Khaarthiyaa Lokanathan, B.S. Optometry
Research Optometrist, Medical Research Foundation, Sankara Nethralaya, Chennai
The natural camera, our eye, captures several images from our surroundings via an aperture, which controls the amount of light passing through the ocular media and reaching the light-sensitive layer of the retina. This aperture, the pupil (a small central opening in the iris), is magnificently designed to respond within a span of milliseconds to constrict and dilate primarily with accordance to the illumination intensity (Pupillary Light Reflex- PLR). The sphincter and dilator muscle aids in pupillary constriction and pupillary dilatation respectively. Pupil constriction is mediated by the parasympathetic nervous system’s innervation of the pupillary sphincter (Oculomotor nerve supply) and occurs as a result of stimulation by either light or near target 1. Pupil dilatation is facilitated by the sympathetic nervous system’s innervation of the pupillary dilator (Trigeminal nerve supply) 1.
Pupil plays a crucial role not only as an aperture, but also as a window that opens up possibilities to remarkably understand the dynamics of our neurological system. Hence, there is no wonder why the pupil evaluation becomes a significant element of a primary eye examination. Clinicians routinely observe the inter-pupil symmetry with respect to the size, shape, light and near reflex. Pupil diameter (PD) is measured using a pupil gauge/ruler and the Relative Afferent Pupillary Defect (RAPD) is identified using the swinging flashlight test. Though the conventional pupillary assessment is a simple and rapid approach, it might not be sensitive enough to pick a subtle/early defect, and can have an influence from multiple external factors such as room lighting, intensity and direction of light stimulation and inter-observer dependency. These were the foremost reasons behind the development of well-controlled pupillometry systems. The quantitative pupillometry (QP) systems offer a comprehensive quantitative estimation of static and dynamic pupil parameters. QP uses pre-fixed stimuli and background illumination, allowing an objective and accurate measurement of PLR parameters. It consists of a light stimulus, an infra-red camera, a digital interface for processing and recording of pupil data 2. It captures not only the magnitude related changes of the pupil (PD) but also the speed/velocity and duration taken for the response. A few commercially available pupillometers are EyeKinetix by Konan Medical (USA), NeurOptics Pupillometer (California), and PupilX (Germany).
Quantitative Parameters of Pupil:
The following figure describes the various quantitative measurements during PLR.
Figure 1: Schematic representation of Pupil Light Response and its quantitative parameters 2, The light stimulus at time zero results in a rapid reduction in pupil diameter. Latency (tL) is calculated as the elapsed time between light onset and the start of constriction. The pupil then rapidly constricts (Maximal Constriction Velocity; MCV) from the baseline (D0) pupil diameter to the minimum (Dmin) pupil diameter; the constriction time (tC) and Maximum Constriction Amplitude (MCA) are calculated as the time interval and size difference between these two values, respectively 2. At offset of light stimulus or during sustained light stimulation, the pupil undergoes a period of rapid redilatation or pupillary “escape” to a partially constricted state. Subsequently the pupil slowly returns to the baseline diameter 2.
Dilatation of the pupil following a light stimulus corresponds to the recovery phase of the PLR 2.
The above figure depicts the pupillary reflex to light, the response of pupil to near target can also be quantified. Pupil Near Response includes Latency, Amplitude and Recovery time (time needed for the pupil to reach its baseline PD post stimulus presentation) 3.
Significance of QP:
Even though QP is more beneficial than the conventional methods, the high cost and lack of awareness are its pitfalls. They have to be addressed soon, for more widespread practicality. It is an important screening/diagnostic cue to identify pupillary abnormalities from retinal to cortical level. It can provide insights about the clinical conditions like RAPD 4, Anisocoria 4, any lesion/infarct at the chiasmal level 5 (early detection by pupil responses) and neurodegenerative conditions (e.g. Alzheimer’s and Parkinson’s disease) 6 . The functional aspect of any pupillary defect can be analysed at a primitive stage too- quick diagnostic marker. The progression/prognosis of any disease can easily be predicted, unlike the conventional manual method. Its accurate, reliable and reproducible feature complimenting its swift efficacy has made it a desirable diagnostic technique.
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