The pupil is a biological anomaly in the sense that it isn’t a structure at all. Rather, it is a black hole in the center of the iris, its size indicative of how much the iris is opened or closed. Its dimensions are therefore reflective of the push-pull dynamics of the iris sphincter and dilator muscles, mediated by the balance between sympathetic and parasympathetic innervation (image via Truong and Ciuffreda).
All this is well-known to first year Optometry students studying neuroanatomy, who are then taught to buy into the clinical fiction of “PERRLA” – the acronym for Pupils Equal and Round, Responding to Light and Accommodation. Much as light is mediated by the iris sphincter and dilator, accommodation is mediated by the ciliary muscle. The pupil is therefore more of a descriptive bystander, a window (or perhaps an eyewitness) to the neurology happening all around and behind it (image via NeuroOptics).
But let’s play along with the clinical fiction of “pupillary function”, since that illusion has served us well. The clinical significance of shining a light into the eye to observe pupillary responses, and specifically in comparing right and left eyes with regard to miosis and mydriasis is vital in determining the presence of an afferent pupillary defect, or APD. But here I want to focus on the beginning of the PERRLA acronym, the expectation that the pupils will be equal and round. What if they aren’t?
The equal part is easy, with asymmetry evident in anisocoria. But what about the “round”? Even for those who study dynamic pupillometry in comparatively benign conditions such as amblyopia, pupillary diameter is treated as minimum, average, or peak circumference presumptive of perfect roundness.
A pupil that is more ovoid than round doesn’t have a distinct name, particularly when we want to denote a difference in the dimensions of roundness between the right and left eyes. A pupil that is oval-shaped or shows segmental constriction is usually subsumed as part of phsyiological anisocoria, but I really don’t think that does it justice. Nor am I speaking of corectopia as in the case of congenitally displaced pupils or induced trauma via accident or surgery. This is a more subtle departure from roundness, so for the moment let’s call it APD prime, or an APD of a different kind indicative of asymmetric monocular pupillary dimensions.
Although I don’t routinely do pupillometrics, we observe pupillary dimensions qualitatively when doing objective auto-refraction in dim illumination. Here is an example of a child I saw with a round pupil of the right eye (top photo) but an oval shaped pupil of the left eye (bottom photo):
What is the significance of this? Certainly there may be ominous implications with regard to neuroanatomic alterations due to infarct, lesion, or tumor in the visual pathways, as reviewed in this article on the dynamic oval pupil in Frontiers in Neurology. One must also entertain departure from roundness as a potential sign of drug abuse, or of mild Traumatic Brain Injury (mTBI).
It is the latter category that is most likely to escape detection barring an overt change in pupillary function secondary to concussion or other insults, or some form of acquired toxicity. I previously suggested that at least a subset of the children we deal with having learning issues likely have some sort of brain dysfunction, as originally elaborated by Gardner.
Much as we make the notation of “NKA” for no known allergies, a notation of “NKmTBI” might be suitable for this subpopulation. Something has apparently gone awry in their developmental neurology. I suggest that one aspect of this, once other potential causes have been ruled out, is revealed in the asymmetry of pupillary dimensions. It is an atypicality, though I’m not always sure what to make of it. While this asymmetry might be congenital, we usually don’t have any earlier clinical records to substantiate it. I’m tossing this out there as much as a placeholder to think about and observe it more, as I am for any of our readers to share their thoughts.
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