Pudendal Nerve Entrapment Syndrome

Etiology

Pudendal neuralgia can arise from mechanical or non-mechanical injuries. The mechanical injury can be due to compression, transaction, or stretching. Amongst the mechanical causes, compression caused by pudendal nerve entrapment is the most common etiology. Non-mechanical causes of pudendal neuralgia include viral infections (herpes zoster, HIV), multiple sclerosis, radiation therapy, and diabetes mellitus, among others.[7]

The first reported case of pudendal neuralgia was due to cycling, which resulted from continuous pressure on Alcock's canal.[8]

Causes of Pudendal Neuralgia

1. Pelvic surgery - The surgery for repair of prolapse of pelvic organs is reportedly the most common cause of pudendal neuralgia. The incidence increases if a mesh is used. Chronic persistent pain may require mesh removal in some cases.[9][10] It can also develop after mid-urethral sling surgery, hysterectomy, and anterior colporrhaphy.
2. Direct trauma to the buttocks or back can result in pudendal neuralgia.[11]
3. Childbirth - Vaginal delivery causes a significant stretching of pelvic floor muscles by the fetal head, which sometimes results in pudendal nerve damage.[12][13]
4. Chronic constipation
5. Excessive cycling - The condition is presumably because of chronic perineal microtrauma, which causes fibrosis in the pudendal canal as well as the sacrospinous and sacrotuberous ligaments.[7]
6. Prolonged sitting can also contribute to this condition.

Epidemiology

Pudendal nerve entrapment is a rare syndrome, and its true prevalence is unknown. As estimated by the International Pudendal Neuropathy Foundation, the incidence of this condition is 1 per 100,000, but the actual prevalence is believed to be substantially higher than reported.[14] Pudendal nerve entrapment syndrome may affect 1% of the general population and accounts for about 4% of all patient consultations for pain control, with women affected more than twice as often as men.[15][16]

Pathophysiology

Direct compression of the pudendal nerve is often visible to the surgeon at the time of decompressive surgery. While repetitive actions such as sitting and cycling are primarily responsible, nerve compression may be congenital or the result of scarring after pelvic surgeries. Sports enthusiasts who regularly have significant and repetitive falls, such as skiers and snowboarders, may develop similar pelvic scarring due to frequent pelvic bruising and hematoma formation. 

History and Physical

History

The classic symptom is perineal pain which is exacerbated on sitting and relieved on standing or sitting on a toilet.[7] Other presenting features of pudendal nerve entrapment are discussed below.[6] These include:

1. It causes pain, numbness, and dysfunction in the distribution of the pudendal nerve. (This includes the genitalia, perineum, rectum, and lower urinary tract.)
2. Urinary issues such as urgency, frequency, and painful ejaculations in men.[17] 
3. Sexual dysfunction, including persistent arousal dysfunction, dyspareunia, vulvodynia, and male erectile disorders.[17]
4. Sphincteric dysfunction presents as constipation, dysuria, fecal incontinence, and urinary hesitancy.[18]
5. There may be a foreign body sensation in the anus, rectum, urethra, or vagina.

Physical Examination

The physical examination in patients with pudendal nerve entrapment is relatively normal, except for pain reproduction.[19] The symptoms depend on the precise site and severity of the entrapment. If the nerve is compromised at the ischial spine or the sacrospinous ligament, it causes pain medial to the ischium—similarly, tenderness over the greater sciatic notch results when the nerve gets entrapped at that location. Entrapment at the piriformis leads to spasms and tenderness of the piriformis muscle. Lastly, entrapment in Alcock's canal results in tenderness and spasms of the obturator internus muscle.

Rectal and vaginal examinations are suggested to exclude other diagnoses such as prostatitis and to identify any intrapelvic entrapment.

Evaluation

Pudendal nerve entrapment is a potentially challenging condition to diagnose because there are no specific diagnostic tests. The clinician needs to realize that it is exceedingly mandatory to get a thorough history and perform a detailed physical examination to reach a diagnosis.[20] Dr. Roger Robert published the "Nantes" criteria to diagnose pudendal nerve entrapment, which appears in detail below.[20] This criterion has been validated by many European physicians who have substantial experience treating similar conditions.

