The first stricking feature in this boy is
the left hypertropia.
A paralytic hyperdeviation
is usually difficult to diagnose because we have
to differentiate between a pair of elevators or
depressors in each eye. As in this case there is
a left hypertropia in primary position
, this means the following possibilities
a. Paralysis of one of the
depressors of the left eye
b. paralysis of one of the elevators
of the right eye
( Left IR , Left SO
Vs Right SR
, Right IO )
it must be recognized
whether the hypertropia is greater on right
or left gaze. If the
left hypertropia
increases in left gaze , as we are seeing
here , this may be caused by paralysis of the
left inferior rectus or the right inferior
oblique muscles.
In patients who have a
paresis of longer standing, as in our patient ,
the head tilt test could be used to differentiate
between a paretic elevator muscle in one eye and
a paretic depressor muscle in the other.
As we see , the left hypertropia is greater on
head tilt to the right shoulder ( the opposite
side of the hypertropia ) ,
Keeping all this in mind with the Park’s 3 step
test this means that this
patient has definitely an isolated left inferior
rectus paralysis .
When the paralysis is of
recent onset, the diagnosis is made on the basis
of incomplete duction in the field of action of
the suspected rectus or oblique muscles , this
makes the differentiation is usually easy .
However, it is a common mistake in recent
paralysis to depend on the examination of
ductions to confirm a paresis of a specific
muscle , since the patient may overcome the
muscle weakness by maximal innervational effort
when fixating with the paretic eye. More
revealing is the examination of versions, for
under these circumstances the patient will show
marked overaction of the yoke muscle of the
paretic muscle in the contralateral eye when
fixating with the paretic eye.
Finally , concerning the head posture ,
Anomalies of head posture should alert the
ophthalmologist to the presence of
paralytic strabismus, this sign is of limited
value in ascertaining the nature of the
underlying pathology except with paralysis of the
oblique muscles. In these conditions of oblique
muscle palsy , the head is inclined toward the
opposite side in superior
oblique
paralysis and toward the paretic side in inferior oblique
paralysis ,
although a head tilt toward the paralyzed side
( paradoxical head tilt ) may occur occasionally
with paralysis of the superior oblique muscle.
The direction of compensatory head position
varies more frequently with paralysis of the
vertical rectus muscles, when the head may be
tilted toward the involved or noninvolved side
.
Causes of isolated IR
paralysis
Idiopathic
Thyroid Eye Disease
Trauma with or without fracture floor of orbit
Extra-ocular muscle cysticercosis
Myositis
Midbrain Infarct
Ocular Myasthenia
Most of these causes however will have a definite
history of pain, redness ,watering at some time or
other .
EOM myositis is however less likely as there is
no tendon involvement as mentioned before in the
details .
Thyroid eye disease may also have an insidious onset
and there is a predilection for Medial Rectus and
inferior rectus involvement in these cases. However,
patients may be euthyroid on systemic examination
and it is more important to look for any other
evidence of ocular involvement such as lid lag,
Lagophthalmos and lid retraction .
Ocular myasthenia usually tends to present with
involvement of the superior rectus and LPS muscle
rather than with a involvement of Inferior rectus .
There may also be a variability of the symptoms of
diplopia .
Isolated inferior rectus palsy due to a midbrain
lesion may result from involvement of the inferior
rectus subnucleus of the oculomotor nuclear complex.
At the most rostral part of the midbrain, the dorsal
cell column innervating the inferior rectus is
located in relative isolation. Therefore, a lesion
selectively involving this area may cause isolated
inferior rectus palsy.
However patients
have midbrain infarction are more likely to be
elderly patients with a definitive history of ischaemic risk factors.
Therapy
For paralysis
of the of the inferior rectus, 4-mm
resection of the paretic muscle without recession of
its antagonist may suffice. The question arises
whether surgery should be performed on the fixating
or nonfixating eye. With rare exceptions, if the
horizontal or vertical rectus muscles are paralyzed,
I prefer to operate on the paretic eye regardless
of whether it is the dominant or nondominant eye.
The amount of surgery that is necessary varies, of
course, depending on whether the paretic eye
(secondary deviation) or the nonparetic eye (primary
deviation) habitually fixates.
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