Straight From the Cutter’s Mouth: A Retina Podcast, hosted by Dr. Jay Sridhar, is a wonderful weekly podcast for ophthalmologists, featuring insights from both prominent, as well as up-and-coming retina specialists, about their lives, passions, and work.
In this episode, Dr. Fisher discusses a variety of tips for distinguishing the retina from posterior hyaloid. For the link to the full episode, click here.
What are your thoughts about using the ultrasound to identify retinal tears? Secondly, how do you distinguish the retina from posterior hyaloid?
It depends on the degree of hemorrhage and the degree of clotting. It also depends a great deal on how much practice the person has had in using ultrasound.
Check the far periphery for retinal tears.
If you’re looking for retinal tears, they almost always occur in the far periphery. They can occur posteriorly, but most of the time, they’re about 2 millimeters behind the insertion side of the muscles. I prefer longitudinal imaging or radial imaging where the manufacturer’s mark is centered towards the pupil and the rest of the probe goes more posteriorly on the globe, just gently pressing against the lid.
Focus on lubrication over pressure.
You don’t need much pressure with ultrasound, it’s a very big mistake to push hard. The pressure isn’t so important; the lubrication to get the sound into the eye is what’s important. You can start at six o’clock and aim towards twelve, and literally, by tilting the probe medially and laterally, walk around the vitreous base.
If the vitreous has come off and there is a tear, you will see the vitreous — which is mildly reflective — usually go right up to the flap of the tear (and it’s the flapped tears that we worry about the most) you’ll be able to trace them out. If you correlate them with the insertion site of the four rectae muscles, or use a scleral depressor in your other hand, and gently indent the sclera, you’ll be able to delineate exactly where the tears are located by clock hour.
Pay attention to the movements.
It gets more complicated at the bottom of the eye where blood and clot collect. It’s harder to differentiate the retinal tear from the normal position of the vitreous face, but it can be done by movements of the eye, and you can actually see the tear move under the influence of the vitreous gel.
We’re less worried about the inferior tears, than we are the ones that are more superior because they tend to detach with gravity and movement. But the motion of the vitreous leading right up to a flapped tear is what we picked up.
Remember that the resolution of an ultrasound with 10 megacycles which is the usual transducer level, is about 150 microns axially, so if the tear is elevated more than that from the retina, it won’t pick it up, but they usually are.
The lateral resolution, which nobody usually talks about at all, relates to the size of the beam at 19 or 20 millimeters back from the surface of the transducer, and that is around 200 microns. So if a tear is smaller than that, you probably will not pick them out unless you can angle your beam sufficiently to stay in the tear longer. That’s a little bit more subtle.
So large retinal tears are fairly easy to pick up, and I try to do them again in clear media cases so I keep my practice up as to what I can pick up, because now I can grade myself immediately afterwards by putting out an indirect and seeing if I got them all. Round holes you won’t pick up unless they’re quite large, but flapped tears, you pretty much pick up most of them. Maybe perhaps not the pseudophakic ones; they’re very tiny and are much more subtle.
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