| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (MH) Class of 2020, Baylor College of Medicine, Houston, Texas |
| Transcript |
Today we're going to be talking about skew deviation. Skew means not parallel and not at right angles; usually it means at an oblique. In the context of neuro-ophthalmology, it usually means that we have a double image that is usually vertical and has some oblique components to it, thus the name skew. A skew deviation is simply a deviation that does not localize to a single muscle or muscle group, or single nerve or nerve pattern. It's usually defined by the company it keeps because it usually means there's a structural lesion in the posterior fossa. It can be rarely cerebellum, but it can arise from anything from the thalamus all the way down to the medulla. Any lesion from the thalamus down to the medulla, and including rarely the cerebellum, could produce a skew deviation. The way we diagnose a skew is similar to the way we diagnose any deviation. We're going to do a three-step test. For example, the normal three-step test for a left hypertropia where the left eye is higher - in this particular example the left eye is higher - we're trying to figure out is it worse in right or left gaze. In a fourth nerve palsy it would be worse in the contralateral gaze, so in left height it would be worse in right gaze, and in which head tilt. In this case, for a fourth, it would be it would be LEFT, RIGHT, LEFT, a marching palsy. A left hypertropia, worse in right gaze, and left tilt. However, you should be suspicious of skew if you do any part of three-step test and it doesn't land on superior oblique muscle. If we get to the end of the three-step test and we don't get the fourth, we should be worried that that skew or myasthenia or thyroid or other cause - and today we're trying to figure out if it's skew - and that means that we have to do a fourth step in the three-step test. The fourth step of the three-step is a double Maddox rod. A Maddox rod, as you know, is a series of cylinders. What that double Maddox rod does is, it allows a pinpoint of light to be projected as a line perpendicular to that pinpoint area of light. What we're looking for in that fourth nerve palsy is ipsilateral excyclotorsion. If we get the wrong torsion, or we have bilateral torsion, or we have torsion towards hypotrophic lower eye, that's really suggestive that it's skew. There's a fifth step of the three-step test, which is we can compare the deviation supine versus upright. When a patient has a fourth nerve palsy, the fourth nerve really doesn't care whether you're sitting up or you're lying down; hypertropia is going to be the same. But in skew - because skew deviation is abnormal input from the otolith - in the supine position, when you're lying down, that takes away the otolith input and that's going to reduce the amount of hypertrophia. If we see in the supine position a decrease in the hypertropia, that also is suggestive that it is a skew deviation. The ductions and the versions usually are normal in skew, and the deviation is usually commented but it can alternate. If we don't see a clear ductional deficit, that suggests the patient has a third nerve palsy, and we have a double vision that is vertical and oblique, we're going to do the three-step test. If we don't get the three-step test to deep fourth, we're going the fourth step, which is the double Maddox rod test - and the fifth step, which is the supine versus upright test. If any of these five steps in a patient who has an isolated hypertropia, don't suggest fourth. You should really be thinking about myasthenia gravis, thyroid opthomopothy, or another restrictive process, and skew. That means looking for the other localizing findings, a posterior fossa disease, and imaging the patient. |
