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Strabismus: background and surgical techniques.

Introduction

The visual pathways and especially the visual cortex develop in infancy and childhood, influenced by the visual experience of the individual. Visual acuity, along with the other modalities of contrast sensitivity, colour, pattern and motion perception, and binocular function with stereopsis develop in this same critical period. (Adams and Sloper 2003) An important study showed that this proper visual experience depends on the correct functioning of both eyes and proper ocular alignment early in life (Hubel and Wiesel 1965).

Strabismus and amblyopia

Ocular misalignment is known as strabismus. This term is derived from the Greek word strabismos: 'to squint, to look obliquely or askance.' Strabismus may eventually lead to amblyopia by denying proper visual experience to both eyes if affected during the critical period of growth. Amblyopia, otherwise known as lazy eye, is the term describing a decrease in best-corrected visual acuity in an otherwise structurally healthy eye. It is the result of a shrinkage of neurons in the neural pathways leading to the visual cortex of the brain, resulting from insufficient stimulation of these nerves due to lack of visual experience. Visual problems, including ocular misalignment, that occur after the age of 5 years do not cause amblyopia as development of the visual pathways is complete by this age (Donahue 2007).

Not only can strabismus cause amblyopia, but it may also be the result of poor vision. The brain locks the two eyes together only if there is a corresponding image from the two eyes. If there is a degraded image from one eye, this eye can drift and cause a squint. Causes may for example include anisometropia (large difference in the spectacle correction between the two eyes) or congenital cataract. Squint and amblyopia are both common conditions: about one in fifty children have a squint, and up to 5% of the population have amblyopia (Adams and Sloper 2003).

Strabismus may be caused by various anomalies and many terms and classifications are used to describe them. Orthophoria means straight eyes or ocular balance. Misalignment can be described as latent or manifest. Latent squints result in the ocular misalignment being suppressed by the ocular fusional mechanisms (the mechanisms by which both eyes lock onto the same target when a corresponding image reaches both eyes), whereas manifest squints are not. The directional nature of the deviations may be convergent, divergent, vertical or rotational. Each of these directional squints may be latent or manifest. The magnitude of the ocular misalignment is measured in prism dioptres, which relates to the angle between the two eyes.

At birth, a baby's vision is quite poor and most children have a mild divergent squint. The strabismus usually resolves by 3 months of age. If greater than 15 degrees or if persisting beyond 3 months, it can be considered abnormal (Archer et al 1989, Donahue 2007, Adams and Sloper 2003). Paediatric strabismus must be treated early to maximize the potential for binocular vision and decrease the risk of amblyopia. Treatment goals include good vision in each eye (no amblyopia) and straight eyes (orthotropia). Both conditions are necessary to produce stereopsis, which is a third goal (Archer et al 1989, Donahue 2007).

Most patients with convergent squints present before school age, usually between the ages of 2 and 3 years. These are intermittent initially but within a few weeks become constant. An intermittent deviation does not preclude the development of stereopsis; however, failure to realign the eyes of children whose eyes are constantly convergent results in lifelong abnormal depth perception. Patients with divergent squints most commonly present between the ages of 1 and 4 years; the condition nearly always remains intermittent and is therefore associated with good binocular vision. Vertical ocular deviations occur in children, in isolation or associated with horizontal strabismus, but are much less common and are not discussed here (Donahue 2007).

New onset of strabismus in school-aged child is uncommon and necessitates full neurological examination. Most cases however are usually a recurrence of a partially treated strabismus earlier in life, which recurs because of a relative deficiency in fusion (ability to maintain binocular vision) (Donahue 2007).

Adult strabismus is different from paediatric strabismus in that there is no risk of developing amblyopia, and binocular vision can be restored when the strabismus is corrected. Most adult strabismus represents recurrence of childhood strabismus, even after years of orthophoria. Where the childhood strabismus resulted in a lack of binocular vision and stereopsis or where there is untreated amblyopia, recurrence is more likely as there is reduced stimulus to maintain orthophoria. This underscores the importance of aggressive treatment in childhood amblyopia and strabismus, aiming to restore binocular vision, expand visual field and potentially preserve stereopsis (Quah and Kaye 2004, Donahue 2007).

Strabismus may also occur in adults after an insult to the ocular motor system, either directly to the central nervous system or the cranial nerves themselves. Important causes include vascular (eg stroke) and inflammatory disease, infiltrative processes (including Graves' disease), myasthenia gravis and trauma. Patients are usually symptomatic with diplopia (Donahue 2007).

Pre-operative considerations

Before surgery for correction of strabismus is considered, a thorough history must be taken. This must include details of the pregnancy, delivery, development, medical and ophthalmic history and family history. Detailed motility evaluation, often repeated at subsequent visits must be carried out along with full ocular examination.

Treatment of significant refractive errors and amblyopia must precede surgery because failure to correct these problems may compromise a stable surgical alignment of the eyes.

