Ketamine-Occasioned Mystical Experience in Veterans with Treatment-Resistant Depression: A Retrospective Exploratory Analysis

Abstract

Introductions:

Mystical experiences are powerful experiences that may have therapeutic value. Ketamine, a dissociative anesthetic, has shown potential to induce mystical experiences. However, little research has explored this phenomenon, particularly in treatment-resistant depression.

Methods:

We analyzed clinical data from 60 veterans with treatment-resistant depression receiving a total of 189 ketamine treatments. Veterans either received intranasal esketamine or racemic parenteral ketamine (intravenous or intramuscular). The Revised Mystical Experience Questionnaire (MEQ-30) was administered following ketamine treatments to assess the occurrence of mystical experience. A linear mixed model was used to examine the association between MEQ-30 scores and several treatment-related variables including gender, age, treatment number, dose, comorbid post-traumatic stress disorder, and pretreatment depression rating scores (PHQ-9).

Results:

Complete mystical experience was reported in 17.02% of esketamine treatments and 18.19% of racemic ketamine treatments. In the esketamine group, a greater number of treatment sessions was associated with higher MEQ-30 scores (p = 0.05). In the racemic ketamine group, higher doses were associated with higher MEQ-30 scores (p = 0.002).

Discussion:

These findings suggest that ketamine can occasion mystical experiences in veterans with treatment-resistant depression. Future studies should further explore the mystical-type effects of ketamine as a potential contributor to its therapeutic effect in treatment-resistant depression.

Introduction

A mystical experience is a powerful experience often characterized by feelings of connectedness with others, transcendence of space and time, bliss, and ineffability.¹ Such experiences, reported by people throughout human history, can have numerous potential causes including deep meditative states, illness-induced delirium, and near-death experiences.^2,3 The relevance of mystical experiences in psychiatry has recently been highlighted, as they are often occasioned by classical psychedelics, some of which are promising candidates for the treatment of psychiatric disorders.^4–6

Psychedelic-occasioned mystical experiences have been shown to have beneficial effects. In a seminal study on such phenomena, participants who had psilocybin-occasioned mystical experiences described it as one of the top five most meaningful experiences of their lives.⁷ Several other studies have since corroborated these findings, and further suggest that such experiences may have enduring effects on an individual’s life perspective, prosocial tendencies, and overall well-being.^8,9 Moreover, studies have shown that mystical experiences may be predictors of treatment response in people with depression receiving psychedelic therapy.^10–12 The degree to which mystical experiences contribute to the antidepressant effect of psychedelic drugs, however, remains a heavily debated area of research.^13,14

Ketamine is a dissociative anesthetic that shares neurophysiological and phenomenological properties with psychedelics.¹⁵ The mechanism of action of the rapid antidepressant properties of ketamine is thought to primarily result from noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonism at inhibitory gamma-aminobutyric acid (GABA)-ergic interneurons, leading to disinhibition of glutamate release and subsequent alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor activation.¹⁶ Although this mechanism is distinct from the serotonin 2A receptor agonism of classical psychedelics, there are significant similarities in downstream effects. These include activation of cellular pathways promoting neuroplasticity, alterations in brain oscillation patterns, reduced intranetwork functional connectivity and increased internetwork connectivity.^17–19 Moreover, while research on the acute subjective effects of ketamine has largely focused on its psychotomimetic and dissociative effects,²⁰ a growing body of research suggests that ketamine may induce subjective effects similar to psychedelics. For example, recent research has shown that ketamine may elicit increases in empathy,²¹ feelings of connectedness with others, spirituality, and profound changes in life perspective.²²

Few studies have directly examined mystical experiences occasioned by ketamine and none, to our knowledge, have done so in veterans with treatment-resistant depression. Studies have explored this question in healthy adults, frontline health care workers with burnout, and sufferers of substance use, with findings suggesting that ketamine can occasion mystical experiences.^23,24 Additionally, these experiences may be part of ketamine’s therapeutic effect.^25,26 A previous study found that mystical experience, but not the dissociative effects, of ketamine correlated with decreased at-risk drinking.²⁵ Another study similarly found mystical experience, rather than other acute subjective effects of ketamine, mediated decreased cocaine use.²⁶ A recent study found that feelings of awe, but not dissociation were associated with antidepressant effects of ketamine.²⁷ Thus, there is evidence to suggest that ketamine-induced mystical experiences may have unique therapeutic value during ketamine treatment for psychiatric disorders.

