in the current issue of critical care , paulus and colleagues investigate whether the need for additional analgesia to prevent pain from endotracheal suctioning can be predicted by measuring the pupillary dilatation reflex ( pdr ) response to tetanic stimulation in deeply sedated patients . this study touches on two important issues : objective pain assessments and pre - emptive analgesia . pain in the critically unwell can lead to significant physiological and psychological consequences , and may affect patient outcome . assessing pain in the critically ill is challenging . when patients are unable to self - report , assessment is based on behavioural and physical signs . unfortunately physiological variables are not predictive of the presence or absence of pain , and recommended behavioural tools ( critical care pain observation tool and behavioural pain scale )   only identify patients who are experiencing pain . this pain is common and poorly managed , explained in part by the inability of behavioural tools to predict those patients who require additional analgesia for painful procedures . apart from chest drain removal , only low - grade evidence supports the use of pre - emptive analgesia for procedural pain   and over - administering opioids to patients is not without potential consequence . management at present is limited to either administering analgesia blindly before every procedure or taking a reactionary approach to treat pain after it has occurred . the predictive utility of pdr to guide pre - emptive analgesia for painful procedures might be a possible solution to an important clinical question . loewenfeld summarised this work and her own investigations in the late 1950s , and copious literature now exists exploring this phenomenon in adults and children under general anaesthesia , in awake volunteers , in organ donors   and in the postoperative period   as a means of assessing analgesic requirements . larson and sessler summarised in their editorial other influencing factors that clinicians need to consider when using pdr in pain assessment , including the effect of drugs and technical difficulties such as patient movement . however , there are a number of questions that remain unanswered concerning the use of pdr in sedated patients , which should be considered . paulus and colleagues are the first group to investigate pdr in response to tetanic stimuli in deeply sedated patients . pdr is a supraspinal parasympathetic reflex in patients under general anaesthesia   but is sympathetically mediated in awake patients . does the mechanism differ dependent on the depth of sedation and how does this affect pdr ? recently the pdr response to light has been shown to reduce with increased levels of sedation , and this needs to be explored in response to tetanic stimuli as well . furthermore , pupil size oscillates naturally ( pupillary hippus ) in the awake state . this is due to the spontaneous fluctuation between the tone and activity of the sympathetic and parasympathetic nervous systems . whether different depths of sedation make a similar contribution to pupil size and affect pdr remains elusive . finally , the influence of different sedative pharmacological agents on the pdr response requires further investigation . the use of pdr to predict additional analgesic requirements for endotracheal suctioning , in deeply sedated patients , shows promise . the use of a non - invasive , innoculus stimulus in conjunction with pdr potentially provides an objective measure to guide pre - emptive analgesia . however , to make pdr a truly useful clinical tool , further research needs to address some of the questions raised above . cb is funded by the higher educational funding council for england and chelsea and westminster hospital nhs foundation trust .