If the patient fulfills all the "Nantes" criteria, no further investigation is generally needed to make the diagnosis. However, if the patient lacks any of the criteria, further evaluation should be pursued. 

The "Nantes" Criteria 

Inclusion criteria:

1. Pain co-relates with the anatomical distribution of the pudendal nerve. (The pudendal nerve supplies the external genitalia and perineum. The pain can be superficial or deep in the vulvovaginal and anorectal areas as well as in the distal urethra.)
2. Pain is predominantly in the sitting position. This symptom favors nerve compression because a decrease in nerve mobility makes it vulnerable to compressive trauma when pressed against hard ligamentous structures. This aspect of pain is dynamic as the pain results from compression and not from a sitting position alone. 
3. The patient does not get up at night due to pain, although many patients may experience difficulty going to sleep because of pain.
4. There is no identifiable sensory loss. The presence of superficial perineal sensory impairment indicates a sacral root-lesion rather than pudendal nerve entrapment.
5. Relief of pain occurs with a pudendal nerve block. This essential criterion is not specific, as any perineal disease other than entrapment can cause pain in the anatomic region of the pudendal nerve. A negative block also doesn't necessarily exclude the diagnosis of pudendal nerve entrapment if the block is placed incorrectly or performed too distally.

Complementary Diagnostic Criteria

1. Pain is described as burning, shooting, or stabbing in nature and is associated with numbness
2. Allodynia or hyperpathia
3. Foreign body sensation or heaviness in the rectum or vagina
4. The pain progressively increases and peaks in the evening but stops when the patient lies down and sleeps
5. Pain is more on one side
6. Pain is more prominent posteriorly and is triggered minutes or hours after defecation
7. Tenderness felt around the ischial spine during a digital vaginal or rectal examination
8. An abnormal result on neurophysiological tests

Exclusion Criteria

1. Pain exclusively in the territory not served by the pudendal nerve, such as the hypogastrium, coccyx, pubis, or gluteus areas
2. Pain is associated with pruritus (more suggestive of a skin lesion)
3. Pain that is entirely paroxysmal in nature
4. An imaging abnormality that identifies the cause of the pain

Associated Signs

1. Pain in the buttock 
2. Referred sciatic pain
3. Pain in the medial thigh (indicates obturator nerve involvement)
4. Pain in the suprapubic region
5. Increased frequency of urination or pain with a full bladder
6. Pain after ejaculation
7. Pain worsens hours after sexual intercourse
8. Erectile dysfunction
9. A normal result on electrophysiological tests

Diagnostic Tests

The following tests can help in the diagnosis:

1. Quantitative warm sensory threshold testing works on the principle that compressed nerves cannot efficiently detect and transmit changes in vibration and temperature sensation. Thus patients with nerve injuries are unable to detect gradual temperature changes.[21] The most commonly used test of this type is the warm sensory threshold detection test. Many patients cannot detect changes in probe temperature until it is hot enough to cause a painful burning sensation.[22] The test can easily be performed in a physician's office or clinic. Biothesiometry, which tests vibratory sensation, can also be used similarly to identify any loss of vibrational sensitivity.
2. Pudendal nerve terminal motor latency testing is a neurophysiological examination that measures the time it takes for a nerve signal to travel from the ischial spine to the anal sphincter. It is more invasive, challenging to perform, and uncomfortable for the patient than warm sensory threshold testing.[23]
3. High-frequency ultrasonography is helpful in the detection of the site of compression. Compressed nerves and associated veins appear flat, whereas inflamed nerves appear edematous.[24]
4. Doppler ultrasound has a role in the diagnosis of pudendal nerve entrapment. As the pudendal nerve and vessels course together in a neurovascular bundle, it can be assumed that if there is nerve compression, venous compression will also occur, which is diagnosable with a doppler ultrasound.[24]
5. MRI (magnetic resonance imaging) of the pelvis is recommended as it can help rule out other causes of chronic pain. The advancement of MRI techniques in evaluating peripheral nerves provides a detailed description of the anatomy, fascicular details, the blood supply of the nerve, and detailed 3-D anatomy.[25] It also helps in localizing the exact site of entrapment. MRI testing is strongly recommended prior to any surgery for pudendal entrapment.[26]
6. Functional MRI assesses nerve integrity based on its biological properties. Currently, it is considered experimental and not considered definitive.[27] There are no specific and consistent radiological findings in patients with pudendal nerve entrapment, and further research is necessary.
7. Diagnostic nerve blocks: In females, an unguided block can be performed vaginally, and in males, transperineally. If there is pain relief immediately following the procedure, it indicates that pudendal nerve pathology is the likely cause of pain. The absence of pain relief doesn't necessarily mean that the patient doesn't have pudendal entrapment, as there may be a technical or operative error as well. Image guidance (using fluoroscopy, ultrasound, or CT scan imaging) significantly increases the reliability of the nerve blocks.[28]
8. Pudendal nerve block injections with a local anesthetic have been recommended to help confirm the diagnosis of pudendal nerve entrapment, especially if the injection is done directly into Alcock's canal using image guidance. This is often used as an indicator of which patients are most likely to benefit from decompressive surgery.[26][29] About 20% of pudendal nerve blocks fail for various reasons, often due to physician error, inexperience, or lack of adequate training.[30][31]

