Pudendal Nerve Terminal Motor Latency Testing
For suspected pudendal nerve injury causing loss of bladder sensation after fistulotomy, pudendal nerve terminal motor latency (PNTML) testing is the most appropriate diagnostic test to rule out nerve damage. 1
Diagnostic Approach
Primary Test: PNTML Measurement
- PNTML testing directly assesses pudendal nerve function by measuring the conduction time from nerve stimulation to muscle response, with normal values typically less than 2.2 milliseconds 2, 3
- The test can be performed using the St. Mark's pudendal electrode via either rectal or vaginal approach (in women), with vaginal stimulation showing equivalent diagnostic accuracy and reproducibility 3
- Bilateral prolongation of PNTML (>2.2 ms) is significantly associated with neurogenic dysfunction, while unilateral prolongation alone is less clinically meaningful 2
Complementary Neurophysiologic Testing
- External anal sphincter electromyography (AEMG) should be performed alongside PNTML, as these tests provide non-redundant information about different aspects of pudendal nerve injury 1
- AEMG is actually more sensitive than PNTML for detecting neurogenic injury (64% vs 51% abnormality rate) and correlates more strongly with functional impairment 1
- The combination of both tests allows comprehensive assessment of pudendal nerve integrity and helps distinguish between neuropathic and myopathic causes of dysfunction 1
Clinical Context and Interpretation
Important Caveats
- PNTML has significant limitations: the majority of patients with neurogenic dysfunction may have normal PNTML results, and abnormal results do not always correlate with symptom severity 2, 4
- An increased PNTML does not predict response to treatment interventions such as nerve blocks 4
- The test primarily evaluates the distal terminal branches of the pudendal nerve and may miss more proximal nerve injuries 5
When to Suspect Pudendal Nerve Injury
- Stereotyped symptoms following perineal surgery include sensory changes, bladder/bowel dysfunction, and sexual dysfunction, which should prompt electrophysiologic evaluation 5
- Loss of bladder sensation specifically suggests involvement of the sensory branches of the pudendal nerve supplying the bladder neck and proximal urethra 5
Additional Considerations
- MRI with neurography may be considered for anatomic evaluation of peripheral nerves in cases of chronic or recurrent symptoms after pelvic floor surgery, though this is more commonly used for chronic pain syndromes 6
- Clinical diagnosis remains paramount, as electrophysiologic findings must be interpreted in the context of symptoms and physical examination findings 4
- Most pudendal nerve injuries following surgical procedures have good outcomes within 6 months, though permanent sequelae can occur 5