Quantitative Assessments of Pupillary Light Reexes in Hospital-Onset Unconsciousness

Background Patients who develop hospital-onset unconsciousness should be promptly managed in order to avoid clinical deterioration. Pupillary examination through pupillary light reex is the mainstay method in the initial evaluation of unconscious patients, but the current method of light shedding and subjective description often shows poor reliability. The objective of this study is to explore whether a quantitative measurement of pupillary light reexes is useful in detecting brain herniation syndrome and predicting neurological outcomes in non-neurological patients who developed hospital-onset unconsciousness. Methods This was a registry-based observational study on patients who activated the neurological rapid response team at Asan Medical Center (Seoul, Korea). Hospital-onset unconsciousness was dened as a newly developed stupor or comatose state during admission. Demographics, comorbidities, pupillometry parameters including Neurological Pupil index, brain herniation syndrome, in-hospital mortality, and modied Rankin Scale at 3-months were analyzed.


Introduction
Unconsciousness is one of the most common complaints in patients during emergency department visits and hospitalizations [1]. Various causes underlie unconsciousness, including metabolic encephalopathy, brain structural lesions, meningoencephalitis, and seizure disorders. Early management of the underlying causes as well as the clinical symptoms should be promptly provided in order to avoid clinical deterioration, morbidity, and mortality [2]. Hospitalized patients generally have more comorbidities and are sicker than the general population; hence, the etiologies and outcomes of sudden unconsciousness may differ between in-hospital patients and outpatients. Nevertheless, comprehensive studies on the early evaluations and long-term outcomes of hospital-onset unconsciousness (HOU) are lacking.
Pupillary examinations are the mainstay in the initial evaluation of unconscious patients [3]. The assessments of pupils include diameter, shape, symmetry, and light re exes. Evaluation of the pupillary light re exes (PLR) is conventionally performed by shedding light into the patient's eye and subjectively described by examiners. However, such conventional assessments of PLR have low inter-rater and intrarater correlations [4,5]. Recently, quantitative pupillometery (QP) was introduced to the neurocritical care eld to provide objective assessments of the PLR. QP is able to detect subtle early changes in the size and light re exes of pupils [6]. This bedside tool is becoming popular in the routine evaluation of unconscious patients who are admitted to intensive care units and require serial measurements of pupils, especially for those with brain herniation syndrome (BHS) [7][8][9][10]. Moreover, QP was shown to be useful in neurological prognostication in unconscious patients after cardiac arrest [11][12][13][14][15][16][17]. Because the PLR is regulated by the autonomic nervous system, QP may also have a role in the assessment of autonomic dysfunction in critically ill patients [18,19]. However, studies on the QP ndings in patients who are hospitalized outside intensive care units are lacking. Moreover, the clinical implication of the ndings of sluggish pupils remains largely uninvestigated in unconscious patients, although xed dilated pupils inarguably suggest urgent situations and grave prognosis [3].
Here, we investigated whether quantitative assessments of the PLR through QP measurement indicates the presence of BHS in non-neurological patients who developed HOU. We also explored which QP parameters are related to clinical outcomes in such patients.

Patients
This registry-based longitudinal observational study was performed between September 1, 2017 and November 30, 2018 at Asan Medical Center, a tertiary hospital located in Korea. All data for the current study have been collected and documented on the registry of the Neurological Alert Team (NAT), a roundthe-clock neurologist-led rapid response team organized for the improvement of hospital-wide performance in response to in-patient medical emergency. [20] For this study, we included patients who were (1) 18 years of age or older, (2) admitted to non-neurological departments, and (3) activated the NAT due to hospital-onset unconscious state (ie, stupor or comatose state) as assessed by the ACDU (Alert, Confused, Drowsy, and Unresponsive) scale, which is a simple four-point scale for assessing patients presenting with altered mental status. [21] The NAT neurologists or clinical nurse specialists routinely measured the patients' pupils by using QP for initial neurological assessment during o ce hours. We excluded patients with no available QP measurements, incomplete demographic data, or unconsciousness following cardiac arrest. This study was approved by the institutional review board of Asan Medical Center and the need for written informed consent was waived considering the retrospective nature of the study.

