The pudendal nerve terminal motor latency (PNTML) indirectly assesses the integrity and patency of the terminal portion of the pudendal nerve, its neuromuscular junction, and the muscle it serves. Using a specialized electrode affixed over the index finger, the pudendal nerve is electrically stimulated near the ischial spine (either transrectally or transvaginally), and the resulting muscular response is measured. The response, termed a compound muscle action potential (CMAP), is detected at the anal sphincter. The interval between the stimulation and the onset of the CMAP is measured. A prolonged latency is noted with injury to large and heavily myelinated axons. The latency time may be within the normal range when only smaller nerve fibers are affected; thus, neurologic dysfunction may exist in the presence of a normal latency time.
Only the distal efferent arm of the pudendal nerve is analyzed in the PNTML. Similar to the clinically obtained anal wink or bulbocavernosus reﬂex, electrically induced sacral reﬂexes can gather information about both the afferent and efferent arc in the pelvic nerves. A short train of dual impulses delivered next to the clitoris and measured at the anal sphincter is termed the clitoroanal reﬂex and provides information about the integrity of the afferent and efferent arm of the somatic pudendal nerve. A stimulating electrode placed in the bladder sends these signals along the visceral, autonomic fibers to the spinal cord, and a reﬂex signal will return along the pudendal nerve to the anal sphincter.
Normal pelvic ﬂoor and pelvic organ function ultimately is controlled by higher centers in the central nervous system, including the cerebral cortex. Recording electrodes located on the scalp near the motor cortex allow the signal transmission speed between a skeletal muscle and the brain to be measured. Repeated electrical stimuli, called somatosensory evoked potentials, at a muscle of interest are used to assess the integrity of the central afferent limb. In a reverse fashion, electrical or magnetically induced stimuli can be delivered at the motor cortex (or along the spine), and the induced muscle action potentials can be detected. Prolonged latencies not attributable to the peripherally studied nerves (such as with a PNTML or sacral reﬂex) are evidence of a central nervous system conduction ﬂaw.
Electromyography (EMG) assesses the inherent electrical potentials generated during neuronal activation of skeletal muscle. It can be performed using surface electrodes or needle electrodes. Surface EMG measures the summation of muscle activity in the general area of the applied electrode. It is best used for simply describing the pattern and coordination of muscle activity but is less useful in providing more specific assessments. Needle EMG of the pelvic ﬂoor can “map” the anatomic location of muscles but is mostly replaced by ultrasonography. The major value of needle EMG is its ability to assess nerve injury and determine whether the injury is acute and ongoing or chronic. Single-fiber EMG can quantify the ratio of muscle fibers to nerve fibers (the so-called fiber density). An increase in fiber density is evidence of previous nerve injury with successful reinnervation. Concentric needle EMG is more widely available and allows for further neurophysiologic evaluation. Abnormal electrical activity associated with acute injury may be seen, and motor unit action potentials (MUAPs) can be assessed and quantified. Following nerve injury and reinnervation, MUAP parameters—such as duration, amplitude, number of phases, and turns—are typically larger.
Positron emission tomography and functional magnetic resonance imaging studies are yielding preliminary insights into the neural control of continence; these technologies are used in the research setting only.