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1) OPHTHALMOLOGIA CASE REPORT

 

A 19-month-old girl was referred to orthoptics by the local health centre, following observation of a right convergent squint (RCS). The family had not noticed the squint until it was mentioned by the health care worker, after which, they noted that it was most obvious when the child was tired. Her past medical history was uneventful. At the date of presentation her general health was good, though she had eczema on her right elbow. She was not on any medications. She had no past ocular history. Her two brothers had keratoconus and her mother is myopic and had a squint, as did 2 of her cousins.

   On initial examination, her visual acuity was 6/7.5 in both eyes with Cardiff Acuity Cards[1]. A slight RCS with slight right over left (R/L) hypertropia were seen[2]. It was noted that she was not taking up central fixation with the right eye and objected to the occlusion of her left eye. Using Hirschberg’s test she was estimated to have a 20Δ esotropia. The remainder her examination was unremarkable.

   Part-time occlusion therapy was started when she was 22 months old. The left eye was occluded for three weeks from 7.30 am-8 pm followed by one week off.

   Following this treatment, her visual acuity improved to 6/6 right and left with persistence of her previous deviation. Her ocular eye movements were estimated as:

 

 

 

 Her fixation had improved and her right eye’s visual acuity had increased. At this point occlusion therapy was stopped and a follow-up appointment made 10 weeks later.

At that appointment her visual acuity was 6/7.5 in both eyes. Her ocular movement was estimated as illustrated in figure 2, demonstrating a marked increase in inferior oblique overactivity compared to that seen when she presented 7 months previously.

 

 

 

 

On examination her squint and R/L hypertropia had increased. Using Hirschberg’s test she was estimated to have a 20/25Δ esotropia.

   She was seen one month later, at 27 months old. At this appointment her visual acuity had deteriorated to right 6/18 and left 6/9. This marked difference suggested that strabismic ambylopia had developed. Her mother felt that her squint had worsened. Her ocular movements were unchanged. Using Hirschberg’s test she was estimated to have a 25Δ esotropia with a 6Δ R/L hypertropia. Following these findings, occlusion therapy was restarted with the patch to be worn every evening.

   Following 6 weeks of treatment, her visual acuity was 6/9-1 bilaterally. Her conditioned was otherwise unchanged. Surgical intervention was discussed and occlusion therapy continued for 1 hour each day, to maintain the visual acuities. She was observed to hold right fixation well with stimulus.

   When she was seen one month later, at 30 months old, her visual acuities were right 6/12 and left 6/12-1. Her ocular movements were unchanged. Her Prism Cover Test for near measured an esotropia of 25Δ with 10Δ of height.  She starting to freely alternate and was advised to continue with the occlusion therapy for an hour every day. After dilation of both eyes with cyclopentalate, her cycloplegic refraction was recorded as:

 

 

 

 

PRESCRIPTION[3]: +1.00/ +0.25 x 90                          +0.50/+0.50 x 90

 

 

 

 

This showed a slight hypermetropia and astigmatism. It was thought that glasses would not greatly improve her squint and she was therefore not prescribed any.

Surgery for the squint was discussed and she was listed.

   Her pre-operative orthoptic assessment recorded her visual acuity as right 6/6-1 assessed using Single Sheriden Gardner test.  She was noted to have a moderate right alternating convergent squint with a slight right over left. Her ocular movements were estimated as shown in figure 4. Her other measurements were unchanged.

 

 

   

 

Intra-operative forced duction test revealed no restriction. Miss Smyth performed the right sided squint surgery, in which the medial rectus was recessed 4 mm, the lateral rectus resected 6 mm and the inferior oblique recessed, and the muscle secured 4 mm behind the temporal insertion of the inferior rectus.

