Fourth nerve palsy / superior oblique underaction
 

This is a popular examination case. You will either be asked to perform the three step tests or the ocular motility examination. Remember that the patient may have asymmetrical bilateral fourth nerve palsy. On inspection, the patient may have an abnormal head posture in the primary position with the head tilt ot the  unaffected side with chin depression. Alternatively, the patient may be wearing glasses with prism. In ocular motility examination. There is overaction of the inferior oblique on the affected side when the patient looks  to the unaffected side. On looking in and down, the affected side has incomplete  depression (see picture above). In the cover/uncover test (Combine with the three-step tests see picture below. The patient has a right fourth nerve palsy).   First step Second step Third step In the primary position, there is hyperdeviation of the affected eye (first step).  Left fourth nerve palsy With the patient looking to either side, the test shows increased deviation when the eyes look to  the unaffected side (second step).  Left fourth nerve palsy With the head tilt, the test shows increased deviation when the head is tilted to the affected side (third step). Left fourth nerve palsy Additional features: beware that the patient may have bilateral fourth nerve palsy

Questions:

1. What ocular sequelae can occur with long-standing fourth nerve palsy?

answer
Overaction of the contralateral inferior rectus (Herring's law)
contracture of the ipsilateral inferior oblique which is often the most obvious feature in a congenital palsy (Sherrington's law)
secondary inhibitional palsy of the contralateral superior rectus, sometimes making it difficult to differentiate the primary site..

2. How can a unilateral palsy be differentiated from a bilateral palsy?

answer
The following features suggest a bilateral palsy:
Excyclotropia in excess of 10 degrees is common
V pattern is common
Chin depression without head tilt
Positive Bielschowsky head tilt test on tilting the head to either shoulder.

3. How can an unresolved fourth nerve palsy be treated surgically?

answer
There are three options:
strengthen or tuck the affected superior oblique muscle
weaken the antagonist ipsilateral inferior oblique muscle
weaken the yoke contralateral inferior rectus muscle.
 

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