Clinical Testing

Clinical Testing - Right click to download

There is a difference between observing an extraocular muscle’s normal function and testing its strength or its innervation.

With respect to normal muscle function, if one or both visual gazes drifts, this is called strabismus, which is caused by the inability of the antagonistic muscle to maintain visual gaze. When a muscle is unable to move in the normal direction, this is called gaze paralysis. If the eyeball twitches (which are rapid involuntary movements) while it is moving its gaze or when it is stationary, this is called nystagmus. Some degree of nystagmus is normal.

With respect to muscle strength and innervation testing, it is important to consider that multiple muscles can accomplish the same actions about the three principle axes. For example, both the superior rectus and inferior oblique muscles will elevate the eyeball; both the inferior rectus and superior oblique will depress the eyeball. To discriminate muscle or nerve functions from one another, the clinician can trap one muscle and test its functional pair. Muscles are trapped from performing their function when the visual gaze axis is perpendicular to the muscle fiber direction.

To test superior rectus from the inferior oblique, the clinician asks the patient to first look out (or lateral) to orient the visual gaze axis perpendicular to the inferior oblique muscle fiber direction, then up. After the inferior oblique is trapped, the only muscle that can mediate elevation is the superior rectus. This test discriminates upper division of oculomotor nerve innervation from inferior division of oculomotor nerve innervation. Likewise, to test the inferior oblique from the superior rectus, the clinician asks the patient to first look in, then up.

To test the inferior rectus from the superior oblique, the clinician asks the patient to first look out (or lateral) to orient the visual gaze axis perpendicular to the superior oblique muscle fiber direction, then down. After the superior oblique is trapped, the only muscle that can mediate depression is the inferior rectus. Likewise, to test the superior oblique from the inferior rectus, the clinician asks the patient to look in, then down. The latter test discriminates trochlear nerve (CN IV) innervation from inferior division of oculomotor nerve (CN III) innervation.

Since both the lateral rectus and medial rectus move the eyeball in once principle direction, the clinician asks the patient to look laterally in the axial plane to test the lateral rectus and medially to test the medial rectus. This test discriminates abducens (CN VI) innervation from inferior division of oculomotor nerve (CN III) innervation.