This retrospective exploratory analysis aimed to investigate ketamine-induced mystical experiences in a clinical cohort of veterans receiving intranasal esketamine or racemic parenteral ketamine (intravenous or intramuscular) for treatment-resistant depression. Specifically, we aimed to (1) investigate the frequency of mystical experience occasioned by esketamine and racemic parenteral ketamine, respectively and (2) explore factors associated with mystical experience as measured by the Revised Mystical Experience Questionnaire (MEQ-30).

Methods

Study design and participants

This study utilized a naturalistic dataset comprised of veterans with treatment-resistant depression who received ketamine treatment at the VA San Diego Healthcare System Neuromodulation program (consisting of outpatient ketamine, TMS, and ECT) between November 2021 and September 2022, during which 86 veterans received a total of 1067 ketamine treatments. Veterans were selected for this study based on convenience sampling. Specifically, veterans were selected when staff was available to administer study surveys, and veterans were willing to participate in filling out surveys. Veterans included in this study either received intranasal esketamine or racemic parenteral ketamine (intravenous or intramuscular) in conjunction with other psychotropic medications they were prescribed. Ketamine treatments were administered in an outpatient clinic setting. Veterans received treatment in a private room equipped with a reclining chair and were provided with eyeshades and headphones to listen to music without lyrics. Veterans met criteria for treatment-resistant depression, defined as nonresponse to at least two trials of traditional antidepressants, consistent with the general clinical criteria for advancing Veterans to treatment in our program. A diagnosis of treatment-resistant depression and the decision to treat with ketamine was made by a psychiatrist in the program. All veterans were initiated on intranasal esketamine. They were switched to racemic ketamine if they did not show therapeutic response (as determined by the clinician) within the first two sessions. Some veterans were directly started on racemic ketamine if they had previously received racemic ketamine outside of VA San Diego. Route of administration of ketamine did not affect pretreatment preparation or support. Prior to treatment, all veterans participated in a phone orientation, during which they were given information about what to expect during ketamine treatment, including the following statement: “during the treatment you will likely experience sedation and/or dissociation (the “Trip” or “out of body experience”).” Please see Supplementary Data for the entire script. Beyond this, there was no pretreatment preparation. There were no assessments of expectancy prior to treatment.

Clinical data included in this analysis were from veterans that either received intranasal esketamine at 84 mg or racemic parenteral ketamine administered with doses ranging from 0.5 mg/kg to 1 mg/kg. The decision to titrate dose was determined by the clinician and involved assessing efficacy and tolerability. If the veteran was tolerating ketamine but lacking significant improvement, then dose was increased. The dataset included variable frequency of repeated measures per participant, with assessments completed anywhere between treatment one and treatment 50.

Ethical considerations

The retrospective analysis was deemed Institutional Review Board (IRB) exempt by the VA San Diego IRB committee (IRB # E240057).

Revised mystical experience questionnaire

MEQ-30 is a 30-item scale designed to assess the intensity and quality of mystical experience. The MEQ-30 includes four domains: (1) mystical, (2) positive mood, (3) time and space, and (4) ineffability. Each item is rated on a 5-point Likert scale, with a maximum score of 150. A “complete mystical experience” as defined by the MEQ-30 is defined as greater than or equal to 60% on each subscale.²⁸ The MEQ-30 was given by clinicians who were part of the neuromodulation program to better understand the experiences of veterans receiving ketamine treatments. Participation was voluntary, and there were no additional inclusion or exclusion criteria to determine who was offered the survey. The administration of the MEQ-30 was part of the ongoing quality improvement processes within the clinic rather than a formal randomized controlled study. MEQ-30 was administered roughly two hours following ketamine administration and just prior to discharge from the treatment session, regardless of administration route. The MEQ-30 was worded in past-tense, prompting participants to reflect on their experience during ketamine dosing. Ketamine treatment was not contingent upon MEQ-30 completion, which led to substantial variability in the collection of the MEQ-30 within and across subjects.