Clinical Assessments
According to our protocol, the neurologists performed initial neurological assessments as soon as possible following NAT activation. Systematic evaluations including vital signs, ACDU scale, Glasgow Coma Scale (GCS), laboratory tests, computed tomography (CT) and/or magnetic resonance imaging (MRI) of the brain, and electroencephalography (EEG) were conducted. Clinical outcomes were assessed by the survival status at hospital discharge and the modi ed Rankin Scale (mRS) at 3 months after the NAT activation. Neurological status was categorized into good (mRS = 0-3) and poor (mRS = 4-6) [22]. All data were prospectively documented in our registry and were reviewed and adjudicated by the investigators at weekly conferences.
Pupillometry Measurements QP was measured at bedside using the NPi-100 pupillometer (NeurOptics, Irvine, CA, USA). The detection thresholds were 10.00 mm for the maximum pupil diameter (Max), 1.00 mm for the minimum pupil diameter (Min), and 0.03 mm for the change in size. The QP parameters included Max (mm), Min (mm), percentage of change (CH = 100 × [Max -Min]/Max), constriction velocity (CV, mm/sec), latency of constriction (Lat, msec), dilation velocity (DV, mm/sec), and Neurological Pupil index (NPi). NPi is a scale with values ranging from 0.0 to 5.0, and its algorithm is derived from a normative model with values measured from healthy volunteers. [8] Low NPi value can be interpreted as less distinct PLR (ie, decreased pupillary reactivity). All QP assessments were performed at least twice in each eye. Among the repetitive QP measurements, the dataset with the maximum NPi value content was documented for each eye. From the results measured on each eye, lower (NPi, CH, CV, DV) or higher (Max, Min, Lat) values for either eye were used for analysis.
The presence of BHS was radiographically and clinically evaluated. We de ned BHS as the development of acute brain lesions that were relevant to the unconsciousness, including midline shift, brainstem compression, hydrocephalus, and diffuse cerebral edema [23]. All CT scans and MRI sequences were jointly interpreted by two investigators who were blinded to the QP data and clinical outcomes. A third investigator was consulted in cases of disagreement.

Statistical Analyses
Data are summarized as mean ± standard deviation for normally distributed continuous variables, medians and interquartile ranges for non-normally distributed continuous variables, and frequencies with percentages for categorical variables. Univariable analyses were carried out to identify the relationship between each variable and BHS or clinical outcomes; categorical variables were compared with Pearson's χ 2 test or Fisher's exact test, and continuous variables were compared with the Student's t-test. Variables with p values < 0.1 in the univariable analysis were included as candidate variables in the multivariable logistic regression model and removed by backward stepwise selection. We further performed all analyses using a forward selection procedure to con rm the nal model. All QP variables signi cantly associated with BHS or clinical outcomes in the univariable analysis were individually added to the nal clinical models to generate the odds ratio (OR) and the 95% con dence intervals (CIs) thereof. To analyze the usefulness of QP for the prediction of BHS, we calculated the predictive accuracy by sensitivity, speci city, negative predictive value, and positive predictive value. Two-tailed p values < 0.05 were considered statistically signi cant. All statistical analyses were performed using R version 3.6.2 (R Foundation for Statistical Computing, Vienna, Austria) and GraphPad Prism, version 8.3.1 (GraphPad Software, La Jolla, CA, USA).

