Strabismus

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For the protein Strabismus, see Strabismus (protein).
Strabismus
Classification and external resources
Strabismus.jpg
Strabismus prevents the eyes from aiming at the same point in space
ICD-10 H49H50
ICD-9 378
OMIM 185100
DiseasesDB 29577
MedlinePlus 001004
MeSH D013285

Strabismus (/strəˈbɪzməs/, from Greek strabismós[1]), also known as heterotropia, is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space, thus hampers proper binocular vision, and which may adversely affect depth perception. Strabismus is primarily managed by ophthalmologists, optometrists, and orthoptists. Strabismus is present in about 4% of children. Treatment should be started as soon as possible to ensure the development of the best possible visual acuity[2][3] and stereopsis.

Diagnosis and classification[edit]

During an eye examination, a test such as cover testing or the Hirschberg test, are used in the diagnosis and measurement of strabismus and its effect on vision. Several classifications are made when diagnosing strabismus.

Latency[edit]

Strabismus can be manifest or latent. A manifest deviation, or heterotropia, is present while the patient views a target binocularly, with no occlusion of either eye. The patient is unable to align the gaze of each eye to achieve fusion. A latent deviation, or heterophoria, is only present after binocular vision has been interrupted, typically by covering one eye. This type of patient can typically maintain fusion despite the misalignment that occurs when the positioning system is relaxed. Intermittent strabismus is a combination of both of these types, where the patient can achieve fusion, but occasionally or frequently falters to the point of a manifest deviation.

Onset[edit]

Strabismus may also be classified based on time of onset, either congenital, acquired, or secondary to another pathological process. Many infants are born with their eyes slightly misaligned, and typically is outgrown by six to 12 months of age.[4] Acquired and secondary strabismus are developed after normal binocular vision has developed. In adults with previously normal alignment, the onset of strabismus usually results in double vision.

Laterality[edit]

Strabismus may be classified as unilateral if the one eye consistently deviates, or alternating if either of the eyes can be seen to deviate. Alternation of the strabismus may occur spontaneously, with or without subjective awareness of the alternation. Alternation may also be triggered by various tests during an eye exam.[3]

Direction[edit]

Horizontal deviations are classified into two varieties. Eso describes inward or convergent deviations towards the midline. Exo describes outward or divergent misalignment. Hyper is the term for an eye whose gaze is directed higher than the fellow eye while hypo refers to an eye whose gaze is directed lower. Torsional strabismus occurs when the eyes rotate around the anterior-posterior axis to become misaligned, which is quite rare.

Naming[edit]

The directional prefixes are combined with -tropia and -phoria to describe various types of strabismus. For example, a constant left hypertropia exists when a patient's left eye is always aimed higher than the right. A patient with an intermittent right esotropia has a right eye that occasionally drifts toward the patient's nose, but at other times is able to align with the gaze of the left eye. A patient with a mild exophoria can maintain fusion during normal circumstances, but when the system is disrupted, the relaxed posture of the eyes is slightly divergent.

Other considerations[edit]

Strabismus can be further classified as follows:

  • Paretic strabismus is due to paralysis of one or several extraocular muscles.
  • Nonparetic strabismus is not due to paralysis of extraocular muscles.
  • Comitant strabismus is a deviation that is the same magnitude regardless of gaze position.
  • Noncomitant strabismus has a magnitude that varies as the patient shifts his or her gaze up, down, or to the sides.

Nonparetic strabismus is generally concomitant.[5] Paretic strabismus can be either comitant or noncomitant.

When the misalignment of the eyes is large and obvious, the strabismus is called large-angle, referring to the angle of deviation between the lines of sight of the eyes. Less severe eye turns are called small-angle strabismus. The degree of strabismus can vary based on whether the patient is viewing a distant or near target.

Pseudostrabismus is the false appearance of strabismus. It generally occurs in infants and toddlers whose bridge of the nose is wide and flat, causing the appearance of esotropia due to less sclera being visible nasally. With age, the bridge of the child's nose narrows and the folds in the corner of the eyes become less prominent.

Signs and symptoms[edit]

Aligned vergence; how one ideally views objects
Aligned vergence; how one ideally views objects
Arrow/dotted line indicates fixation distance: All three patients are fixating with their right eye (assuming an overhead view).

When observing a patient with strabismus, the misalignment of the eyes may be quite apparent. A patient with a constant eye turn of significant magnitude is very easy to notice. However, a small magnitude or intermittent strabismus can easily be missed upon casual observation. In any case, an eye care professional can conduct various tests, such as cover testing, to determine the full extent of the strabismus.

This symptom can be seen in Down syndrome, cerebral palsy, and Edwards syndrome.