Patient health questionnaire

The patient health questionnaire (PHQ-9) is a psychometrically validated self-report scale widely used to screen for and measure severity of depressive symptoms. It consists of nine items which correspond to the diagnostic criteria for major depressive disorder in the DSM-5. As part of clinic protocol, veterans were required to complete a PHQ-9 within the 48 h preceding each ketamine treatment session. The pretreatment PHQ-9 refers to the scores prior to each treatment obtained before initiating ketamine treatment.

Statistical analysis

Statistical analyses were conducted using SPSS (Version 29.020). Means/medians, standard deviations, and frequency distributions are reported for participant demographic characteristics and PHQ-9 scores at baseline in Table 1. Wilcoxon signed-rank test (for continuous variables) and chi-squared tests (for categorical variables) were used to calculate differences in these baseline variables between patients in esketamine versus racemic ketamine treatment.

Table 1.

Baseline Patient Characteristics

Characteristics	Esketamine treatment	Racemic ketamine treatment	p value
Gender
Male (n, %)	100 (69.9%)	30 (65.2%)	0.55
Female (n, %)	43 (30.1%)	16 (34.8%)	0.55
Age (mean, ± SD)	45.23 ± 12.70	49.78 ± 12.38	0.032
Comorbid PTSD	54.23%	52.17%	0.871
Pretreatment PHQ-9
Mean, ± SD	13.45 ± 5.25	15.41 ± 4.73	0.037
Median	13.00	15.00

SD, standard deviation; PTSD, post-traumatic stress disorder; PHQ-9, patient health questionnaire. Bolded values indicate statistically significant findings.

For inferential analysis, a linear mixed model was employed to analyze the relationship between the MEQ-30 scores and various predictor variables. The model accounted for the repeated measures within individuals by using a compound symmetry covariance structure for the repeated measures. The fixed effects included in the esketamine group were pretreatment PHQ-9 score, comorbid post-traumatic stress disorder (PTSD), treatment number, gender, and age. The fixed effects included in the racemic ketamine group were pretreatment PHQ-9 score, comorbid (PTSD), treatment number, gender, age, and dose. PTSD was determined by clinical evaluation and score on the Post-Traumatic Stress Disorder Checklist, with score greater than or equal to 33 indicating patient meets criteria for PTSD. Dose was not included in the esketamine group because all patients in that group received a fixed dose of 84 mg. Patient identifier was included as a random effect with a covariance structure of variance components to account for individual variability. Interactions thought to be clinically relevant were tested; however, none were statistically significant, so only main effects were included in the final analysis. A sensitivity analysis was performed by removing variables, removing outliers, and altering model assumptions. The final model was considered the best fit model, as it produced the lowest Akaike Information Criterion score.

Power analysis

A formal power analysis was not conducted prior to analysis.

Results

Patient characteristics

The study involved 60 patients and 189 ketamine treatments (143 esketamine and 46 racemic ketamine treatments) (see Table 1). Of those involved in the study, the esketamine group was younger on average (45.23 ± 12.70 vs. 49.78 ± 12.38 years; p = 0.043) and had a significantly lower mean pretreatment PHQ-9 score (13.45 ± 5.25 vs. 15.41 ± 4.73; p = 0.025) compared to the racemic ketamine group. The frequency of comorbid PTSD did not differ significantly between treatment groups, with it being present in 54.23% of patients receiving esketamine and 52.17% of patients receiving racemic ketamine. Gender distribution did not significantly differ between the groups.

Mystical experience frequency

There was large variance in MEQ-30 scores in both ketamine treatment groups (see Fig. 1). In the esketamine group, the mean MEQ-30 score was 59.95 (standard deviation [SD] = 41.032), with scores ranging from 0 to 146. In the racemic ketamine group, the mean MEQ-30 score was 61.52 (SD = 38.784), with scores ranging from 0 to 129.

Fig. 1.

Distribution of Mystical Experience Scores. The top figure shows the distribution of scores on the Revised Mystical Experience Questionnaire (MEQ-30) associated with intranasal esketamine. The mean score was 59.95 with standard deviation of 41.032. The bottom figure shows the distribution of scores on the MEQ-30 associated with racemic parenteral ketamine (intravenous and intramuscular). The mean score was 61.52 with standard deviation of 38.784.

The frequency of complete mystical experiences, defined as a total MEQ-30 score greater than or equal to 60% on each subscale, was similar between ketamine treatment groups, with 17.02% of patients in the esketamine group and 18.19% of patients in the racemic ketamine group reporting a complete mystical experience (see Fig. 2).