Patient Characteristics
A total of 1,442 patients activated the NAT during the study period. We included 404 (28%) patients who were in an unresponsive state as assessed by the ACDU scale at the time of on-call consultations to the NAT; and excluded 190 patients who either did not have QP measurements due to NAT activation during after-hours (n = 129), showed unconsciousness following cardiac arrest (n = 46), or had incomplete demographic data (n = 15). Thus, the remaining 214 patients were included in the nal analysis; their mean age was 63.9 ± 14.9 years, and 127 (59%) were men. The baseline characteristics of the 214 patients are presented in Table 1. Brain Herniation Syndrome BHS was found in 37 (17%) patients, who showed higher in-hospital mortality rate compared with those without BHS (57% [21/37] vs. 21% [38/177]; p < 0.001). The diagnoses of the patients with BHS were hemorrhagic stroke (n = 10), ischemic stroke (n = 6), acute liver failure (n = 6), brain tumor or leptomeningeal seeding with acute hydrocephalus (n = 6), hypoxic-ischemic brain injury (n = 3), meningoencephalitis (n = 2), subdural hemorrhage (n = 2), cerebral venous thrombosis (n = 1), and unknown etiology of diffuse cerebral edema (n = 1). In univariable analysis, younger age, good functional status before admission, absence of a history of hypertension or diabetics, and lower GCS score at NAT activation were signi cantly associated with the risk of BHS (Table 2). Among the QP parameters, lower NPi, larger Max, and larger Min showed independent association with the risk of BHS on multivariable analysis. The NPi cut-off value of < 1.6 provided the maximum area under the receiver operating characteristic (ROC) curve (0.750) for predicting BHS, with a sensitivity of 49% (95% CI = 32 − 66) and a speci city of 91% (95% CI = 86 − 95). In addition, the positive predictive value and negative predictive value of this NPi cut-off value for predicting BHS were 53% (95% CI = 39 − 67) and 89% (95% CI = 86 − 92), respectively.

Neurological Outcomes
A total of 201 (94%) patients had available data on mRS at 3 months. Poor neurological outcome was noted in 152 (76%) patients. Table 3 shows the factors associated with poor neurological outcomes in patients with HOU, which included age, pre-admission mRS ≥ 4, diabetes mellitus, cancer, and previous stroke. The following QP Parameters were also associated with poor neurological outcomes at 3 months: NPi, CH, CV, Lat, and DV. Multivariable analysis showed that pre-admission mRS ≥ 4, cancer, previous stroke, NPi, CH, Lat, and DV were signi cantly associated with poor neurological outcomes at 3 months.
The area under the ROC curve for poor neurological outcomes at 3 months was 0.762 in the model with clinical variables (pre-admission mRS ≥ 4, cancer, and previous stroke). Upon addition of the QP variables (NPi, CH, Lat, and DV), the area under the ROC curve for poor neurological outcomes at 3 months was increased to 0.837 (Fig. 2).