Refraction and, if possible, correction with spectacles must be attempted first as optical problems can result in strabismus. For example, in intermittent divergent squints even a correction of a mild myopia (short-sightedness) may improve control of the ocular deviation. Also, children with severe hypermetropia (long-sightedness) may be unable to sustain the accommodative effort for a clear image, and the lack of accommodative effort produces a blurred image and manifest divergent squint (Simon et al 2005-6).

Second, assessment by an orthoptist is usually performed. Orthoptists are trained in the assessment of patients in particular with problems of ocular motility, binocular vision, amblyopia and strabismus. Orthoptic correction with prisms or patching can be attempted. Patching has been performed for 250 years, but with increasing research more is now known about which pathologies will benefit from this therapy. This treatment is usually performed with commercially available adhesive patches, spectacle-mounted occluders or atropine drops to blur near vision. This is applied to the eye with better vision thus forcing the use of the poorer eye by limiting the stronger eye. The number of hours of patching per day should reflect the degree of amblyopia (Freeman and Isenberg 1989). Correction of refractive error may be sufficient in some cases (Simon et al 2005-6, Stewart et al 2002, Cotter et al 2007).

Although surgery is a final resort when optical and orthoptic correction has failed, the challenge is to perform surgery, where necessary, as early as possible for the best sensory outcome. The decision will depend on a number of factors but particularly on the type of squint. For example, infantile (congenital) convergent squints will rarely resolve without surgery which can therefore be planned at an earlier stage (Simon et al 2005-6). Surgery is performed to improve function, appearance and well-being. Double vision in one or all fields of gaze is a common complaint in adult-onset strabismus, and surgery is performed to relieve this. Alignment of the two eyes can restore stereopsis (binocualar vision) or allow development of stereopsis in some patients. Asthenopia (eye strain) is common in patients with latent or intermittent squints and again may need surgical correction. Some patients develop abnormal head posture to relieve diplopia or improve vision. Correction of this may improve the field of vision and reduce habitual head posturing, which may in turn improve the patient's self-image.

Strabismus surgery

Strabismus surgery is usually performed by an ophthalmologist with a specialist interest in paediatric ophthalmology or strabismus, although it is part of the general training required for all ophthalmologists to have experience of performing such procedures. Seven per-cent of strabismus surgery is performed on adults, and therefore it is important to have an operating list that has sufficient provision for paediatric cases (Adams and Sloper 2003).

General anaesthetic is almost always used and the patient draped as for cataract surgery. A forced-duction test is performed to rule out any mechanical barrier to movement. If normal, the conjunctiva is dissected providing good access to the relevant rectus or oblique muscles of the eye.

Six muscles control eye movements: four recti and two oblique muscles. The superior and inferior recti rotate the eye up and down respectively. The medial and lateral recti rotate the eye nasally and temporally. Their origins are posteriorly in the bony orbit and they insert anteriorly onto the eyeball or 'globe.' The positions of the insertions are at fixed distances from the corneal limbus and they can therefore be identified at the time of surgery (Fig.1).

The effect of a muscle can be weakened or strengthened. Strengthening procedures are achieved by resection of a segment of muscle. Weakening procedures are performed by moving the insertion of the muscle (the position at which it attaches to the globe) posteriorly along its line of action. Pre-determined formulae determined from empirical data are used to determine the amount of 'strengthening' or 'weakening' that should be performed for a particular squint and given angle of deviation. Different procedures can be performed for the same operation and the importance is the angle between the eyes, not which eye is operated. So for a divergent squint, bilateral lateral rectus recessions can be performed such that the origin of the lateral rectus muscles are moved posteriorly thus rotating both eyes inwards. Alternatively one eye can have a lateral rectus recession and a medial rectus resection so a segment of the medial rectus is removed and the muscle is reattached to its original insertion, hence strengthening this muscle. Both may give a good postoperative result.

Two important complications can arise at this most critical stage of the operation. First, when the muscle is dissected from the globe it is tethered by two sutures (usually 6-0 Vicryl) (Fig. 2). If the muscle slips off these sutures it can slip behind the globe and is impossible to recover. Further complex surgery must be performed at this stage to prevent the patient developing a secondary squint. The results are poor and this is an extremely unsatisfactory situation. Second, on reattaching the muscle to its new origin, there is a risk of perforation of the globe. As the sclera is only 0.3 mm in some places this would seem a difficult complication to avoid. However, it is thankfully uncommon as there is a risk of developing a secondary retinal detachment.

[FIGURE 1 OMITTED]

Once the muscle is reattached to its new origin, the conjunctiva is closed (usually with 8-0 Vicryl). The benefits of the absorbable Vicryl sutures are that there is no need for removal or manipulation, which could be quite challenging in young children. The eye will appear red and feel uncomfortable for up to 2 weeks after surgery although pain is uncommon.