Fig. 2.

Frequency of complete mystical experience. The frequency of complete mystical experience, defined as a Revised Mystical Experience Questionnaire (MEQ-30) score of greater than 60% on each subscale. 17.02% of esketamine treatments were associated with complete mystical experience. 18.19% of racemic parenteral ketamine treatments were associated with complete mystical experience.

Linear mixed-effects model analysis

In the esketamine group (see Table 2), the association between treatment session number and MEQ-30 scores was found to be statistically significant (ℬ = 0.73, p = 0.05), in that a higher number of completed treatment sessions was associated with higher mystical experience scores. Other variables, such as pretreatment PHQ-9 score (ℬ = −0.99, p = 0.20), comorbid PTSD (ℬ = −3.09, p = 0.66), age (ℬ = −0.35, p = 0.41), and gender (ℬ = −6.36, p = 0.54), were not significant predictors of MEQ-30 scores.

Table 2.

Esketamine Group

Fixed effects	Estimate (β)	Standard error	p value	95% CI lower	95% CI upper
Intercept	78.82	31.33	0.012	17.41	140.24
Gender	−6.36	10.44	0.54	−26.82	14.10
Age	−0.35	0.42	0.41	−1.18	0.48
Treatment Number	0.73	0.37	0.05	0.01	1.47
Comorbid PTSD	−3.09	7.02	0.66	−16.84	10.66
Pretreatment PHQ9	−0.99	0.77	0.20	−2.50	0.51

CI, confidence interval. Bolded values indicated statistically significant findings.

In the racemic ketamine group (see Table 3), ketamine dose displayed a statistically significant association with MEQ-30 scores (ℬ = 80.38, p = 0.002), with higher doses being associated with higher mystical experience scores. Pretreatment PHQ-9 score (ℬ = −1.21, p = 0.40), comorbid PTSD (ℬ = 12.33, p = 0.24) age (ℬ = −0.26, p = 0.77), and gender (ℬ = 5.54, p = 0.83) were not significant predictors of MEQ-30 scores.

Table 3.

Racemic Ketamine Group

Fixed effects	Estimate (B)	Standard error	p value	95% CI lower	95% CI upper
Intercept	4.47	58.73	0.94	−110.63	119.58
Gender	5.54	26.59	0.83	−46.57	57.66
Age	−0.26	0.86	0.77	−1.95	1.43
Treatment Number	−0.10	0.62	0.87	−1.31	1.10
Dose	80.38	25.70	0.002	30.02	130.75
Comorbid PTSD	12.33	10.42	0.24	−8.10	32.76
Pretreatment PHQ9	−1.21	1.44	0.40	−4.03	1.60

Discussion

This study aimed to investigate the frequency and factors associated with ketamine-occasioned mystical experience in veterans with treatment-resistant depression. Our findings contribute to the growing body of research exploring the acute subjective effects of ketamine and their potential role in treatment.

Our results demonstrate that the frequency of complete mystical experiences was 17.02% and 18.19% in veterans who received intranasal esketamine or racemic parenteral ketamine, respectively; moreover, there was large variation in MEQ-30 scores in both groups. These finding suggests that a substantial number of veterans may experience a mystical experience during ketamine treatment, but there is significant variability in the intensity of this experience. There are several possible contributors to the variability in MEQ-30 scores that we observed. MEQ-30 questionnaires were administered at various time-points during the course of treatment, and administration was contingent upon willingness of veterans to participate. This led to significant variability in the administration of MEQ-30, and likely contributed to the variability in MEQ-30 scores. Another possible contributor to the variability in scores is the wide array of potential acute subjective effects of ketamine. As previously mentioned, ketamine has been shown to produce dissociative and psychotomimetic effects.²⁰ Thus, variability in scores could simply be due to the inherent variability in the effects of ketamine. Lastly, nonpharmacologic factors, including mindset and environment (set and setting), and baseline characteristics of participants, which have been posited to contribute to differences in subjective effects of psychedelics^29–31 may have contributed to the variability in mystical experience effects of ketamine. Although we attempted to explore the relationship between mystical experience and certain potential predictive variables, there are numerous other potential variables that we did not account for, which could have affected the mystical experience.

