Strabismus - Fundamentals of Clinical Ophthalmology.pdf

STRABISMUS
There is a simple way of determining the
suspect muscle using a modification of the
3–step test.
Step 1: If the right eye is higher then it is a
weakness of depressors of the right eye or
elevators of the left eye.
Step 2A: If the deviation on tilt is worse on the
right side, it is an oblique dysfunction.
Step 2B: If the deviation is worse on left gaze, it
is the right superior oblique; and if worse on
right gaze, then it is the right inferior oblique.
Step 3A: If deviation on tilt is worse on the left
side, then it is a rectus dysfunction
Step 3B: Deviation that is worse on left gaze is
the right inferior rectus, and if worse on right
gaze, it is the left superior rectus.
Investigation of paretic strabismus
Investigation and measurement of horizontal
muscle function is readily obtained, whereas
vertical muscle dysfunction may require further
clinical tests.
Head tilt
In the presence of paretic strabismus, head tilt
is presumed to obtain fusion and to avoid
diplopia. Simple tests to demonstrate fusion
without demonstrating head tilt should be carried
out. The head is then tilted to the opposite side to
see if fusion is lost and diplopia evoked. If there is
vertical muscle paresis, measurement with the
paretic muscle will produce larger deviation of the
other eye (Hering’s Law). In adults, the patient
will normally fix with the normal eye, unless the
paretic eye has the better vision. By contrast, in
developmentally determined strabismus with
binocular vision, the abnormal head posture is
not uncommon as a consequence of fixing with
the eye with restricted movement in the early
years.
Vertical muscle imbalance can be measured
with the Parks 3-step test. The use of coloured
spectacles with the red glass in front of the right
eye and green glass in front of the left eye, then
asking the patient to fix a light at 6 m means that
the coloured glass dissociates the eyes and makes
it easier to make measurements.
Forced duction test
The forced duction test is useful in differentiating
defective movement due to mechanical
restriction of the muscle or muscle paresis. In
children, it should be carried out under general
anaesthetic at the commencement of surgery.
In adults, forced duction tests can be performed
in the clinic using topical anaesthesia. The
restriction in oblique muscles will be made more
evident by pushing the eye back into the orbit,
and in recti muscles by pulling forwards. 7 For
example, if there is a lateral rectus palsy,
grasping the eye in the adducted position and
asking the patient to straighten the eye allows the
examiner to make some assessment of residual
muscle action.
Saccadic velocities
The patient is asked to change fixation
between two fixation targets 20–30 º apart. The
eye with a weak muscle usually shows a slow
saccade in the direction of action of the weak
muscle, compared with the normal side.
Restrictive causes of adult strabismus
Thyroid eye disease
In thyroid eye disease, the extraocular muscles
may be involved in an infiltrative process and can
increase their volume by up to six times. The
most frequently involved muscles are medial recti
and inferior recti. There is an inflammatory
infiltrative disorder of the muscle, resulting in
fibrosis and restriction of eye movement. One or
both eyes may be involved and a range of
restrictive muscle disorders may result. It is
important to consider thyroid eye disease in any
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