Treatment / Management

Conservative: Avoidance of painful stimuli is one of the most important components of treatment. For instance, if cycling causes pain, the patient should use proper padding or cease the activity. Other activities to avoid might be hip flexion exercises, jogging, rowing, gymnastics, skiing, and snowboarding. Similarly, patients who present with pain on prolonged sitting should adopt lifestyle modifications to minimize that activity, such as using a standing workstation. Roughly 20% to 30% of patients will see relief from conservative measures alone.

Physical Therapy: Pelvic floor physical therapy works best for patients in whom pain results from muscle spasms such as levator ani syndrome and similar myofascial disorders. Physical therapy helps in the relaxation of pelvic floor muscles by releasing spasms and muscle lengthening. A course of 6 to 12 weeks is commonly recommended. Adding transcutaneous electrical nerve stimulation (TENS) to physical therapy appears to be helpful.[32][33][34] TENS is low risk, relatively inexpensive, readily available in most locations, and non-invasive.(A1)

Cognitive Behavioral Therapy: Behavioral therapy has been useful for various types of chronic pelvic pain syndromes, even though it has not been specifically tested for pudendal nerve entrapment.[35][36] It is generally recommended as an adjunctive treatment when there is evidence of psychological issues such as anxiety, depression, hopelessness, emotional instability, etc.[26][37] Frustration and depression are particularly common in patients with chronic neuropathic pain who have not enjoyed any relief from earlier treatments.(A1)

Pharmacologic therapy: The drugs used are analgesics, muscle relaxants, and anticonvulsants. There are no randomized trials to study and evaluate the efficacy of these drugs or which combinations might be most effective. Often, several medications from different drug classes are used. A typical combination would be a tricyclic antidepressant (amitriptyline), an SSNRI (duloxetine), and a neurotransmitter analog (gabapentin and/or pregabalin). Opioids are avoided as much as possible. Commonly used medications include:[38]

1. Amitriptyline, starting at 10 mg HS and gradually increasing to 50 mg. 
2. Duloxetine (a selective serotonin-norepinephrine reuptake inhibitor) starting at 30 mg daily for seven days, then increasing to 60 mg daily. No benefit is seen from further dosage increases
3. Gabapentin (with or without pregabalin), starting at 300 mg TID and gradually increasing up to a maximum of 900 mg TID
4. Pregabalin (with or without gabapentin), starting at 75 mg BID and gradually increasing up to 300 mg BID
5. Clonidine 0.1 mg HS not only helps with pain but is useful as a sleep aid

