Pupil to limbus ratio: Introducing a simple objective measure using two-box method for measuring early nisocoria and progress of pupillary change in the ICU

Abstract

Introduction:Measurement of static pupillary size in the ICU is of importance in cases of acutely expanding intracranial mass lesions. The inaccuracies with subjective assessment of pupillary size by medical personnel preclude its use in emergent neurological situations.Objective:To determine if the ratio of pupil to limbus diameter (PLD ratio) measured by a two-box method is a reliable measure of pupil size for detecting early anisocoria and measuring pupillary changes.Materials and Methods:The PLD ratio was defined as the ratio of the pupillary diameter measured at a para-horizontal axial plane with the limbus diameter measured at the same or parallel axial plane. A two-box method was used to estimate the diameters of imaged pupils. Eyes were imaged using an iPhone 4S cellphone camera. Background illumination was measured and kept constant. The pupils of a 78-year-old woman, who presented with a large intra-axial parenchymal hemorrhage, were imaged. The patient had left pupillary miosis in dark but not in bright light. After presenting this case along with the images of the pupillary examination, a group of 21 medical staff were asked several questions on the pupillary examination. Reliability of PLD ratio were assessed via standard error of mean (S.E.M) of PLD ratios for 3 different subjects each imaged under constant illumination and fixation but from different angles to the optical axis.Results:Analysis of questionnaire data together with PLD ratios revealed that ~ 14% and 10% of participants could estimate the pupillary size in darkness and bright light respectively but none were simultaneously accurate indicating that subjective assessment of pupillary size was unreliable. The approach towards a systematic pupillary examination was inconsistent among the participants. The PLD ratio was found to be a reliable measure of pupillary size with standard error of mean below 0.1 mm for the three subjects tested.Conclusion:Static pupillary sizes can be objectively and consistently evaluated using PLD ratios using a two-box method. PLD ratios are resistant, within limits, to changes in imaging angle or choice of para-horizontal axes for measurement.

Pupillary abnormalities in non-selected critically ill patients: an observational study

“…Our findings should be cautiously compared with those from previous studies that almost uniquely included neurosurgical patients (4)(5)(6)(7)(8)(9). The prevalence of PAs may vary according to the quality of the pupillary evaluation, which can be challenging at the bedside (2,(10)(11)(12)(13)(14). Indeed, the difference in pupillary diameter between the two eyes should be measured under equal illumination and with vergence and accommodation (2).…”

“…Therefore, we cannot rule out the fact that some of areflexia were false positives in our study. Some PAs could also have been missed because of inadequate examinations or monitoring intervals (12,13,17). To date, the best way to limit misdiagnosis of PAs is probably to objectively measure the size and reactivity of pupils by using a handheld automated pupillometer (11,12,15 17).…”

Conceptual advantages of using pupil to limbus ratio over absolute pupil diameter

“…We have read with interest the article of Mojumder et al on the use of pupil to limbus ratio to measure anisocoria [1] and we would like to share our experience on this topic:…”

The Relationship Between Blood Pressure and Pupil to Limbus Diameter Ratio in Hypertensive Women: A Pilot Study

“…PLD ratio was measured by two-box method as described in the literature [7]. PLD ratio was measured at constant illuminance for all the participants.…”

“…By measuring pupil diameter as an indicator of autonomic nervous function, pupil to limbus diameter (PLD) ratio may correlate with alterations in BP [6]. PLD ratio is defined here as "the ratio of the pupillary diameter measured at an axial plane with the limbal diameter measured at a same or parallel axial plane" [7]. Recent studies show that there is a positive correlation between BP and PLD ratio of right and left eye and significant positive correlation between pulse rate and PLD ratio of the right and left eye [8].…”