||Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX; Professor of Ophthalmology, Weill Cornell Medicine; Daniel Rodricks, Baylor College of Medicine Class of 2023
||So today we're going to be talking about the temporal crescent. It's called that because it's in the temporal portion of your visual field. And it's a crescent because it represents a crescentic shape. So normally they draw the visual fields as circles. But really they're not circles, they're ovals. And that's because the temporal portion of your field is actually bigger than the nasal portion of your field. And what that means is this part of your field, this temporal portion of your field, has no correlate in the nasal half of the visual field of the fellow eye. So if this is the right eye and this is the left eye, the temporal crescent is a monocular visual field problem. And so as you recall from prior lectures, the visual pathway on the afferent side is composed of the retina. And we have the temporal and the nasal retina, the optic nerve, the chiasm, the optic tract to the geniculate body, through the radiations to the cortex. So when we have a temporal field defect, that represents the nasal fiber. The nasal fiber is actually the crossing fiber, and when the nasal crossing fiber crosses, it meets up with its correlate from the fellow eye which is the temporal fiber. So the nasal crossing fiber and the temporal uncrossed fiber meet in the optic tract. And that's why lesions behind the chiasm produce contralateral homonymous hemianopsias, because they're the temporal field of one eye, and the nasal field of the fellow eye. And to the geniculate, the radiations of the cortex, that topographical lateralization is preserved. However, the nasal fiber that is controlling this temporal crescent also must travel in the optic tract, and travel to the radiations and ultimately to the cortex. And when it reaches the cortex, the temporal crescent is located in the most anterior portion of the medial portion of the occipital cortex. And so the nasal fiber that came from the contralateral eye carrying temporal crescent is in the contralateral anterior portion of the occipital cortex. And what that means is we can have two special types of visual fields. We can have a homonymous hemianopsia that spares this occipital portion of the occipital lobe. And that means we can have sparing of this temporal crescent. So in every patient with a homonymous hemianopsia we want to test for this temporal crescent. And normally in automated perimetry we only have a 24 or a 30 degree visual field. And this temporal crescent is way out here in the temporal region and so it's 60 to 90 degrees out there. And so you really can't use a 24 or 30 degree field to capture that. So when we have a homonymous hemianopsia that looks complete, we're going to put the target,like your finger,into this blind field and say "when do you see the finger?" So if they see it over here then you cross the midline-"no, no, no"-if all of a sudden they see it again, that is the test we're going to be using with confrontation. "No, no, no, no, yes" to detect the temporal crescent, and that temporal crescent being preserved means the lesion is occipital, but also that it is preserving that anterior portion of the medial occipital cortex. The more difficult field is when there isn't a homonymous hemianopsia and you only have the temporal crescent now, and that means you're only involving this portion of the occipital cortex. So the most common causes of a field defect in the temple of one eye only is normally a retinal problem in that ipsilateral eye. However, there's going to be a relative afferent pupillary defect in this affected left eye, and normally we will be able to see in the retina if there's a retinal detachment or some retinal cause for this. So if we don't see an RAPD, and we have a normal fundus, but the patient is complaining about the temporal crescent and they map it out as a temporal crescent in one eye, that lesion is actually on the contralateral side in the occipital cortex in the anterior medial portion of the occipital lobe. And that means we should do an MRI scan of this patient here. So, you need to know about the temporal crescent. It is a crescent of temporal field. It has no correlate in the fellow eye in the nasal field. It is crossed with the nasal fiber in the chiasm. And then travels to the most anterior portion of the medial occipital cortex. And you can either have a homonymous hemianopsia that spares the temporal crescent, or you can just have the temporal crescent alone. And both of those lesions need an MRI of the occipital lobe.