While a significant number of veterans in both groups experienced complete mystical experience, this number is likely lower than that associated with classical psychedelics such as psilocybin. One study of healthy adults found that psilocybin produced complete mystical experience per the MEQ-30 in 72% of participants.³² Another study of individuals who received psilocybin for treatment-resistant depression reported 58% of participants experienced complete mystical experience as measured by the Oceanic Boundlessness subscale of the 5-Dimensional Altered States of Consciousness Scale.¹² Research suggests that dextromethorphan, which like ketamine, is an NMDA-receptor antagonist induces significantly less potent mystical-type experiences compared to psilocybin, indicating that drug specific differences between ketamine and psilocybin may contribute to variations in subjective experience.^33,34

Nonpharmacological factors may also contribute to these perceived differences. Veterans have comorbidities that differ from populations assessed in other studies, namely high incidence of comorbid PTSD. 54.23% and 52.17% of veterans receiving esketamine and racemic ketamine respectively were suffering from comorbid PTSD. The cooccurrence of depression and PTSD is known to cause greater morbidity and is associated with differing risk factors than depression alone,³⁵ which may have contributed to their treatment experience.

Additionally, psychedelic treatment studies have typically included psychological support during the treatment sessions; the presence of supportive guides or therapists during a psychedelic experience is believed to augment the potential for the occurrence of mystical experiences.³⁶ For our clinical cohort, involvement in psychotherapy outside of the ketamine program was not systematically measured. Moreover, preparatory psychotherapy was not offered as part of the ketamine treatment program. Thus, it is possible that attention to set, setting, and support could have increased the occurrence of mystical experiences in ketamine treatment.

In the esketamine group, treatment number positively correlated with mystical experience scores, suggesting that veterans experienced greater mystical experience further along in their treatment. This finding contrasts from research on the dissociative effects of esketamine, which have shown that dissociation dissipates over time. Multiple factors may explain our finding. It has previously been hypothesized that decreased dissociation may be due to neuroadaptation involving alterations in neurotransmitter activity and receptor expression over time. The fact that mystical experience increased over time in our study, may point to a distinct neurobiological mechanism driving mystical experience. Dissociation has been linked to the NMDA receptor antagonism of ketamine; however, ketamine can also increase extracellular serotonin in the prefrontal cortex.³⁷ Given that classical psychedelics primarily induce mystical experiences through potent serotonergic activity, it is possible that ketamine-induced mystical experiences are also driven more by serotonin receptor activity rather than its NMDA receptor antagonism. It is important to note that some research on classical psychedelics suggests that effects may also dissipate over time.³⁸ Beyond potential neurobiological mechanisms for this finding, there are other possible explanations. First, intranasal esketamine is self-administered by veterans and relies upon absorption through nasal mucosa.³⁹ Inexperience or unfamiliarity with optimal treatment administration may be more likely at early stages of treatment, thus leading to suboptimal dose of esketamine. Second, the observed relationship between mystical experience scores and treatment number may have been related to decreased anxiety surrounding treatment, and stronger therapeutic alliance through longitudinal relationship with nurses and clinicians, increasing openness to experience as the treatment course progressed. Previous research suggests that psychedelic-occasioned mystical experience is associated with lower anxiety and greater openness to experience surrounding the treatment.^40,41 Moreover, therapeutic alliance has been shown to be an important predictor of both mystical experience and treatment response with psilocybin-assisted therapy.^10,42 Similarly, studies have shown that pretreatment anxiety is a predictor of poor response to ketamine⁴³ while therapeutic alliance is thought to enhance treatment response.⁴⁴ Finally, it is possible that repeated esketamine treatments resulted in neurobiological and antidepressant effects that increased the likelihood participants might have mystical experiences. Studies of serotonergic psychedelics suggest that healthy subjects may be more likely to experience mystical experience than those suffering from psychiatric disorders, at frequencies of over 70%.⁶ Given that our study is exploratory and findings contrasts with previous research, replication and controlled studies are required for a more definitive interpretation.