Discussion
In this registry-based observational study, we showed that the application of QP was useful for detecting BHS and predicting clinical outcomes in patients with HOU. When the patients' altered mental status activated the neurological rapid response team, the initial detection of reduced pupillary reactivity (lower values of NPi) as well as enlarged pupillary size (Max and Min) were signi cantly associated with later identi cations of BHS. The speci city and negative predictive value of the nding of NPi < 1.6 were as high as ~ 90% in predicting the presence of BHS, while the sensitivity and positive predictive value were relatively low at ~ 50%. QP parameters indicating reduced pupillary reactivity were also signi cantly associated with in-hospital mortality (lower values of NPi, CH, CV, and DV, and higher values of Lat) and poor neurological outcome at 3 months (lower values of NPi, CH, and DV, and higher values of Lat). The addition of QP in considering the patients' medical conditions enhanced the prediction value (larger area under the ROC curve) of mRS at 3 months. To the best of our knowledge, this is the rst study to show the prognostic value of measuring QP in patients who are admitted for non-neurological illness and develop sudden unconsciousness.
In terms of the etiologies of HOU, we found that metabolic encephalopathy was the most common cause, followed by seizure-related events, ischemic and hemorrhagic strokes, and metastatic and primary brain tumors. Approximately 43% of our study population had acute diseases of the brain such as stroke, intracranial bleeding, brain tumor, meningoencephalitis, and hypoxic-ischemic brain injury. BHS, one of the most severe life-threatening neurologic emergencies, was found in 17% of patients with HOU and was caused by massive cerebral infarcts, subdural hemorrhage, or diffuse brain swelling related to metabolic conditions (eg, acute liver failure). Seizure-related events including convulsive seizure, nonconvulsive status epilepticus, and postictal state were also as frequent as 29%.
We found that the use of QP may be useful for the early detection of BHS. The pupillary examination is a crucial component in the initial evaluation of unconscious patients, but early signs of pupils relevant to BHS can be nonspeci c or elusive despite careful examination. In emergency situations, it is challenging to quickly perform and properly interpret neurological examinations, especially in hospitalized patients with multiple comorbidities. According to the experience of our rapid response team, QP is a practical tool when used in addition to routine neurological examination, because it is portable and can be quickly performed at bedside. In a previous preliminary observational study, midline shift and increased intracranial pressure were associated with a decrease of CV below 0.6 mm/sec; however, patients with diffuse brain edema and without midline distortion did not show such a decrease in CV until the intracranial pressure exceeded 30 mmHg [10]. In our study, 13 of 37 patients with BHS had diffuse brain swelling but did not have midline shifts, which may account for the absence of differences in CV between the BHS and non-BHS groups. Nevertheless, we demonstrated that the NPi value of less than 1.6 has a high speci city and a high negative predictive value for detecting BHS in patients with HOU. Collectively, these ndings suggest that patients who develop unconsciousness during the hospital admission and show decreased NPi value on QP measurements would bene t from undergoing expeditious neuroimaging studies to detect BHS.
Traditionally, the PLR has been rather subjectively assessed by using a variety of non-standardized light sources by practitioners with varying levels of skills in neurological examinations. Recent studies have shown that such subjective assessments of pupillary reactivity have subpar reliability, with low inter-rater and intra-rater correlations [4,5]. Recently, the QP has become popular as it provides a non-invasive, hand-held implementation of neuro-monitoring. QP also offers an objective and standardized measurement of the PLR, and mean QP values in healthy volunteers (Max 3.5-5.3 mm, Min 2.4-3.4 mm, CH 29-36%, Lat 0.22-0.27 sec, CV 1.5-2.9 mm/sec, and DV 0.9-2.2 mm/sec) have been suggested in prior studies [7,10,[24][25][26]. Additionally, the automated algorithm of QP provides an NPi value derived from a combination of QP variables. An NPi score below 3.0 is generally considered an abnormal nding of sluggish light re ex [8]. Preliminary studies conducted in intensive care units reported the usefulness of QP for detecting a wide range of conditions including increased intracranial pressure [7][8][9][10]. response to osmotherapy [27], discrimination between compressive lesions and microvascular ischemic oculomotor nerve palsy [28], assessment of disease severity of aneurysmal subarachnoid hemorrhage [29], the depth of sedation and analgesia [30], and neurological prognostication in comatose resuscitation-of-spontaneous-circulation following cardiac arrest [11][12][13][14][15][16][17]. The prognostic implication of QP has been only investigated in patients with cardiac arrest [11][12][13][14][15][16][17], in whom a decrease in NPi values less than 2.0 was associated with unfavorable neurological outcomes [13]. Our results showed that the QP was useful in detecting BHS and predicting clinical outcomes in patients with HOU, and that decreased NPi values and increased pupillary sizes were signi cantly related to BHS. Decreased NPi values were also associated with in-hospital mortality and unfavorable neurological outcomes at 3months. In terms of QP variables, the pupillary size (Max and Min) was not signi cantly related to mortality rate or neurological outcomes, whereas other QP variables such as CH, Lat, and DV were associated with clinical outcomes including in-hospital mortality and neurological status at 3-months.
