||Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX; Professor of Ophthalmology, Weill Cornell Medicine; Sami Younes, Baylor College of Medicine Class of 2022
||We are going to be doing the three-step test. The three-step test is the test that we use to determine if we can isolate a single muscle causing a vertical deviation a hypertrophy, and so the first step of three-step test is to identify which eye is the higher eye. Is it the right eye that's higher? Which is a right hypertropia, or is it the left eye that is higher? That is a left hypertropia. So, if we got a right hypertropia, the right eye is higher, but the left eye is lower which is a left hypertropia. And so in this H-diagram, which represents the muscle from the diagnostic positions of gaze, we know that we don't have to worry about the medial rectus muscle or the lateral rectus muscle because those are the horizontal muscles. We only have to worry about the vertically acting muscles. And so in this diagram, the superior rectus on the side and the inferior rectus, they act primarily vertically in the abductive position. That's why they're in that position of the H diagram. In contrast, the obliques, being left inferior oblique and the left superior oblique, act vertically best in the adduction position, and the reason for that is just mechanical properties of the insertion and origin of the muscle. ; When we have an eye with a superior oblique palsy, the superior oblique, its effective origin is actually at the trochlea, not at its real origin. And because of this angle of insertion and being posterior to the equator, when this muscle fires its primary action is going to be intorsion. However if the eye is put into the adductive position, the superior oblique will become primarily a depressor because it's in the plane of action of the muscle belly and will depress. If you're in the abductive position it will just abduct more and be an intorter. So it depends when you're the superior oblique muscle whether you're in the adducted position or the abducted position what its primary, secondary, and tertiary action are. So in the primary position it's going to be an intorter but in the adducted position, it will become a better depressor. And that's why the superior and inferior oblique are on this adducted position in the H diagram. In the abducted position the rectus muscles are going to be better at elevating and depressing because the obliques have been pushed away from their mechanical advantage based on their insertion and origin on the eye. So once you've determined which eye is higher, for example in this eye example, the right hypertropia. That means if the right eye is high, the depressors are weak, and I got two depressors in the right eye, the right inferior rectus and the right superior oblique. Or the left eye is low, a left hyprotropia. And if the left eye is low, that's going to be the left inferior oblique and the left superior rectus. The second step of the three-step test is going to be which gaze direction in the H diagram makes the deviation worse, and I already told you that the obliques work better at vertical action in the adducted position and the recti work better vertically in the abducted position. So when we look in this example to left gaze, and it made the deviation worse, that's going to mean the left superior rectus or the right superior oblique because those two muscles act vertically better in left gaze because the left eye is in the abducted position and the right eye is in the adducted position. In contrast, if the deviation was worse on gaze to the right, it would be the exact opposite. Right inferior rectus and left inferior oblique. But because of the second step of the three-step test, we have eliminated two of the initial four muscles that we were considering and we're ready for the third step now, which is the head tilt test. So when we tilt our head to the right, the right eye has to intort to maintain the vertical orientation and the left eye has two extort. Otherwise everything would tilt every time you tilted your head and it doesn't because you have counter rolling mediated by the vestibular ocular reflex. So if I'm worse on my right hypertropia on head tilt to my right, that means a right intorter is weak or a left extorter is weak. But I only have two choices left, the right superior oblique and the left superior rectus, which are both intorters. And so what that means is, if I have to choose between a right intorter and a left extorter, I'm going to choose the right intorter, which in this case is the right superior oblique. The three-step test has localized this to a right fourth nerve palsy. Now there is a fourth step to this three-step test. The fourth step is to measure the torsion and we can measure the torsion incyclotorsion or excyclotorsion with a double maddox rod, which we don't have time to talk about right now. But the expected torsion in a right superior oblique palsy, because it's primarily an incyclotorter, would be right-sided excyclotorsion. But what if we found that it was the opposite torsion or the wrong torsion? Then we would be thinking that it's a mimic of a fourth nerve palsy, like a skew deviation or myasthenia or thyroiditis. And the fifth step of the three-step test, if we're thinking it's skew. Skew is mediated by the vestibular ocular reflex, it is otolith dysfunction. And so if we put that reflex at rest by lying flat, it will make the vertical deviation less than if you're sitting up. But a fourth nerve palsy doesn't care whether you're sitting up or lying down, but a skew does care. And so the fifth step of the test is to make sure that it's not skew by lying the patient down and rechecking the vertical deviation. And if there's a change of the vertical deviation by lying down or sitting up, that also suggests it's skew. Which is super important because most fourth nerve palsies are benign in isolation, congenital, ischemic, traumatic, idiopathic. But skew is almost never benign and that means you have a posterior fossa lesion until proven otherwise.