Symptoms of strabismus include diplopia and/or eye strain. To avoid double vision, the brain may adapt by ignoring one eye. In this case, often no noticeable symptoms are seen other than a minor loss of depth perception. This deficit may not be noticeable in someone who has had strabismus since birth or early childhood, as they have likely learned to judge depth and distances using monocular cues. However, a constant unilateral strabismus causing constant suppression is a risk for amblyopia in children. Small-angle and intermittent strabismus are more likely to cause disruptive visual symptoms. In addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading, and unstable or "jittery" vision.

Pathophysiology[edit]

The extraocular muscles control the position of the eyes. Thus, a problem with the muscles or the nerves controlling them can cause paralytic strabismus. The muscles are controlled by cranial nerves III, IV, or VI. An impairment of cranial nerve III causes the associated eye to deviate down and out and may or may not affect the size of the pupil. Impairment of cranial nerve IV, which can be congenital, causes the eye to drift up and perhaps slightly inward. Sixth nerve palsy causes the eyes to deviate inward and has many causes due to the relatively long path of the nerve. Increased cranial pressure can compress the nerve as it runs between the clivus and brain stem.[2] Also, if the doctor is not careful, twisting of the baby's neck during forceps delivery can damage cranial nerve VI.

Evidence indicates a cause for strabismus may lie with the input provided to the visual cortex.[6] This allows for strabismus to occur without the direct impairment of any cranial nerves or extraocular muscles.

Strabismus may cause amblyopia due to the brain ignoring one eye. Amblyopia is the failure of one or both eyes to achieve normal visual acuity despite normal structural health. During the first seven to eight years of life, the brain learns how to interpret the signals that come from an eye through a process called visual development. Development may be interrupted by strabismus if the child always fixates with one eye and rarely or never fixates with the other. To avoid double vision, the signal from the deviated eye is suppressed, and the constant suppression of one eye causes a failure of the visual development in that eye.

Also, amblyopia may cause strabismus. If a great difference in clarity occurs between the images from the right and left eyes, input may be insufficient to correctly reposition the eyes. Other causes of a visual difference between right and left eyes, such as asymmetrical cataracts, refractive error, or other eye disease, can also cause or worsen strabismus.[2]

Accommodative esotropia is a form of strabismus caused by refractive error in one or both eyes. Due to the near triad, when a patient engages accommodation to focus on a near object, an increase in the signal sent by cranial nerve III to the medial rectus muscles results, drawing the eyes inward. If the accommodation needed is more than the usual amount, such as with people with significant hyperopia, the extra convergence can cause the eyes to cross.

Psychosocial effects[edit]

People of all ages may experience psychosocial difficulties if they have noticeable strabismus.[7][8] Both large-angle and small-angle strabismus can negatively affect self-esteem, as it interferes with normal eye contact, often causing embarrassment, anger, and feelings of awkwardness.[9] Adult and children observers perceive a right heterotropia as more disturbing than a left heterotropia, and children observers perceive an esotropia as worse than an exotropia.[10] Successful surgical correction of strabismus is known to have positive effects on psychological well-being, even when implemented with adult patients.[11][12]

Management[edit]

Surgery to correct strabismus on an eight-month-old infant

As with other binocular vision disorders, the primary therapeutic goal for those with strabismus is comfortable, single, clear, normal binocular vision at all distances and directions of gaze.[13]

Whereas amblyopia (lazy eye), if minor and detected early, can often be corrected with use of an eye patch on the dominant eye and/or vision therapy, the use of eye patches is unlikely to change the angle of strabismus. Strabismus is usually treated with a combination of eyeglasses, vision therapy, and surgery, depending on the underlying reason for the misalignment. Surgery does not change the vision[citation needed]; it attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles and is frequently the only way to achieve cosmetic improvement and restoring[citation needed][clarification needed] binocular vision. The procedure can typically be performed in about an hour, and requires about one or two weeks for recovery. Adjustable sutures may be used to permit refinement of the eye alignment in the early postoperative period.[14]

Double vision can rarely result, especially immediately after the surgery,[citation needed] and vision loss is very rare. Glasses affect the position by changing the person's reaction to focusing. Prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position.[15]

Early treatment of strabismus in infancy may reduce the chance of developing amblyopia and depth perception problems. Most children eventually recover from amblyopia if they have had the benefit of patches and corrective glasses. Amblyopia has long been considered to remain permanent if not treated within a critical period, namely before the age of about seven years;[4] however, recent discoveries give reason to challenge this view and to adapt the earlier notion of a critical period to account for stereopsis recovery in adults.

Eyes that remain misaligned can still develop visual problems. Although not a cure for strabismus, prism lenses can also be used to provide some temporary comfort for sufferers and to prevent double vision from occurring.

Botulinum toxin therapy is used for treating strabismus in certain circumstances. In 1989, the Food and Drug Administration approved botulinum toxin type A (BT-A) as a treatment for strabismus in patients over 12 years old.[16][17] Most commonly used in adults, the technique is also used for treating children, in particular children affected by infantile esotropia.[18][19] The toxin is injected in the stronger muscle, causing temporary and partial paralysis. The treatment may need to be repeated three to four months later once the paralysis wears off. Common side effects are double vision, droopy eyelid, overcorrection, and no effect. The side effects typically resolve also within three to four months.

