refraction and refractive adaptation
can be caused amblyopia where tend to be large in myopia (more than 2 D
difference between the eyes) than astigmatism (difference of >1.5) and same
hyperopia (difference of1D) (Weakley 2001). Generally, myopic anisometropia less
in incidence of amblyopia than hypermetropic anisometropia. In hypermetropic
anisometropia patients at all distances used accommodation to focusing. The eye
with greater hypermetropia will stay constantly blurred due to a greater
accommodative need for best focus, which will lead to the absence of a clear
image on the retina. However, this not occur in myopia because clear image may
be obtained without accommodation (Jampolsky et al. 1955).
amblyopia is more common and bilateral amblyopia can also occur. Isoametropic
amblyopia result from bilateral high refractive errors which occurring in
children with hyperopia greater than +4.5D (Klimek et al. 2004). Accommodation
in patients with high hyperopic refractive error lead to strabismic amblyopia. Myopic
with symmetric myopia rarely to develop amblyopia which decrease distance to
focus image (Wu and Hunter 2006). Meridional amblyopia caused by uncorrected
bilateral astigmatism which lead to blurred image in a specific meridian
(Dobson et al. 2003). Oblique maradiane most cause astigmatismic amblyopia (Abrahamsson
and Sjostrand 2003).
refractive treatment in anisometropic amblyopia including elimination of the
unilateral by correction of refractive error, which should be enhance neuronal
sensitivity and therefore stimulate visual recovery. However,
correction of refractive error in strabismic amblyopia decrease the angle of
strabismus without change cortical inhibition /suppression from the
deviating eye and therefore the mechanism of improvement is unclear (Stewart et
al. 2004a; Wu and Hunter 2006). The period of treatment is 18 weeks, which the
maximum refractive error should be prescribed for full-time wear. If acuity
through this refractive adaptation period still improve may be no need for further
treatment (occlusion) (Chen et al. 2007; Moseley et al. 2002; Stewart et al.
2004a). A clinically, improvement of corrected visual acuity being after refractive
adaptation period up to 18 weeks with spectacles in amblyopic patients in
different types (anisometropic 3-4, strabismic 4-6 or mixedage 8-6 of age
years) (Stewart et al. 2004a). 21% of the patients not require further
treatment by occlusion therapy.
and optical penalisation
penalisation is an alternative to patching for treat amblyopia. The percent of
drop is 1% used to b blocks parasympathetic innervations of the iris and
ciliary muscle lead to dilation of pupil and loss of accommodation (Holmes and
Clarke 2006). Atropine action prevents the non-amblyopic eye from accommodating,
near vision in that eye become blurred which forcing the other eye (amblyopic)
to used instead (Wu and Hunter 2006).
popular more than atropine in past because considered that patching more
effective. For this cause, atropine used
for cases that compliance with occultation is very weak and poor or patching
cannot be undertaken for any reasons. However, atropine treatment has several
advantages comparing to patching: It’s not as annoying to the patient’s
appearance, compliance obtained by once the drop is instilled (patch can be
easily removed by the child), peripheral binocularity is allowed with atropine where
completely impaired by patching and distress to the child through treatment is
rare with atropine. In other side, some disadvantages such as effect of atropine
last up to 2 weeks, while patching immediately stopped if required. Side effect
of atropine rarely occur such as dry mouth, flushing, tachycardia,
hyperactivity, possible seizures and photophobia and allergy, compared to just
minor irritation or mild allergy with patching (Holmes and Clarke 2006).
randomized study, make comparing between 1% atropine and patching fore at least
6 hours a day for treatment of moderate amblyopia (PEDIG 2002). The study
conducted on 419 children aged 3-7 years with VA of 6/12-6/30. Patching group
was faster improvement. At 6 months, visual acuity of amblyopic eye improved by
three lines approximately in both groups, which treatment effect did not vary
with type of amblyopia, age or initial visual acuity. At age 2 years follow up
examination, found visual acuity in amblyopic eye had similarly improvement 3-4
lines in both groups (PEDIG 2002, 2003).
Also, conducted another
randomised trial by PEDIG to comparing the use atropine on a daily basis versus
weekend use in children aged 3-7 with moderate amblyopia visual acuity
6/12-6/24. The improvement in visual acuity with weekend atropine was similar
to its improvement with daily atropine in both groups by more than two lines (Repka
et al. 2004).
conducted on two groups with sever amblyopia (visual acuity 20/125-20/400). Subjects
with amblyopia involve strabismus, anisometropia or both, aged 3–12 years old in this study. Group1
include 60 children ages between 3-6-year-old which randomised to weekend
atropine plus a plano lens or weekend atropine plus full spectacle correction
for the normal eye. In group 2, 40 children ages 7-12 years old randomised to
weekend atropine or 2 hours of daily patching. Visual acuity assessed at 18
weeks in group1 and 17 weeks in group2. Visual
acuity of amblyopic eye improved by similar result with both types of
treatments in group1(4-5lines) and group2 (1.5 lines) (Repka et al. 2009).
Occlusion and dose
performed on strabismic, refractive and mixed amblyopia reported that more than
80% of the improvement occurred within the first 6 weeks of treatment and full
improvement occurring after 12 weeks. In this study found that a dose greater
than 2 hours daily did not affect the final outcome (Stewart et al. 2004b).
to number of hours of cumulative dose, most of improvement occurs after150-250
hours, which 0.2 logMAR improvement requiring a dose of 170 hours (Stewart et
al. 2007a). Cleary et al. found maximum improvement after 400 hours of
occlusion. Pediatric Eye Disease Investigator Group (PEDIG) investigated the
number of hours of prescribed occlusion in USA for for strabismic,
anisometropic and mixed amblyopia which found no significant difference in the
efficacy between 2 hours and 6 hours of daily occlusion for moderate levels
(20/40–20/80) of amblyopia (Repka et al.
2003). Also, there were no differences in efficacy for 6 hours against 12 hours
of daily occlusion for severe amblyopia (20/100–20/400) (Holmes et al. 2003a).