Under normal physiological conditions, men experience multiple nocturnal penile tumescence episodes during rapid eye movement (REM) sleep.¹ The precise mechanism for these normal nocturnal erections is not well understood;² however, it is likely that when normal nocturnal erections occur, the balance between the parasympathetic and sympathetic systems shifts toward net parasympathetic tone. Consequently, activation of the efferent pelvic nerve during REM results in cavernosal helicine arteriole dilation and cavernosal erectile tissue smooth muscle relaxation surrounding the lacunar spaces. This increase in cavernosal arterial blood inflow and activation of the corporal venoocclusive mechanism during REM results in rigid and sustained nocturnal erections. The visceral afferent branch of the pelvic nerve (S2, 3, 4) conveys intracavernosal distension pressure, providing pleasurable stretching and tumescence sensation during erotically aroused erection.³ Normal nocturnal erections do not awaken the individual from sleep, but the increased pressure and penile distension sensation of the nocturnal erection is felt after awakening (Figure 1).

In contrast, sleep-related painful erection (SRPE) is a rare form of nocturnal penile tumescence where the patient’s sleep is disrupted (parasomnia), awakening with a bothersome painful erection.^4–6 SRPE was first described by Karacan et al.⁷ Epidemiologically, SRPE has been reported in 1% of men presenting with sexual dysfunction.⁴ The mean age at SRPE diagnosis is 40 years.⁴ In our experience these painful nocturnal erections range from severe to uncomfortable. Such patients may experience these erections several times per night, each episode lasting from minutes to hours. The most bothersome night erection is commonly experienced in the early morning.⁴ The disrupted sleep leads to irritability, anxiety, depression, and disturbance of daytime performance. SRPE episodes are often not predictable and may occur every night or at intervals with several weeks to months of absent SRPE episodes. SRPE patients report that staying in bed usually makes the pain worse, and that to achieve detumescence they exercise (eg, jog in place, run, walk) or take cold showers, which activates the sympathetic (adrenergic) system. In general, patients with SRPE do not have any penile anatomical abnormalities identified on physical examination.⁴ Patients with SRPE do not have painful erections when awake during erotically aroused penile erection associated with intercourse and/or masturbation, although some may experience various degrees of penile discomfort. In general, erotically aroused penile erections are easily maintained and prolonged.

While several mechanisms have been proposed,^4–6 the pathophysiology of SRPE remains unknown. Based on the successful innovative management of several patients with SRPE, we speculate on a novel pathophysiological basis for SRPE. We hypothesize that SRPE occurs due to sacral irritative radiculopathy of the visceral afferent and efferent pelvic nerve roots in the cauda equina from lumbosacral disc disease, in conjunction with the reduced sympathetic tone that likely occurs upon awakening from REM sleep.

In our sexual medicine practice, a series of SRPE patients have undergone a multidisciplinary step-care management algorithm.^8,9 One purpose of the algorithm is to help the clinician localize the origin of the triggers responsible for the SRPE. Such triggers may exist in any of 5 regions including end organ (Region 1), pelvis/perineum (Region 2), cauda equina (Region 3), spinal cord (Region 4), and/or brain (Region 5). We have found our SRPE patients to have a pattern of abnormal neurogenital test findings consistent with pathology in the cauda equina (Region 3), negative Regions 1 and 2 anesthesia testing, abnormal findings on lumbosacral MRI consistent with intervertebral disc annular tear (Figure 2), and transforaminal epidural spinal injection (TFESI) anesthetic administration resulting in clinically significant symptom reduction (attenuation of SRPE). Based on these findings, we have diagnosed our SRPE patients as having lumbosacral annular tear-induced sacral radiculopathy (Region 3), and they subsequently underwent lumbar endoscopic spine surgery.¹⁰

We herein propose that SRPE can result from a combination of a lumbosacral annular tear that induces a sacral radiculopathy resulting in irritation of both afferent and efferent components of the pelvic nerve, in conjunction with awakening from REM sleep with low sympathetic and high parasympathetic nervous system tone. This “perfect storm” that involves these 3 processes may account for the unique pathophysiology of SRPE. First, irritation of the pelvic nerve (visceral) afferents could result in a penile pain response to the normally pleasurable sensation of the distension of the corpora cavernosa during erection. Second, the parallel irritation of the pelvic nerve efferents could result in increased duration and intensity of arteriolar and lacunar penile smooth muscle relaxation leading to amplification (prolongation) of the nocturnal erection. Finally, during REM sleep, while it is known that the balance between sympathetic and parasympathetic tone fluctuates widely,¹¹ it is likely that upon awakening from REM sleep, there is net parasympathetic tone. Normally, when sympathetic tone is elevated relative to parasympathetic tone, pain thresholds are elevated, ie, pain sensitivity is reduced, largely due to increased norepinephrine release at the spinal cord “pain-gate” proximal to the lumbosacral annular tear via the descending brainstem-spinal cord pathway.¹² Thus, in patients with SRPE, when they awaken with low sympathetic tone, they have reduced pain thresholds, and experience painful erections (Figure 3). By contrast, should SRPE patients experience erotic arousal during the day, sympathetic tone would likely be relatively high (along with the parasympathetic tone producing the erection), in which case the pain threshold would be higher than upon awakening and the erection not painful (Figure 4).

A 53-year-old male in our SRPE series presented with a 3.5-year history of awakening during sleep with the awareness of a sustained, rigid erection, making a full night’s sleep impossible. Erections were characterized as uncomfortable, with a 5/10 discomfort. These episodes most commonly occurred between midnight and 5 a.m., and were not associated with erotic arousal. At time of presentation, he reported increasing frequency and intensity of these episodes, 7/7 nights, over the last 7 months. Once awake, he was unable to fall back asleep easily. This sleep deprivation caused extreme fatigue with a significant deleterious effect on his ability to work and quality of life. The patient otherwise reported normal libido, arousal, erectile function, and orgasms during waking hours. He had no history of priapism, illicit drug use, blood dyscrasia or disorders, or pelvic trauma or surgery. Physical exam was unremarkable. The patient tried using muscle relaxers, benzodiazepines, and antihistamines with little improvement, but with side effects including daytime drowsiness. Using a wearable sleep tracking biometric device, the patient correlated these erection episodes with the onset of REM sleep. The patient reported low back pain that worsened with weightlifting. He was diagnosed with SRPE and underwent our multidisciplinary step-care management algorithm including neurogenital testing. A lumbar spine MRI revealed right L4-5 annular tear, left L5-S1 annular tear, and L1-2 focal central disc herniation. A TFESI at L4-5 and L5-S1 resulted in “much better” clinically significant symptom reduction; an L1-2 TFESI resulted in no relief. He was thus found to have lumbosacral annular tear-induced sacral radiculopathy. He underwent lumbar endoscopic spine surgery at L4-5 and L5-S1, and has had resolution of his SRPE with 18 months of follow-up. He has no more sleep deprivation, fatigue, or daytime drowsiness, and has improved quality of life. He is also actively undergoing postoperative physical therapy and lifestyle modification.