In the racemic ketamine group, higher doses were correlated with greater mystical experience scores. This dose-response relationship could not be tested in the esketamine group, as all veterans received the same dose (84 mg). The dose-response relationship of mystical experience in the racemic ketamine group is consistent with previous research on the acute subjective effects of ketamine, which has shown greater potency of acute subjective effects at higher dose.^45,46 Moreover, this mirrors the findings of classical psychedelics, where higher doses are correlated with greater mystical experiences.^47,48

Clinical implications

Ketamine-occasioned mystical experiences introduce intriguing implications for optimizing ketamine treatment protocols for depression. Some studies of classical psychedelics have shown that mystical experiences may correlate with antidepressant outcomes following treatment.^10–12 To our knowledge, the antidepressant effects of ketamine-occasioned mystical experience are unknown. Most of the literature related to ketamine has focused on whether the dissociative effects (typically measured using the Clinician-Administered Dissociative States Scale) have predictive value for treatment response; however, data so far have been mixed.⁴⁹

Now that we and others have shown that ketamine can induce mystical experiences, more research is needed to examine the relationship between mystical experiences and antidepressant outcomes. If ketamine-occasioned mystical experiences have therapeutic value, clinicians can attempt to alter treatment protocols to maximize the potential for mystical experiences from ketamine. This might include optimizing protocols to incorporate factors shown to predict and potentiate mystical experiences, including psychological preparation before treatment, intention-setting, comfortable environment for sessions, psychological support during and after sessions, appropriate music, and encouragement of meditation and/or spiritual practices.^50–52 Psychotherapeutic approaches to decrease anticipatory anxiety, improve the ability to surrender to the ketamine experience, and increase openness and acceptance would also be prudent.^41,53 It is important to acknowledge that mystical experiences may carry certain psychiatric risks, such as psychosis, which could be heightened by treatment protocols designed to enhance such experiences.⁵⁴ Future research should carefully consider this potential risk factor to ensure safety.

Even if ketamine-occasioned mystical experiences do not produce immediate therapeutic value,⁵⁰ the content of these experiences may inform and support future psychological growth. This is a principle commonly held in other areas of psychedelic treatment, but which is still under development in the use of ketamine. Moreover, it has been theorized that the acute subjective experiences associated with ketamine, including mystical experiences, may correlate with neuroplasticity; thus, mystical experiences may be used as markers of neuroplasticity, which could in turn inform models for ketamine-assisted psychotherapy.^55,56

Limitations

There are several important limitations to this study. Firstly, this study is retrospective and exploratory in nature. Thus, the findings are subject to bias related to patient selection and data collection that was not able to be controlled. This may limit the overall generalizability of our findings. We caution that these findings should be taken as exploratory and preliminary.

Second, MEQ-30 surveys were administered in a somewhat ad hoc manner at various time-points during the course of treatment. This may have created greater vulnerability to bias. Moreover, this prevented us from analyzing questions related to the relationship between mystical experiences and subsequent antidepressant effects.

Additionally, no formal power analysis was conducted prior to data collection. Thus, it is unclear whether the study was adequately powered to detect meaningful associations.

Lastly, the small sample particularly within the racemic parenteral ketamine group is a limitation of the study. While we accounted for repeated measures in both groups independently for our analysis, the irregular frequency of these measures may introduce variability that is difficult to control.

Future directions

Several avenues for future research could expand upon the findings of this study. First, conducting studies with larger sample sizes would improve statistical power and allow for more robust conclusions. Additionally, future research should explore other components of the acute subjective experience, including dissociative, psychotic, and anxiety-related symptoms. Importantly, future studies should explore the effect of mystical experiences, or more broadly acute subjective effects of ketamine, on short- and long-term treatment outcomes.

Authors’ Contributions

K.V.B.: Writing—original draft (lead) and formal analysis (lead). J.N.C.: Writing—review and editing (equal), investigation (equal), and resources (equal). E.E.: Project administration (equal) and resources (equal). J.M.: Writing—review and editing (equal) and formal analysis (supporting). D.P.: Writing—review and editing (equal) and investigation (equal). J.P.: Writing—review and editing (equal) and resources (equal). D.R.: Conceptualization (equal), data curation (equal), and formal analysis (equal), investigation (lead), methodology (equal), writing—review and editing (equal), Supervision (lead), and resources (lead). A.B.: Conceptualization (lead), data curation (lead), formal analysis (equal), investigation (lead), methodology (equal), supervision (lead), and writing—review and editing (equal).

Disclaimer

The contents of this article do not represent the view of the U.S. Department of Veterans Affairs or the United States Government.

Author Disclosure Statement

Authors report no financial disclosures or conflicts of interest on areas related to this article.

Funding Information

This research received no specific grant funding.