 

 

 

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 2) DISCUSSION- THE MANAGEMENT OF STRABISMIC AMBYLOPIA

 

 

INTRODUCTION

From the findings of her orthoptic examinations she had an acquired non-accommodative esotropia with inferior oblique overaction. This type of esotropia develops within the first 6-24 months of life and is often associated with strabismic ambylopia [1]. Ambylopia is a type of cerebral visual impairment resulting from abnormal vision that occurs during the first 7 years of life in which the developing brain is sensitive to such changes. Treatment is therefore only effective when given during this sensitive period. The prevalence of ambylopia is approximately 2-4% [2]. It is the most common visual defect in children in the UK [3]. The most frequent forms being monocular, due to an uncorrected refractive error or a squint (strabismic ambylopia). This impairment can affect binocular vision and stereopsis, thus barring certain professions and causing considerable disability should the normal eye suffer any disease or trauma. The corollary of this is that all children are screened pre-school with the aim of detecting unilateral ambylopia at a stage when effective treatment can be given. Those presenting with a visible squint or those with a visual acuity of <6/9 are referred to the orthoptic clinic [2].

 

AETIOLOGY & PATHOPHYSIOLOGY

Strabismic ambylopia is thought to be caused by the development of a dominant fixing eye. The non-fixing eye subsequently becomes amblyopic. This occurs more often in smaller-angle esotropics than larger-angle esotropics, in whom cross-fixation may occur as an adaption to their strabismus preventing the development of ambylopia. However, when fixation preference is established in a child under 6, the non-fixing eye is at risk of developing ambylopia. Research suggests that it involves a defect in the perception of form and shape within the cortex; this is seen as a decreased ability to see small objects especially with low contrast [4].

 

DIAGNOSIS

Strabismic ambylopia is diagnosed when the vision in one eye is two lines, or worse, compared to the other eye. Visual acuity may be decreased to 6/30 but is rarely below this. If visual acuity is <6/60, anisometropic ambylopia or other ocular disease should be suspected [4].

   Accommodation is reduced in amblyopic patients who can subsequently have the same visual acuity in the dark as they do in the light, with their amblyopic eye, in contrast to patients with retinal pathology and healthy individuals who would lose several lines of vision in the dark [4].

   Ambylopic patients may be able to identify single optotypes, however, they will find it difficult to distinguish several similarly sized optotypes if they are grouped together- this is known as the crowding phenomenon [4].

  

Visual Acuity Measurement

The gold standard in the evaluation of amblyopia is accurate assessment of visual acuity. Snellen and Logmar measurements are preferred, as other methods tend to underestimate the amblyopia, however preverbal children cannot read these charts chart [4].

   Since near visual acuity may improve before that for distance, visual acuity for both near and distance vision should be assessed, if possible, during the treatment [4].

 

Fixation Reflex

In a large number of strabismic patients the only way to evaluate and detect ambylopia is the use of the cover test [4]. The patient is asked to look directly at a light or target held at a central position, one eye is covered throughout the test but the cover is moved from eye-to-eye and the initial movement of the eye to take up fixation is observed as the cover is taken off that eye [5]. Freely alternating strabismic patients with no ambylopia will show a steady and central fixation of the light. Amblyopic patients may exhibit central, steady fixation under monocular viewing but under binocular conditions, fixation is immediately re-established by the dominant prefixing eye with immediate deviation of the amblyopic eye. The amblyopic eye is thus described to have central, steady but unmaintained fixation. In more severe ambylopia the non-fixing eye may be unable to take up fixation, even under monocular viewing conditions, and may even show nystagmus. Ambylopia is said to exist if anything other than the combination of central, steady and maintained fixation is observed [4].

 

Differential Diagnosis

Monocular visual loss may be secondary to intraocular pathology, refractive error, optic disc atrophy or small lens opacities. Thus an extensive ophthalmic and orthoptic examination, including cycloplegic refraction, should be carried out to exclude any of these before the diagnosis of ambylopia is made [4].

 

MANAGEMENT

The Management of Amblyopia Pre- & Post- Strabismus Surgery by Occlusion Therapy

This aims to equalize the visual acuities [1] and to centralise fixation [3]. If binocular single vision is restored by the surgical re-alignment of the visual axis visual acuity is more likely to be maintained [1].

