Correspondence - ICO Library

-55-
Binocular Vision & Inferior Oblique Muscle Palsy with ‘Paradoxical’ V-Pattern Strabismus FIRST Quarter of 2011
Strabology Quarterly© E. Khawam, MD and D. Fahed, MD Volume 26 (No.1)
A Medical Scientific e-Periodical Pages 51-60
abduction of the eye, the angle between the Y-
axis of the globe and the muscle plane
increases. Therefore the IO produces,
increasingly, an excycloduction. Since the IO
muscle is an excycloductor, an elevator, and
an abductor, its paralysis therefore results in
weakness of these three components:
1. Torsional Deviation: The result is
an intorsion of the eye. However, the absence
of incyclotropia could be due to cyclofusion,
or to monocular sensory adaptation whereby
a reordering of the spatial response of the
retinal elements along new vertical and
horizontal retinal meridians, analogous to
abnormal retinal correspondence (ARC). It
can also be due to spread of comitance to the
synergistic muscle (8) which has an opposite
torsional movement: the SR muscle that has a
secondary incycloduction movement.
2. Vertical Deviation: The paretic eye
is hypotropic if the uninvolved eye fixates. If
the involved eye fixates, a hypertropia of the
uninvolved eye is present. The hypotropia
may become especially pronounced in
adduction and depression, associated in such
instances with overaction/contracture of its
direct antagonist, the SO muscle.
3. Horizontal Deviation: The normal
abducting effect of the IO muscle is greatest in
upgaze. Its paralysis, therefore, results in
greater esodeviation in upgaze. And, with
increasing abducting action, mainly in
downgaze, of its antagonist, the SO muscle, an
A-pattern esodeviation is encountered. An
A-pattern exodeviation, not infrequently
encountered, may develop due to the marked
overaction of the SO muscle(s).
4. Findings on Rotations: Weakness of
the vertical action of the IO muscle, especially
in the adduction position is found. However
minimal weakness on rotation does not rule
out the diagnosis of IO palsy.
a. Overaction of its direct antagonist: the SO
muscle. A prominent finding that, in
my experience, is always present.
b. Overaction of the yoke muscle: the SR
muscle of the opposite eye. In case the
affected eye is fixating in adduction, a
“Rising eye phenomenon” of the fellow
eye is seen in abduction.
c. Underaction of the yoke of the antagonist:
described by Chavasse (3) as
“Inhibitional palsy of the contralateral
antagonist”, the IR muscle of the
opposite eye.
5. Associated Secondary Mechanical
Anomalies: With time, vergence-adaptation
followed by muscle-length-adaptation take
place: the innervationally-overacting muscles
shorten with actual loss of sarcomeres
resulting in contractures with structural and
fascial restrictions; and the inhibitionally
pseudo-palsied muscle(s) elongate with actual
increased number of sarcomeres (9).
6. Habitual Head Postures:
a: Head Tilt: to the ipsilateral shoulder
so that the affected IO muscle is put at rest
and the tonic impulses, sent by the otolith
apparatus will not have a direct action on it.
Head tilt –we believe- is usually a
compensatory mechanism to reduce the
vertical deviation rather than the torsional one,
and thus to gain fusion (10).