The bulbocavernosus reflex (BCR) is an involuntary polysynaptic spinal reflex used in medical diagnostics. It causes the contraction of certain pelvic muscles following stimulation of the external genitalia. This reflex serves as a method for clinicians to assess the functional integrity of the lower spinal cord. The BCR is used to gain information about the health of the sacral spinal cord segments (S2-S4), which govern many pelvic and lower body functions. Evaluating this reflex helps clinicians pinpoint potential damage to the nervous system, particularly after a traumatic injury.
Anatomy of the Reflex Arc
The pathway of the bulbocavernosus reflex is a structured loop involving sensory input, central processing, and motor output, all mediated by the pudendal nerve. Sensory input begins when the glans penis or clitoris is stimulated, which is detected by nerve endings. These impulses travel along the dorsal nerve of the penis or clitoris, branches of the pudendal nerve, toward the spinal cord.
The sensory signals enter the sacral spinal cord, specifically targeting segments S2 through S4. This region contains the nerve cell bodies that process the signal and relay the motor command back out of the spinal cord. The integrity of this S2-S4 segment is what the reflex ultimately verifies, making it a direct test of this specific lower spinal cord area.
The motor command then exits the spinal cord through the efferent fibers of the pudendal nerve. This motor signal causes a rapid, involuntary contraction of the bulbocavernosus muscle and the ischiocavernosus muscle. Clinically, this contraction is often observed as a brief tightening of the external anal sphincter. The entire process is a reflex arc that bypasses the brain, demonstrating the localized neural function of the sacral cord.
How Clinicians Test the Reflex
Clinicians typically elicit the bulbocavernosus reflex through a physical procedure. In male patients, the glans penis is gently squeezed, while in female patients, the clitoris or labium minus is stimulated. Another method involves gently tugging on an indwelling urinary Foley catheter, which pulls the balloon against the bladder neck to provide the necessary stimulus.
The resulting muscle contraction is usually detected by a gloved finger placed in the patient’s rectum to palpate the external anal sphincter. A positive response is felt as a brief, reflexive tightening of the sphincter muscle around the examining finger. This physical observation is a quick, bedside method used to confirm the presence or absence of the reflex.
Electromyography (EMG) Testing
For a more precise evaluation in specialized neurophysiological settings, the reflex can be tested using electromyography (EMG). This involves electrical stimulation of the dorsal nerve and recording the motor response from the bulbocavernosus or external anal sphincter muscle with electrodes. EMG provides objective data, measuring the time it takes for the signal to complete the arc, known as the reflex latency. An abnormally long latency, typically greater than 45 milliseconds, indicates a delay in the nerve pathway.
Interpreting the Test Results
The interpretation of the BCR test results provides significant diagnostic information, particularly in cases of spinal trauma or neurological disease. The presence of a normal reflex indicates that the S2-S4 spinal cord segments and their associated peripheral nerves are intact and functioning. This normal response confirms the structural integrity of the lower reflex arc.
Conversely, the immediate absence of the reflex following a severe spinal injury is a primary indicator of spinal shock. Spinal shock is a temporary state of depressed reflexes below the level of a spinal cord injury. The return of the BCR, typically within 48 hours, signals the end of the spinal shock phase, which is a crucial prognostic milestone for a patient’s recovery.
If the reflex is absent when spinal shock is not a factor, it suggests a complete lesion or injury to the sacral spinal cord, the cauda equina, or the pudendal nerve itself. This finding helps differentiate between an injury to the conus medullaris, the end of the spinal cord, and the cauda equina. The absence of the reflex is characteristic of a lower motor neuron lesion in this region.
The reflex also has applications in diagnosing pelvic floor conditions and sexual dysfunction. The presence of a reflex alongside a complete spinal cord injury higher up the spine suggests an upper motor neuron lesion, often associated with bladder overactivity and reflex erections. In contrast, a delayed or absent reflex is often seen in men with neurogenic erectile dysfunction or in patients with diabetic neuropathies, pointing to damage in the reflex pathway.