Bulbocavernosus and Anal Reflexes: Clinical Definitions and Assessment
The bulbocavernosus reflex (BCR) is elicited by squeezing the glans penis (or clitoris in females) and observing contraction of the bulbocavernosus muscle and anal sphincter, while the anal reflex is elicited by light pinprick or scratch of the perianal skin causing visible contraction of the external anal sphincter—both reflexes are mediated through the S2-S4 spinal reflex arc and serve to assess the integrity of sacral nerve pathways.
Bulbocavernosus Reflex (BCR)
How to Test
The BCR is tested with the examiner's finger in the rectum directed laterally to where the bulbocavernosus muscle is inserted. A moderate squeeze on the glans penis will cause the bulbocavernosus muscle to contract if the reflex arc is intact 1. The typical latency of this reflex response is approximately 33-36 milliseconds 2, 3.
Clinical Significance
- Pathway: The reflex involves tactile stimulation of the penile shaft activating parasympathetic fibers that travel in the pudendal nerve through the S2-S4 spinal reflex arc 1
- Assessment value: BCR and rectal sphincter tone must be assessed because both are mediated by the S2-4 spinal reflex arc 1
- Neurological implications: In patients with cauda equina lesions, the BCR is either absent or present with prolonged latency 3
Important Caveats
The method of elicitation matters significantly—stimulation of the dorsal penile nerve produces a mean signal transit time of 35 msec, while posterior urethral stimulation produces 60 msec 4. These represent two different reflexes with different afferent pathways, so clinicians must report how the reflex was elicited.
A negative BCR does not always indicate pathway disruption. In spinal cord injury patients, BCR was negative in 45% of cases, yet the anal sphincter reflex was positive in 13% of those with negative BCR 5. Additionally, BCR status changed in 32% of patients over 45 weeks without neurological changes, suggesting difficulty in eliciting the reflex rather than true pathway interruption 5.
Anal Reflex
How to Test
The anal reflex is tested by light pinprick or scratch of the perianal skin, which causes visible contraction of the external anal sphincter 6. The perianal skin should be observed for evidence of fecal soiling before testing the reflex 6.
Clinical Significance
- Pathway: Mediated through the S2-S4 spinal reflex arc, same as BCR 1
- Typical latency: Approximately 55 milliseconds when stimulated from the peri-anal region 2
- Preferred assessment: The anal reflex provides a better first option as a non-invasive clinical assessment of sacral reflex status in spinal cord injury, as testing is less intrusive and already performed as part of standard neurological examination 7
Key Differences from BCR
Stimulation of the peri-anal region produces habituating reflex responses (meaning they diminish with repeated testing), while perineal stimulation produces non-habituating responses 2. These different evoked responses must be distinguished when evaluating sacral nervous system lesions.
Clinical Applications
In Spinal Cord Injury
Both reflexes differentiate upper motor neuron from lower motor neuron injury. The anal reflex is recommended as the primary assessment because it is less invasive and already incorporated into the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) through pinprick stimulation of the S4-5 dermatome 7.
In Constipation and Defecatory Disorders
During digital rectal examination for constipation evaluation, the patient should be instructed to "expel my finger" to assess the integration of expulsionary forces 6. The examination evaluates resting tone of the sphincter segment and its augmentation by squeezing effort, with the puborectalis muscle also contracting during squeeze 6.
In Male Sexual Dysfunction
Assessment of BCR and rectal sphincter tone is essential in the comprehensive physical examination for erectile dysfunction, as both provide information about the integrity of the S2-4 spinal reflex arc that is crucial for sexual function 1.
Common Pitfalls
Do not assume a negative BCR always indicates nerve pathway damage—it may simply reflect difficulty in eliciting the reflex 5. When BCR is negative, also test the anal sphincter reflex to help differentiate true pathway disruption from technical difficulty in elicitation 5.
Recognize that reflex responses differ based on stimulation site—peri-anal stimulation produces habituating responses while perineal stimulation does not 2. This distinction is critical for accurate interpretation of sacral nervous system function.