| |
| |
| |
 |
 |
|
Strabismus (cross-eye): Diagnosis and Treatment.Elena Piozzi - Alessandra Del Longo |
|||||||||||||||||||||
|
Strabismus is a frequent disease in children (3-5% incidence).
Normal eye position is called orthophoria
Convergent strabismus counts 80% of eye deviations with 65%
of unilateral deviations, which always effect the same eye, and
35% of alternating deviations, which affect both one eye and the
contralateral
Corneal Reflexes They exploit the cornea’s mirroring features. The penlight stimulus pointed before the eyes is placed symmetrically in the centre of the pupil in orthophoric patients. A shifted reflection with reference to the centre and its asymmetry are a sign of deviation.(fig.2)
Cover test The apparently staring eye is covered to evaluate the behaviour both of the same eye and of the contralateral one when the screen is removed. No movement is appreciated in conditions of orthotropia. In cases of unilateral strabismic deviation, the deviated eye straightens when the apparently staring eye is covered, but it returns to the strabismic position when the screen is removed. In cases of alternating deviation, when the apparently staring eye is covered the contralateral one straightens and maintains the position after the screen is removed. 4 Dioptre Vision Test A 4 dioptre prism placed before the eye of normal individuals enables to observe the behaviour of the contralateral eye, which first has a rapid movement towards the apex of the prism and a second slow returning movement. In cases of microstrabismus no movement is observed when a prism is placed before the eye that is not staring. (fig.10)
Refraction The refraction test must always be performed after cycloplegia through the administration of colliria such as cyclopentolate, which is currently the most frequently used one. Some authors prefer to use 1% tropicamide. Atropine is currently reserved to certain cases. The examination, which evaluates the presence of refractive defects is called sciascopy. In collaborative patients sciascopy, which remains the main test, is currently combined with autorefractometry, which is a computerised detection method of refraction. Sight defects are hypermetropia, myopia and astigmatism. Either associated with astigmatism or not, hypermetropia is the most common defect that has been found in convergent strabismus. Myopia is more frequently associated with divergent strabismus. The correction of the visual defect when strabismus is present must be complete and consider the values found in cycloplegia. Examination of the anterior segment and of the fundus oculi The evaluation of the anterior segment by means of a slit lamp and the examination of the fundus oculi complete the eye examination and are necessary to rule out eye diseases as a cause of strabismus. Visual Acuity Test The type of test required to detect visual acuity depends on the patient’s age. Along with the well known methods that make use of optotypes with figures, E tests and letters, we wish to mention tests that can be used in a pre-oral age with Teller’s Cards. These cards enable to measure visual acuity in children under 12-15 months. It is a psychophysical test based on the child’s ability to recognise structured stimuli (black and white stripes) against an even grey background. The small patient is presented gradually decreasing stimuli and the evaluation is based on the screen with the smallest black and white stripes that can evoke a visual response. (Fig. 5 - Fig. 6 – Fig. 7) A difference in the response to the test between the two eyes is a sign of amblyopia. Another simple test, which can be used in early infancy, is the occlusion test, which evaluates the child’s reactions when the healthy eye is closed.
Treatment principal Medical Treatment The first treatment envisages complete optical correction of the defect in refraction with permanent lenses. When amblyopia is present the choice treatment is occlusion of a variable duration depending on the patient’s age. (fig. 3)
The occlusion must not be passive but active (games or work for near sight), both to achieve better patient compliance and to obtain satisfactory results in a shorter time. The maintenance treatment after occlusion makes use of penalising methods such as the use of filters applied to lenses. The use of adequately built bifocal lenses is recommended in the forms characterised by excessive convergence (high AC/A ratio). (Fig. 11)
Surgical Treatment The end of surgical treatment is both functional and aesthetical. Surgical treatment is the final action in the diagnostic-rehabilitative course of strabismic patients who require a repeated series of tests and examinations. Surgery must possibly be recommended when there is alternating strabismus and equal vision in both eyes. Surgical treatment aims at removing the visual axes’ relative deviations. Surgery is performed on eye muscles to influence the very muscles’ activity and subsequently eye alignment. Operations are hence distinguished either in surgery focused either on weakening muscle action (withdrawal) or on strengthening it (resection). The choice of the type and extent of surgery depends on the assessment of eye motility, on the characteristics of the deviation and on the measurement of the same. Let us now study some special cases of strabismus. Essential infantile esotropia Strabismus with an early onset (essential infantile esotropia) sets in during the first 4-6 months of life: parents often notice the deviation at birth. This deviation is characterised by: - large deviation angle (often more than 50 dioptres) - crossed fixation - abduction deficit (due to the lateral rectal muscle’s failure to contract) latent nystagmus - ocular stiff neck (not always present) - mild hypermetropia (rarely high) - hyperfunction of the mall oblique muscles (it can be noticed after the child’s 1st year) (Fig. 4) - dissociated vertical deviation (to be distinguished from the hyperfunction of the small oblique muscles).
Diagnosis When there is strabismus an eye examination must be performed as soon as possible to highlight the presence of sensory anomalies, the most important of which is amblyopia (sight deficit due to lack of use). When amblyopia is present early treatment is essential to obtain good results and the total recovery of sight. Treatment correction of the refractive defect occlusion traditional surgery (withdrawal of the median rectal muscles either associated or not with the withdrawal of the small oblique muscles). Botulinum toxin treatment has been recently used as an alternative to traditional surgery. The injection is administered to the median rectal muscles according to a well coded method within the child’s 7th – 8th month of life. Literature has reported satisfactory results in terms of eye alignment (60-70%). (Fig. 8 and Fig. 9)
|
|||||||||||||||||||||
