Cranial nerve number IV, the trochlear nerve has only just one functional component and that is a general
somatic efferent component. This nerve innervates one skeletal muscle. This is a muscle that moves the eyeball,
so it’s an extraocular muscle. That is the superior oblique muscle which we see here. Then we see the tendon
coursing through the trochlea in the upper medial quarter of the eye and then we see the insertion of the
superior oblique into the sclera of the eyeball. If the trochlear nerve is lesioned, the superior oblique will
not function properly. That will then result in an eye movement that is down and in. As a result of the eye
being down and in, it makes it difficult to look down. Some causes of damage to the trochlear nerve
would be due to a microvasculopathy. This could result from damage to the microvasculature due to diabetes.
Hypertension can cause damage to the microcirculation. Atherosclerosis can also impede the ability of the
trochlear nerve to receive an adequate blood flow. Tumors are always a consideration. Then an aneurysm of
an artery that would be impinging upon the trochlear nerve could cause it to lesion as well.