    Cranial nerve palsies

      The presentation of cranial nerve palsies along with common causes are discussed in the table below. Diplopia associated with cranial nerve palsies is usually constant:

      Table 4: Summary of nerve palsies and their causes

      EOM Disease

      A few of the important EOM pathology associated with double vision and squint are described in the table below:

      Table 5: Summary of EOM disease

      Management of non-paralytic squint:

      1. Identify and treat the underlying cause: variable (e.g. diabetes/thyroid control, aneurysm coiling, anti-inflammatories, etc). 
      2. Correction of residual squint/double vision: usually with prisms in glasses. Surgical correction is rarely indicated/useful in this group.

      PSYCHOSOCIAL IMPACT OF SQUINT

      Although the mains aims of managing squint are the reduction of double vision and the prevention of amblyopia, it is important to be respectful and considerate about the holistic impact that squint can have on the individual. Patients may feel extremely self-conscious or develop negative body image. They may feel the stigma of having a squint in other aspects of their life as well, including peer-interaction, job opportunities and relationships. Studies have shown that both children and adults find squints disturbing to look at, irrespective of their degree of personal experience with them.

      Additionally, new onset diplopia has ramifications for driving. Patients are required to inform the DVLA and cease driving, until their diplopia has been adequately corrected by glasses/wearing a patch while driving.

      REFERENCES

      • Jame B, CHew C, Bron A. [Lecture notes series] Ophthalmology. 10th ed. Blackwell Publishing: Oxford. 2007
      • Friedman NJ, Kaiser PK. Essentials of Ophthalmology.1st ed. Saunders Elsevier. 2007.
      • Denniston A, Murray P. [Oxford handbook series] Oxford Handbook of Ophthalmology. Oxford university press: Oxford. 2008
      • Scott O. [Patient.co.uk Professional Reference] Squints. EMIS group. Available online at: http://www.patient.co.uk/doctor/Squints.htm [updated 2011 Jun cited 2012 May]
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