Therefore, a combination of QP variables may be more reliable than the NPi alone in the prediction of neurological outcomes, despite the high predictive value of the NPi. In summary, our results show that measurements of PLR with QP may be useful in the early detection of life-threatening neurological conditions and neuro-prognostication for patients with HOU.
Fixed or dilated pupils are generally ominous neurological signs, as sluggish or absent PLR may indicate the compression or stretching of the dorsal midbrain in which the Edinger-Westphal nuclei is located, or of the efferent oculomotor nerve that carries parasympathetic bers [31]. Some studies suggested that the integrity and function of PLR may also be affected by the perfusion defect to the brainstem or alterations of neurotransmitter release [32,33]. Furthermore, even though pupillary constriction by light stimulus is predominantly integrated by the parasympathetic nervous system, it is possible that sympathetic activities are also engaged in the regulation of the pupillary reactivity. First, the sympathetic nerves are involved in the dilation phase of PLR: the supranuclear inhibition via sympathetic neurons suppresses the pre-ganglionic parasympathetic neurons at the Edinger-Westphal nucleus, resulting in relaxation of the pupil sphincter muscle; also, the sympathetic neurons contract the iris dilator muscle via peripheral sympathetic innervation [31]. Second, the reticular activating system affects the pupil size and PLR by tonic inhibitory input of the Edinger-Westphal complex through releasing norepinephrine [34]. Third, cognitive and emotional processes may result in mydriatic reaction by the input of cortical innervation into the brainstem, although the exact circuits remain poorly understood [34,35]. Thus, the dynamics of the pupil reactivity may be indicative of lesions or dysfunctions of the cortex, subcortex, and brainstem that affect the parasympathetic system, sympathetic system, neurotransmitters, and their complex interactions.
Further studies are needed to clarify the mechanism underlying the association between QP values and clinical outcomes in patients with HOU in the absence of BHS. It is possible that autonomic dysfunction, as well as multiorgan dysfunction and brain dysfunction (eg, metabolic encephalopathy and BHS) may underlie such association. Autonomic dysfunction mediated by in ammatory response likely has an important role in the pathogenesis of the dysfunction of the brain or other organs [36]. Neurotransmitter imbalance such as cholinergic de ciency due to in ammation and multiorgan dysfunction in critically ill patients can lead to pupillary dysfunction [18,33]. In this context, although there is limited evidence on the association between QP values and the severity of encephalopathy [37], QP as an indicator of autonomic and brain dysfunction may play a role in assessing the severity of metabolic encephalopathy as the cause of HOU (Fig. 3).
In addition to the inherent limitations of its single-center retrospective design, the present study has the following limitations. First, we did not assess the pupillary dilation re ex, which may be evoked by sensory stimulation and predominantly mediated by the sympathetic nervous system. Instead, we measured the DV, which could re ect sympathetic activity during the dilation phase of the PLR. Second, concurrent use of drugs that could confound the evaluation of the PLR such as opioids, anticholinergics, or sedative agents was not evaluated. Although a previous study showed that the use of these medications within therapeutic doses does not signi cantly suppress the PLR [38], we cannot exclude the possibility that other medications such as propofol may have affected the PLR [39]. Third, we did not analyze confounders such as underlying pathology of the retina or optic nerve as well as the in uence of ambient lights on the PLR [40].

Conclusions
In conclusion, reduced reactivity and increased size of pupils were related to BHS in patients with HOU.
Moreover, reduced pupillary reactivity was associated with in-hospital mortality and poor neurological outcomes at 3-months. These ndings suggest that QP measurement may be useful for early detection of potentially life-threatening neurological conditions in non-neurological in-patients who develop unconsciousness.

Declarations
Manuscript complies with all instructions to authors. Authorship requirements have been met and the nal manuscript was approved by all authors. This manuscript has not been published elsewhere and is not under consideration by another journal.

Availability of data and materials
The data that support the ndings of this study are available from the corresponding author upon reasonable request.

Ethics approval
This study was approved by the institutional review board of Asan Medical Center and the need for written informed consent was waived considering the retrospective nature of the study.