Spinal cord injury (SCI) results in damage to descending autonomic pathways causing a wide array of autonomic dysfunctions. Autonomic dysreflexia (AD) is a potentially life-threatening condition characterized by an abrupt increase in systolic blood pressure (SBP) more than 20 mmHg due to innocuous or noxious stimuli below the level of injury. 
Neurogenic detrusor overactivity (NDO) is the leading cause of AD events in individuals with SCI and requires routine urodynamic studies (UDS) for surveillance and management of lower urinary tract (LUT) function. AD episodes can be made more serious due to reflexive vagal activation that causes profound bradycardia leading to dysrhythmias, such as atrial fibrillation, sinus pauses, or AV node block. Considering the aforementioned, and the potential risk of cardiovascular complications associated with AD during UDS in individuals with SCI, the need for safe and effective treatments that address both NDO and subsequent autonomic consequences is paramount. 
Heart rate variability (HRV) is a powerful tool used to non-invasively assess autonomic regulation of the cardiovascular system. Reductions in the beat-to-beat variation of R-R interval on the electrocardiogram (ECG) are associated with worsened overall health status in a number of disease conditions due to increasing sympathetic tone. In SCI, HRV provides valuable and reliable feedback on the integrity and responsiveness of autonomic pathways in response to external stimuli that is graded by neurological level of injury (NLI). 
In the present study, we use HRV to assess autonomic nervous system responses to bladder filling during UDS in individuals with cervical and upper thoracic SCI that underwent the trial. We hypothesized that intradetrusor onabotulinumtoxinA injections would improve reflex autonomic regulation of the cardiovascular system in SCI patient undergoing UDS.

This prospective phase IV clinical trial using a pre-post study design was approved by the local ethics committee. Between November 2014 and December 2019, 55 individuals with chronic SCI (>1-year post-injury) at or above T6 with confirmed history of AD and NDO were screened based on these inclusion criteria. 34 individuals were included and assigned to undergo intradetrusor onabotulinumtoxinA injections (200 IU) intended to improve LUT function and ameliorate bladder-related AD. One month following intradetrusor onabotulinumtoxinA injections, UDS were repeated. Of these participants, 13 individuals had incomplete ECG recordings (i.e. not suitable enough for analysis) and two participants were excluded from analysis due to the presence of arrhythmic events that prevented HRV analysis. NLI and completeness (i.e. AIS grade) of SCI were classified according to the International Standards for Neurological Classification of SCI (ISNCSCI).[1]
All UDS were performed in accordance with the International Continence Society. Concurrent to UDS, we continuously recorded beat-by-beat blood pressure, via finger photoplethysmography corrected to brachial pressure, and one-lead electrocardiogram for heart rate (HR) in order to detect autonomic dysreflexia and to analyze HRV, which was assessed using time and frequency domain analysis in accordance with the European task force HRV guidelines. [2]
Main outcomes measures were RR-interval, time and frequency domain metrics of HRV (a surrogate marker of autonomic nervous system activity), and AD pre- and post-intervention.
All data were tested for normality and equal variance by a Kolmogorov-Smirnov test and an F-test respectively. Data were analyzed using two-way repeated measures ANOVA with Tukey post-hoc test as indicated in each figure legend and adjusted mean differences with 95% confidence intervals (CI) were reported in text for each comparison. The assumption of sphericity was tested using Mauchly’s test and the Greenhouse-Geisser correction factor to the degrees of freedom was used for all positive tests.  Correlations between the change in HRV in response to bladder filling during UDS, and SBP changes during UDS and AD were performed using Pearson correlations. P<0.05 was considered to be significant. All data are presented as means with standard deviation [SD].

A total of 19 participants with SCI (5 women; mean [SD] age 42 [8.3] years) with complete ECG and BP data sets were suitable for autonomic analysis. To perform injury-level-dependent analyses, all participants in the present cohort with upper thoracic SCI (n=6 mean [SD]; age, 44 [8.83] years; time-post-injury, 20 [13.7] years) were assigned age matched (within 5 years) cervical SCI controls for comparison (n=6 mean [SD] age, 44.3 [10.2] years; time-post-injury, 14.3 [13.7] years). Accordingly, no significant difference in age was observed between the two groups (mean difference, -0.33; 95% CI-12.6 to 12.0; P=0.95). In the whole cohort, majority of participants had motor-complete SCI in accordance with AIS (A=8, B=8, C=3). 
During baseline UDS, an increase in RR-interval (adjusted mean difference, -0.08; 95% CI, -0.14 to -0.03; P=0.002) as well as time and frequency domain metrics of HRV were detected. Vagally mediated increases in high frequency (HF) power during UDS were larger in participants with cervical SCI compared to upper thoracic SCI (adjusted mean difference, 20.3; 95% CI, 3.3 to 37.2; P=0.013). Intradetrusor onabotulinumtoxinA injections significantly reduced time domain metrics of HRV and HF power (adjusted mean difference, 9.1; 95% CI, 3.1 to 15.1, P<0.01) responses to UDS across all participants.

This study highlights the potential for intradetrusor onabotulinumtoxinA injections to minimize reflex autonomic responses associated with UDS in this cohort. Our study does however have some limitations. It is well known that respiratory rate can have a significant effect of HRV and was not controlled in the present study. While this may have a subtle influence on variability between participants, the pre-post design of the study compares outcomes within participants who’s breathing patters are likely to remain constant from day-to-day. Furthermore, the robust and significant effects of our intervention suggest that the impacts of UDS on HRV are larger than those imposed by alterations in respiratory rate. 
We present two novel findings in the present study which show that UDS increases reflex parasympathetic activation, as assessed by HRV, in response to bladder filling during UDS and that intradetrusor onabotulinumtoxinA injections ameliorate this response in individuals with SCI. Furthermore, we provide evidence that HRV may be a useful monitoring tool for early prediction of AD during UDS.

Changes in HRV during UDS could be a potential indicator of improved autonomic cardiovascular function following interventions such as intradetrusor onabotulinumtoxinA injections.

Kirshblum SC, Burns SP, Biering-Sorensen F, et al. International standards for neurological classification of spinal cord injury (Revised 2011). J Spinal Cord Med. 2011;34(6):535-546.
Electrophysiology TF of the ES. Heart Rate Variability. Circulation. 1996;93(5):1043-1065. doi:10.1161/01.CIR.93.5.1043