 

Occlusion Therapy

 The form of occlusion therapy is dependent upon the difference of vision between the two eyes. Visual acuity of <6/60 indicates that full time occlusion therapy should be initiated. Occlusion to the non-strabismic eye should continue if the better eye fixes following removal of the patch. If the visual acuity improves during therapy it can be changed to part-time [1]. However, if total occlusion is undertaken the physician must remember that ambylopia may develop in the occluded eye, therefore, weekly follow-ups are recommended [4]. If improvement has ceased for over two months alternative treatment may be necessary [1]. In approximately 40% of patients, visual acuity falls in the ambylopic eye following cessation of occlusion therapy, therefore, careful monitoring of patients with amblyopia after cessation of occlusion therapy is essential and patching should be restarted if the visual acuity does decrease in the amblyopic eye. [4]

   Non-compliance is the main problem encountered in occlusion therapy, a study by Foley-Nolan, McCann and O’Keefe (1997) that 45% of their subjects were non-compliant at some stage during the trial [7]. A different study by Searle, Harrad and Vedhara [3] found that only 54% of patients managed to comply fully with the orthoptist’s patching regimens and that the parents had little faith either in the effectiveness of patching or in their ability to do so correctly [3].

   Conventional occlusion therapy is not always possible in all amblyopic children, due to skin sensitivity, latent nystagmus, emotional difficulties in wearing the patch or due to complete lack of co-operation [7].

 

Risks of Occlusion

·        Allergic response

·        Occlusion amblyopia

·        Intractable diplopia (  may occur in strabismic patients treated when they are over 7, unless they have anisometropic ambylopia)

·        Increased and of deviation following cessation of treatment

·        Danger associated with disorientation [6].

·         

Cycloplegic Drugs- Atropine

Atropine (1%) can be used once daily to obscure the vision in the better eye when occlusion is not tolerated, thus increasing the stimulus to the amblyopic eye[6].  Comparison between atropine therapy and occlusion therapy showed that atropine was as effective as occlusion therapy and had a higher degree of patient acceptability. However, the mean duration of therapy for the atropine group was longer, at 7.2 months, compared to 4.3 for the occlusion group. A clear advantage of atropine over occlusion therapy is that the examiner is easily able to check compliance, by checking for the fixed dilated pupil [7].

 

Disadvantages

·        Side effects

·        The visual acuity may not be reduced substantially for the amblyopic eye to take up fixation in dense amblyopia

·        Its effects last 10-14 days [6].

·        It requires frequent visits to the hospital to monitor fixation and visual acuity [6].

·        May cause the development of a myopic refractive error, based on animal studies [7].

 

Lens Penalisation

If occlusion therapy is not tolerated this is another alternative. It involves the reduction of visual acuity of the non-amblyopic eye at one or all fixation distances. This is achieved by changing the spectacle correction and may be enhanced by the concurrent use of cycloplegic drugs in the non-amblyopic eye. [6]

   In total penalisation, a strong convex lens is placed before the non-amblyopic eye. Near penalisation can be achieved by using cycloplegic drugs in the non-amblyopic eye and using a convex lens of up to 3.0DS in front of the amblyopic eye. Distance penalisation is achieved by placing a convex lens before the better eye, thus using the amblyopic eye for distance fixation [6].

 

Neurotransmitter Therapy

Preliminary research suggests that levodopa may play a role in the neural systems involved in ambylopia. A corollary of this is that amblyopia may be partially responsive to these types of chemicals yet at present there is little clinical evidence to back up this claim [4].

 

 References of the discussion:

[1] Ansons AM, Davis H. Diagnosis and Management of Ocular Motility Disorders. 3^rd ed. Oxford: Blackwell Science Ltd; 2001. p221.

 

[2] Clarke M, Wright CM, Hrisos S, Anderson JD, Henderson J, Richardson SR. Randomised controlled trial of treatment of unilateral visual impairment detected at preschool vision screening. BMJ. 2003; 327:1251

 

[3] Gregson R. Why are we so bad at treating ambylopia?  Eye. 2002; 16: 461-462.

 

[4] Good WV, Hoyt CS. Strabismus Management. 1^st ed. Oxford: Butterworth- Heinemann; 1996. p17-27

 

[5]Ansons AM, Davis H. Diagnosis and Management of Ocular Motility Disorders. 3^rd ed. Oxford: Blackwell Science Ltd; 2001. p75.

 

[6] Rowe F. Clinical Orthoptics. 1^st ed. Oxford: Blackwell Science; 1997. p131-135.

 

[7] Foley-Nolan A, McCann Aoife, O’Keefe M. Atropine penalisation versus occlusion as the primary treatment for ambylopia. Br J Ophthalmology. 1997; 81:54-57.