Hello -
In summary, a large central cataract produces visual loss from that
eye leading to cortical (brain) neglect of the visual input from that
eye. With the input from that eye being neglected, the eye will
deviate to it's "unused" position, which is divergent from the
direction of focus of the other eye. This divergence will be
accentuated by focusing on distant objects and bright light.
References and explanations follow.
A large central congenital cataract will decrease the visual input
from that eye leading to cortical suppression of the input from that
eye (http://www.merck.com/pubs/mmanual/section19/chapter273/273a.htm
and http://home.so-net.com.hk/~trevorayyc/Surgery/15Eye/15Eye_Paedi.htm).
This loss of input will produce an ambylopia - eyes looking in
different directions. The characterization of "divergent" versus
"convergent" refers to the directions that the eyes point. A
"divergent" squint (or exotropia) means that the affected eye will
deviate away from the other eye (also described as laterally, or
temporally). "convergen" means that the eye would move towards the
other.
Google search "congenital cataract squint"
A nice tutorial demonstrating the different eye movements associated
with heterotropia/heterophorias - see the demonstration under the
tutorial section:
main page - http://www.city.ac.uk/optics/BVTutor/home.html
tutorials - http://www.city.ac.uk/optics/BVTutor/html/the_cover-test.html
Google search "heterotropia"
So, why is the deviation divergent?
First of all, the eye is most likely to be divergent when the patient
looks at a distant object, as focusing on near objects produces
convergence (both eyes turning inward to focus on the near object -
this works even with a cataract as the muscles of the eye are not
affected) - http://www.eyemdlink.com/Condition.asp?ConditionID=176 and
http://www.eyesite.ca/7modules/Module1/html/fr_Mod1Sec5.html with nice
pictures showing the condition.
Google search " "congenital cataract" exotropia "
The divergence appears to be related to the inability of the affected
eye to focus (due to the visual loss produced by the cataract) and the
resulting cortical neglect of the input from that eye. The eye will
move in a convergent direction when accomadation and convergence help
to alleviate the visual problem - otherwise, they diverge. However,
the natural position of the eye when not being used (or when
anaesthetized) is divergent (see chapter 11 -
http://www.netspace.net.au/~strabism/strablec.htm also
see V-a-3-c in http://www.uic.edu/com/eye/education/material/Microsoft%20Word%20-%20pedophth%2010-27-00.pdf
Google search " "congenital cataract" exotropia physiology "
For further reference, a nice overview of paediatric opthamology:
http://www.aafp.org/afp/980901ap/broderic.html
Please let me know if you have further questions.
synarchy |
