Bulbocavernosus Reflex
Original Editor - Khloud Shreif
Top Contributors - Khloud Shreif, Alexandra Stead and Vidya Acharya
Introduction
Bulbocavernosus reflex (BCR) or bulbospongiosus reflex is a somatic reflex that gives information about the integrity of sacral spinal segments. This reflex involves the contractions of the bulbocavernosus muscles and/or the external anal sphincter in response to squeezing the glans penis or clitoris. When present, it is indicative of intact spinal reflex arcs (S2–S4 spinal segments) with afferent and efferent nerves through the pudendal nerve.[1]
Clinically, the BCR is a valuable diagnostic tool. It is easy to perform and is useful for testing the integrity of the sacral reflex arc. Additionally, it helps distinguish upper motor neuron lesions from lower motor neuron lesions, which have important prognostic and therapeutic implications for bowel, bladder, and sexual function.[1]
Anatomy Background
The bulbocavernosus (bulbospongiosus) muscle is one of the superficial layers of pelvic floor muscles, surrounding the vaginal opening in females and the base of the penis in males.
The pelvic floor muscles receive somatic innervation by nerves from the sacral spinal cord segments S2–S4, the pudendal nerve providing the motor and sensory supply, innervating the levator ani muscles through its perineal branches. In addition, supplying the external urethral sphincter and anal sphincter helps with voluntary contraction and relaxation.
Physiology of the reflex
Reflex Arc Pathway: The bulbocavernosus reflex is a polysynaptic sacral spinal reflex that assesses the connection of the sacral part of the spinal cord (S2–S4).
Afferent Pathway (Sensory Input): When there is mechanical stimulation (a gentle squeeze) to the clitoris or the glans penis, sensory receptors in the skin of the genital area are stimulated. Then afferent sensory feedback travels through the dorsal root of the spinal segment (S2–S4) pudendal nerve to the spinal cord.
Central Processing (Synapse): At the level of the spinal cord (interneurons in the grey matter), the input signals are processed This process activates the motor neurons located in the anterior horn.
Efferent Pathway (Motor Output): The efferent motor impulses travel back through the pudendal nerve to the bulbocavernosus muscle and the external anal sphincter causing contraction of the bulbocavernosus muscle and/or external anal sphincter.[1]
Bulbocavernosus reflex contributes to sexual function, enhancing erection and ejaculation in males and engorgement in women.[2]
Application
BCR can be assessed by both clinical and electrophysiological (EMG)
- It is done by squeezing the glans penis in males or the clitoris in females and observing the contraction of the external anal sphincter, or palpable contraction of the bulbocavernosus muscle. If the reflex contraction is difficult to palpate or visualize during clinical testing, or if more objective assessment is needed, electromyographic (EMG) testing may be required.
- EMG test is done by simulating the dorsal nerve of the penis (males) or the clitoral nerve (females) and recording the motor impulses at the external anal sphincter or Bulbocavernosus muscle, which gives accurate results about amplitude and latency.
Clinical Significance
BCR helps determine whether the sacral lesion is upper or lower motor neuron (UMN / LMN) and helps predict the type of sphincter dysfunction, especially in those patients with a complete SCI, but can not predict outcomes of improvement.[4]
BCR may be linked with sexual dysfunction if neurological causes are suspected or to diagnose sexual dysfunction in patients with neurological problems, such as in Parkinson’s disease, a delayed or absent BCR associated with erectile dysfunction, while an exaggerated reflex may be linked to premature ejaculation in those with Parkinson’s disease.[5]
Reduced BCR amplitude and prolonged latency of pudendal somatosensory evoked potentials (PSEP) reflect peripheral damage to the pudendal nerve; they are additional tests used to diagnose neurogenic bladder in diabetic patients. BCR alone can not indicate clinical improvement, but important and helpful to identify sacral segment involvement.[6]
In surgeries or pelvic surgery, recording of this reflex and the external urethral sphincter reflex is used to monitor the integrity of sacral nerves to help protect urinary, bowel, and sexual functions.[7]
Diagnostic Values
The clinical bulbocavernosus reflex (BCR) alone is insufficient for assessing the sacral or pudendal nerve and is not completely reliable or valid as a diagnostic tool when used in isolation.[8] A negative result of clinical BCR does not often indicate nerve damage, as the results often do not match the findings measured by (EMG) testing of the clitoro-anal reflex (CAR). A negative BCR may not always be due to disruption of the reflex pathway but may also result from difficulty in eliciting the reflex.[8] Furthermore, the presence of BCR does not mean that the sacral reflex is intact.[4] BCR can change spontaneously over time without any neurological changes. Therefore, objective electrophysiologic testing (EMG) is recommended for accurate evaluation.[9][8]
Despite these limitations in clinical testing, BCR can be helpful as a screening tool when assessed through electrophysiological methods. It can help in diagnosing neurogenic bladder and predicting prognosis, especially in patients with unexplained urinary retention, incontinence, or other bladder dysfunction.[10]
References
- ↑ 1.0 1.1 1.2 Previnaire JG. The importance of the bulbocavernosus reflex. Spinal cord series and cases. 2018 Jan 10;4(1):2.
- ↑ Tajkarimi K, Burnett AL. The role of genital nerve afferents in the physiology of the sexual response and pelvic floor function. The journal of sexual medicine. 2011 May;8(5):1299-312.
- ↑ Med Education. Bulbocavernosus Reflex | Spinal Shock & Spinal Cord Injuries . Available from: http://www.youtube.com/watch?v=iiLAI6RrZJ4[last accessed 17/12/2025]
- ↑ 4.0 4.1 Kirshblum S, Eren F. Anal reflex versus bulbocavernosus reflex in evaluation of patients with spinal cord injury. Spinal cord series and cases. 2020 Jan 7;6(1):2.
- ↑ Weber GC, Odin P. Diagnostic work up: Laboratory and biomarkers. InInternational Review of Neurobiology 2022 Jan 1 (Vol. 162, pp. 53-96). Academic Press.
- ↑ Niu X, Wang X, Huang H, Ni P, Lin Y, Shao B. Bulbocavernosus reflex test for diagnosis of pudendal nerve injury in female patients with diabetic neurogenic bladder. Aging and disease. 2016 Dec 1;7(6):715.
- ↑ Silverstein JW, Block J, Olmsted ZT, Green R, Pieters T, Babarevech K, Ballas-Williamson A, Skinner SA, Sciubba DM, Lo SF. Intraoperative monitoring of the external urethral sphincter reflex: a novel adjunct to bulbocavernosus reflex neuromonitoring for protecting the sacral neural pathways responsible for urination, defecation, and sexual function. Journal of Clinical Neurophysiology. 2024 Sep 1;41(6):558-64.
- ↑ 8.0 8.1 8.2 Wyndaele JJ, Quaghebeur J, Wyndaele M. What is the clinical meaning of a negative bulbocavernosus reflex in spinal cord injury patients?. Spinal cord series and cases. 2022 Feb 18;8(1):24.
- ↑ Wester C, FitzGerald MP, Brubaker L, Welgoss J, Benson JT. Validation of the clinical bulbocavernosus reflex. Neurourology and urodynamics. 2003;22(6):589-91.
- ↑ Andrade MJ, Soares TF. The importance of the clinical examination of the lower sacral segments: Four case reports. The Journal of Spinal Cord Medicine. 2019 Jan 2;42(1):123-7.