Prognosis[edit]

When strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated eye. Even with therapy for amblyopia, stereoblindness may occur. The appearance of strabismus may also be a cosmetic problem. One study reported 85% of adult strabismus patients "reported that they had problems with work, school, and sports because of their strabismus". The same study also reported 70% said strabismus "had a negative effect on their self-image".[20] After surgery, the squint can return, so a second operation is sometimes required to straighten the eyes.[2]

See also[edit]

References[edit]

  1. ^ Online Etymology Dictionary
  2. ^ a b c d e Emmett T. Cunningham, Paul Riordan-Eva. Vaughan & Asbury's general ophthalmology. (18th ed.). McGraw-Hill Medical. ISBN 978-0071634205. 
  3. ^ a b c Neil J. Friedman, Peter K. Kaiser, Roberto Pineda (2009). The Massachusetts Eye and Ear Infirmary illustrated manual of ophthalmology (3rd ed.). Saunders/Elsevier. ISBN 978-1437709087. 
  4. ^ a b Nield, LS; Mangano, LM (April 2009). "Strabismus: What to Tell Parents and When to Consider Surgery". Consultant 49 (4). 
  5. ^ Definition by TheFreeDictionary
  6. ^ Lawrence Tychsen (2012). "The Cause of Infantile Strabismus Lies Upstairs in the Cerebral Cortex, Not Downstairs in the Brainstem". Archives of Ophthalmology 130 (8). pp. 1060–1061. doi:10.1001/archophthalmol.2012.1481. 
  7. ^ Denise Satterfield, John L. Keltner, Thomas L. Morrison: Psychosocial Aspects of Strabismus Study, JAMA Ophthalmology, August 1993, Vol 111, No. 8, doi:10.1001/archopht.1993.01090080096024
  8. ^ S.E. Olitsky; S. Sudesh; A. Graziano; J. Hamblen; S.E. Brooks; S.H. Shaha (August 1999). "The negative psychosocial impact of strabismus in adults". J. AAPOS 3 (4). pp. 209–211. 
  9. ^ Strabismus, by All About Vision, Access Media Group LLC
  10. ^ S.M. Mojon-Azzi, A. Kunz, D.S. Mojon: The perception of strabismus by children and adults, Graefes Arch. Clin. Exp. Ophthalmol. 2011 May, Vol. 249, Number 5, pp. 753-757. doi:10.1007/s00417-010-1555-y
  11. ^ J.P. Burke, C.M. Leach, H. Davis: Psychosocial implications of strabismus surgery in adults, J. Pediatr. Ophthalmol. Strabismus, 1997 May-Jun;34(3):159-164, PMID 9168420
  12. ^ J.M. Durnian, C.P. Noonan, I.B. Marsh: The psychosocial effects of adult strabismus: a review, Br. J. Ophthalmol. 2011 Apr;95(4):450-453. doi:10.1136/bjo.2010.188425
  13. ^ Eskridge JB (October 1993). "Persistent diplopia associated with strabismus surgery". Optom Vis Sci 70 (10): 849–53. doi:10.1097/00006324-199310000-00013. PMID 8247489. 
  14. ^ Parikh, RK; Leffler, CT (July 2013). "Loop suture technique for optional adjustment in strabismus surgery". Middle East African Journal of Ophthalmology 20 (3): 225–8. doi:10.4103/0974-9233.114797. PMC 3757632. PMID 24014986. 
  15. ^ a b "Strabismus". MedlinePlus. Retrieved 5 April 2013. 
  16. ^ United States Department of Health and Human Services (April 30, 2009). "Re: Docket No. FDA-2008-P-0061" (PDF, 8.2 MB). Food and Drug Administration. Retrieved 2014-04-06. 
  17. ^ Kowal, L., Wong, E., & Yahalom, C. (2007). Botulinum toxin in the treatment of strabismus. A review of its use and effects. Disability & Rehabilitation, 29(23), 1823-1831.
  18. ^ Thouvenin D, Lesage-Beaudon C, Arné JL (January 2008). "(translated from French) Botulinum injection in infantile strabismus. Results and incidence on secondary surgery in a long-term survey of 74 cases treated before 36 months of age". J Fr Ophtalmol. 31 (1). pp. 42–50. PMID 18401298. 
  19. ^ de Alba Campomanes AG, Binenbaum G, Campomanes Eguiarte G (April 2010). "Comparison of botulinum toxin with surgery as primary treatment for infantile esotropia". J AAPOS 14 (2). pp. 111–116. doi:10.1016/j.jaapos.2009.12.162. PMID 20451851. 
  20. ^ Scribe/Alum Notes Winter 2001 – Template