Point 19
Intro
While at first glance it appears that contraction of the superior oblique turns the eye down and out, the rest of the story (Paul Harvey would love this) is slightly more complicated. If you are interested, read on! The vector diagram resolves the arrow RB into effective components. Vector RA depresses the eye around the lateral axis. Vector RC abducts the eye around the vertical axis and intorts (medial rotation) the eye around the anteroposterior axis. Therefore, vector RB acts to depress, abduct and intort the eye.
When the eye is in the primary position, the superior oblique lies medial to the A-P axis of the globe. However, when the eye is adducted, the line of pull of the tendon of the superior oblique is parallel to the A-P axis of the globe. In this position, none of the actions of the muscle are dissipated in the other actions (abduction and intorsion). Hence the clinical test for the strongest action of the superior oblique is to ask the patient to look in (medially) and then down.
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A 4th nerve lesion causes atrophy of the superior oblique muscle. When looking down and in (medially) with the bad eye there will be DIPLOPIA. The false image will lie below the true image (vertical diplopia) and will be somewhat oblique (torsional diplopia). The weakness of downward movement of the affected eye, most markedly when the eye is turned inward, results in the patient complaining of special difficulty in reading or going downstairs.
The weakness of the superior oblique in the
primary position (looking straight ahead) results in the
"bad" eye being slightly extorted and elevated
due to the unopposed action of the inferior oblique. This
will result in torsional and vertical diplopia. For
instance, if the LEFT superior oblique is paralyzed,
the LEFT eye is extorted and elevated. In order to
get rid of the torsional part of the double vision,
the patient will tilt their head to the side OPPOSITE
the paralyzed muscle, that is to the RIGHT. This
causes reflex (from the otoliths) intorsion of the
normal RIGHT eye (on side of head tilt) so that the
vertical axis of the two eyes become parallel (the eye
associated with the paralyzed superior oblique is already
extorted by the unopposed inferior oblique). To alleviate
the vertical diplopia, the patient will also
FLEX his/her chin when tilted to the RIGHT. In
this position the patient will have to elevate the
normal RIGHT eye in order to look straight ahead. The
"bad" (LEFT) eye is already elevated and when the two
eyes are located at the same vertical (up-down) position in
the socket, the vertical diplopia is ameliorated.