Feasibility of Pupillometry-Based Indices to Assess Nociception in Sedated Critically Ill Patients.

Abstract

To the Editor We have read with interest the article by Vinclair et al.1 We appreciate the research study done by the esteemed authors, as we also use pupillometer in our clinical practice. Patients on treatment with drugs that were known to alter pupil reflex dilation measurements were excluded from the study. It was noted that for sedation, the patients received continuous infusions of midazolam or propofol, sufentanil, and ketamine at the discretion of the physician in charge.1 Also, the doses of sufentanil were higher in the brain-injured group than in the non–brain-injured group. Rouche et al2 studied the effects of depth of sedation on pupillometry indices in mechanically ventilated patients. They compared the maximum contraction velocity of the pupil (Vmax) and variation in pupil diameter (PD) among 3 groups of patients based on the levels of sedation by evaluating the of bispectral index (BIS). The groups were classified as patients under heavy sedation (<40), acceptable sedation (40–60), and light sedation (>60) based on BIS values. There was a significant difference in Vmax between the heavily sedated and the acceptable groups (P < .0001) and also a significant difference in the variation of PD (P < .0001) between the groups.2 Similarly, there was a significant difference in Vmax (P < .0001) and variation in PD (P < .0001) between heavy sedation and light sedation groups.2 Gaillard et al3 evaluated the pupillary dilation reflex (PDR), to a noxious stimulus, for predicting analgesic level before endotracheal suctioning of mechanically ventilated patients under a deep level of sedation by applying tetanic stimulations of 10, 20, 40, and 60 mA.3 There was no significant difference in the area under the receiver operating characteristic (ROC) curves between different intensity levels. It was also noted that the PDR never exceeded 0.6 even with the highest stimulation.3 In the present study, the authors have not mentioned the level of sedation of the patients, and this could have contributed to possible bias in their study. In the present study, the authors found that pupillary pain index measurements during tetanic stimulation positively correlated with behavioral pain scale responses to endotracheal suctioning. However, the authors did not analyze the correlation with the behavioral pain scale during tetanic stimulation. Tetanic stimulation and endotracheal suctioning are 2 different noxious stimuli as one is a central stimulus carried by the cranial nerves, whereas the latter is a peripheral stimulation via the spinal cord. Though the authors have excluded patients with bilateral mydriasis, which happens at a later stage of brain herniation, they did not exclude patients with other pupillary abnormalities. Patients at early stages of uncal herniation will have unilateral mydriasis and patients with mild brain injury can present with sluggish pupillary responses.4 Similarly, the patients with brainstem injury could present with varied pupillary responses, the most notable being the pinpoint pupils encountered in pontine injury.4 We want to point out that there had been no mention of these clinical presentations and the assessment of pupillometry in such patients could result in erroneous findings. Aishvarya Shree Nedunchezhian, MDNeeraja Ajayan, MDAjay Prasad Hrishi, MD, DNB, DMDivision of NeuroanesthesiaDepartment of AnaesthesiologySree Chitra Tirunal Institute for Medical SciencesThiruvananthapuram, Kerala, India[email protected]