29th December 2023

4ᵗʰ nerve palsies

As mentioned earlier, a palsy of the trochlear nerve will result in vertical diplopia. Superior oblique intorts and depresses the eye in the adducted position. So, clinical examination of a patient with a 4ᵗʰ nerve palsy may reveal abnormal head posture (torticollis) in an attempt to compensate for the diplopia, strabismus in the affected eye where there would be a hypertropia and in some cases an esotropia. The trochlear nerve is the only cranial nerve which crosses the midline to innervate the contralateral side and exits the brainstem dorsally. This means that in cases of trauma to the dorsal aspect of the brainstem, a patient can have a bilateral 4th nerve palsy, which presents with vertical diplopia and strabismus.

The Parks 3 Step test can be utilised as a gross bedside test to help determine which extraocular muscle is causing a vertical diplopia. Understanding why the test works requires an in-depth understanding of how the extraocular muscles rotate the eye on 3 axes, which is not covered in this section. The test will not be diagnostic if more than one extraocular muscle is affected.

1. Identify the hypertrophic eye in primary position – ask the patient to gaze ahead and see which eye is hypertrophic. If, for example, the right eye is higher, then the muscles affected are narrowed to either the depressors of the right eye (IR + SO) or the elevators (SR + IO) of the left eye.
2. Identify if the hypertropia worsens in left or right gaze – in a trochlear nerve palsy, hypertropia of the paretic eye will increase on the opposite gaze.
3. Identify if the hypertropia is worse on head tilt to the left or right – in a trochlear nerve palsy, the hypertropia will worsen on tilting to the same side.

There is a very quick and easy diagram you can utilise when identifying which extraocular muscle may be the culprit in presentations of vertical diplopia.

In this example, the patient presents with vertical diplopia and a right-sided hypertropia on examination. Step 1 is to circle the muscles on the bottom of the eye, which is deviated, and circle the muscles on the top on the contralateral side (highlighted in green). The next step of the test is to examine when the deviation worsens in left or right gaze. In this example, the patient’s vision worsens in left gaze, so you circle the muscles utilised for left gaze (amber). The final step is to identify which direction of head tilt worsens the deviation; in this case, it is right-sided head tilt. Highlight the muscles on the diagonal, which corresponds to whichever direction the deviation is greatest. The extraocular muscle, which has been circled 3 times, is the muscle causing the deviation, which in this example is the right superior oblique.