||Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX, Professor of Ophthalmology, Weill Cornell Medicine; Alina Mohanty, Baylor College of Medicine, Class of 2022
||So today we're going to be talking about a different type of fourth nerve palsy. You've already heard about unilateral fourth nerve palsies, and this time we're going to be talking about bilateral. Ironically, bilateral fourth nerve palsy can be a little more difficult to diagnose than a unilateral. As you know from the previous video on unilateral fourth nerve palsy, we need a three step for this - the three-step test. The first step determines which eye is higher. The second step - is it worse on right or left gaze? And the third step - is it worse on right or left head tilt? With a bilateral fourth nerve palsy, because you have a left hypertropia and a right hypertropia, and both eyes are high - it kind of cancels out. And so, in the primary position, you might have little or no deviation and that's going to be deceptive because the patient might not be complaining about double vision looking straight ahead because the two cancel each other out - the left hyper and the right hyper cancel out. The second step of the three-step test is: is it worse on right and left gaze? And normally in a right fourth, we would have a right hypertropia worse in left gaze and a left eye hypertropia in right gaze. And so, if we see that, right hypertropia in left gaze and left hypertropia in right gaze, that is a reversing hypertropia. And that is the big clue that it's a bilateral, because a single right fourth or left fourth cannot reverse like this. It has to stay on the same side; the hypertropia has to be hyper on that same side - right gaze left gaze, right tilt left tilt. So, if it's a reversing hyper on the second step or on the third step (which is the head tilt test), so if we have a reversing hypertropia - a right hypertropia on right tilt and a left hypertropia on left tilt - that is also a reversing hypertropia. And so, the reversing hypertropia in either of the second or the third step of the three-step test suggests it's bilateral. And finally, as you know, we measure the torsion with the Double Maddox Rod Test. And so, in a unilateral fourth nerve palsy, we're really expecting single-digit numbers of the excyclotorsion, and so if you have a number like 8 degrees of excyclotorsion, that's compatible with a unilateral fourth. But if we have 14 degrees of excyclotorsion, that's way too much for a unilateral fourth nerve palsy and suggests that you have a bilateral fourth nerve palsy. And one of the most difficult things about bilateral fourth nerve palsy is it might look like a one-sided right hypertropia worse in left gaze and right tilt; it looks just like a fourth nerve palsy on the right. And then you do surgery on this, and it unmasks the underlying bilateral fourth. And so, all of a sudden now, after surgery, they have a left hypertropia worse on right gaze and left tilt. And the Double Maddox Rod Test may or may not show this degree of excyclotorsions - might be 10 degrees of excyclotorsion. And so, we really need to be thinking about bilaterals in any trauma case because, as you know, the fourth nerve is the only cranial nerve that exits dorsally and crosses in the posterior commissure at the level of the inferior colliculus in the midbrain. And so, if you have a trauma case with a coup contrecoup type of injury, it can cause a bilateral fourth from that crossing point. And so, when we have a bilateral fourth, we would image that. In a trauma case, you have to suspect bilateral fourth even if it looks like a unilateral. So, in summary, as opposed to the unilateral fourth nerve palsy, the bilateral fourth nerve palsy is more difficult to diagnose. it may have little or no deviation in primary because the left hypertropia cancels out the right hypertropia. You're going to be looking for the distinctive sign: a reversing hypertropia on the second stage - right or left gaze - or a reversing hypertropia on the head tilt test - the third step of the three-step test. And on the fourth step of the three-step test, Double Maddox Rod Testing is going to show double digits in terms of excyclotorsion. And beware of the unmasking of the bilateral fourth in any trauma case, even if it looks like a strictly unilateral case.