[FIGURE 2 OMITTED]

Under or over-correction of the squint occurs because the formulae used are worked out empirically based over a large population of patients who have had surgery. In some cases further surgery may be required, however in the experience of the authors a small reduction in squint angle is often enough to resolve symptoms or deformity such that the patient is satisfied with the outcome. Put another way 'the squint operation puts the ball on the green and the brain puts it in the hole' (Jones 2008).

Serious infection is uncommon after strabismus surgery. Most commonly an allergic conjunctivitis to suture material can develop or a foreign body granuloma can develop at the suture site. Conjunctival inclusion cysts can develop if conjunctival epithelium is buried during muscle reattachment. Rarely a preseptal or orbital cellulitis can develop and frank endophthalmitis, a sight-threatening complication, has been reported. Repeat surgeries are more challenging due to postoperative scarring of the conjunctiva and it is ideal to achieve a good result at first attempt (Simon et al 2005-6).

Benefits of strabismus surgery

Before 6 years of age, classmates develop negative attitudes towards classmates with strabismus (Paysse et al 2001). Interpersonal relationships, self-image, schoolwork, and participation in sports can be affected, becoming more marked into adolescence (Satterfield et al 1993). These difficulties are reduced after correction of the squint (Archer 2005). In adulthood, strabismus can have a negative effect on social interaction (e.g., during a job interview) (Coats et al 2000). Aside from the visual and cosmetic benefits, strabismus surgery has a far more holistic effect on the patient than is immediately apparent. From our own experience, patients are more grateful for their squint surgeries than for sight-improving cataract surgeries.

Conclusion

Strabismus is a common condition, the treatment of which can improve a patient's vision if performed at the right time. Surgery for strabismus is technically straightforward, although it is not without significant risks and these should be properly discussed during the consent process. The challenge and the art is in the assessment of patients and the selection of the procedure to be performed.

It is a rewarding procedure to perform for both patient and surgeon, and the postoperative outcome is quickly apparent. The benefit goes further than the eyes alone and there is an improvement in the patient's overall quality of life. Future work might be aimed at the use of botulinum toxin-a denervation treatment which has been shown to be effective and has the benefit of being reversible (Mills et al 2004).

References

Adams GGW, Sloper JJ 2003 Update on squint and amblyopia Journal of the Royal Society Medicine 96 3-6

Archer SM, Sondhi N, Helveston EM 1989 Strabismus in infancy Ophthalmology 96 133-137-Archer SM, Musch DC, Wren PA, Guire KE, Del Monte MA 2005 Social and emotional impact of strabismus surgery on quality of life in children Journal of the American Association for Pediatric Ophthalmology and Strabismus 9 148-151

Coats DK, Paysse EA, Towler AJ, Dipboye RL 2000 Impact of large angle horizontal strabismus on ability to obtain employment Ophthalmology 107 402-405

Cotter SA, Edwards AE, Arnold RW et al 2007 Pediatric Eye Disease Investigator Group Study: Treatment of strabismic amblyopia with refractive correction American Journal of Ophthalmology 143 (6) 1060-3

Donahue SP 2007 Pediatric Strabismus 356 (10) 1040-1047

Freeman RS, Isenberg SJ 1989 The use of part-time occlusion for early onset unilateral exotropia Journal of Pediatric Ophthalmology Strabismus 26 94-96.

Hubel DH, Wiesel TN 1965 Binocular interaction in striate cortex of kittens reared with artificial squint Journal of Neurophysiology 28 1041-1059

Jones DH 2008 Quote Consultant Ophthalmologist, Royal Cornwall Hospital

Mills MD et al 2004 Strabismus Surgery for Adults: A report by the American Academy of Ophthalmology Ophthalmology 111 (6) 1255-62

Paysse EA, Steele EA, McCreery KM, Wilhelmus KR, Coats DK 2001 Age of the emergence of negative attitudes toward strabismus Journal of the American Association for Pediatric Ophthalmology and Strabismus 5 361-366

Quah SA, Kaye SB 2004 Binocular visual field changes after surgery in esotropic amblyopia Invest Ophthalmology Visual Science 45 1817-1822

Satterfield D, Keltner JL, Morrison TL 1993 Psychosocial aspects of strabismus study. Archives of Ophthalmology 111 1100-1105

Stewart CE, Fielder AR, Stephens DA, Moseley MJ 2002 Design of the monitored occlusion treatment of amblyopia study (MOTAS) British Journal of Ophthalmology 86 (8) 915-919

Simon JW et al 2005-6 Paediatric Ophthalmology and Strabismus American Academy of Ophthalmology Basic and Clinical Science Course Section 6 American Academy of Ophthalmology, San Francisco

Correspondence address: Dr N. Modi, Royal Cornwall Hospital, Truro Email: ncmodi@gmail.com

Dr Neil C Modi

Royal Cornwall Hospital, Truro

Mr David H Jones

Royal Cornwall Hospital, Truro
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Title Annotation:CLINICAL FEATURE--STRABISMUS SURGERY
Author:Modi, Neil C.; Jones, David H.
Publication:Journal of Perioperative Practice
Date:Dec 1, 2008
Words:2627
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