Pudendal Nerve Block: Infiltration with a local anesthetic or steroid in an area encircling the pudendal nerve is a mainstay of pudendal nerve pain. The block can be given unguided or with the aid of ultrasonography, fluoroscopy, or computed tomography (CT). The most consistently reported technique is the use of a CT scan.[39] Image guidance is suggested for better and more reliable results. While no standard medication or combination is used, one frequently used mixture includes 1% lidocaine, 0.25% bupivacaine, and a corticosteroid such as triamcinolone. The short-acting anesthetic starts working in 20 minutes or less, the bupivacaine can last much longer, and the steroid effect begins at about 3 to 5 days and can last about a month. At that time, another injection can be given if needed. About 25% of patients report pain relief lasting more than one month following pudendal nerve blocks.[26] While this can be effective, there is evidence that ongoing therapeutic pudendal blocks may lose efficacy after two years.[40] For patients receiving ongoing injections, ultrasonography is suggested to minimize costs and reduce patient exposure to ionizing radiation over time. (See our companion article on Pudendal Nerve Block for more details on the procedure.)[41](B3)

Surgical Decompression: Surgery to directly free the pudendal nerve in Alcock's canal is considered the most effective long-term treatment and potential cure for pudendal nerve entrapment. The four different approaches are transperineal, transgluteal, transischiorectal, and laparoscopic.[42] All methods destroy some nerve fibers but help equivalently by removing the underlying cause of the compressive neuropathy.[43][44] Overall success with surgical decompression is about 70% (60% to 80%).[43] The goal of decompressive surgery is to completely free the nerve from entrapment and compression while allowing it complete mobility.(B2)

Erdogru described a new modification (the Istanbul technique) of laparoscopy using an omental flap in 27 patients. The outcome measurement was defined in terms of pain scores and quality of life. Approximately 80% of patients reported more than an 80% reduction in pain after six months which is quite impressive.[42] Laparoscopy has the advantage of a better visual surgical field with built-in magnification. It allows for the option of leaving a neuromodulation electrode in place as a backup, but it has a steep learning curve.[42][45] Placing a neuromodulation electrode at the time of decompression surgery seems reasonable in selected complicated or severe cases, as the electrode can always be removed later if it's not needed.[42](B2)

Sacral Neuromodulation: This minimally invasive treatment includes using a peripheral nerve stimulator, which causes neural regulation of the pudendal nerve in the ischioanal fossa. The first case report of this technique by Valovska mentioned the successful management of a patient with pudendal neuralgia with minimally invasive transforaminal sacral neurostimulation.[46] A prospective trial of 27 patients with refractory pudendal neuralgia showed promising results by neuromodulation of the conus medullaris, in which twenty out of twenty-seven patients had a positive response (defined as at least a 5-% reduction in pain).[47] Further, of those twenty patients, all had long-term relief.[47] (B3)

Sacral neuromodulation has often been used as a treatment of last resort when patients have failed all other treatments, including surgical decompression. In such cases, about two-thirds will respond favorably to neuromodulation.[47][48] Optimal settings and standards have not been established for this therapy, but there is some evidence that higher frequencies (>20 Hz) may provide better results. Sacral neuromodulation is safe, effective, minimally invasive, widely available, and generally underutilized for pudendal neuralgia. While surgical decompression is generally the preferred long-term curative therapy for most patients with pudendal nerve entrapment, sacral neuromodulation should be considered for those individuals who are not surgical candidates or where the decompressive procedure has failed.(B3)

Pulsed Radiofrequency Ablation: Pulsed radiofrequency ablation is a relatively new method of pudendal neuromodulation. It is considered safer than continuous radiofrequency treatment by reducing heat-related complications. It uses pulsed electromagnetic radiation to cause neuromodulation and appears to be potentially useful for chronic refractory neuropathic pudendal neuralgia.[49] When compared to pudendal nerve blocks, pulsed radiofrequency ablation showed equivalent pain relief, but the benefit extended substantially longer, out to three months.[50] Several recent studies showed persistent pain relief in 89% of the 90 pudendal neuralgia patients followed for six months after pulsed radiofrequency ablation therapy.[51][52] While promising, these studies are not yet sufficient to justify the widespread use of this treatment modality.(B2)

Cryotherapy: Early reports using cryotherapy in small series appear promising, which would be expected based on results from its use in other neuropathies.[53] However, there is currently insufficient evidence of efficacy and safety to justify its routine use outside of a clinical trial. (B2)

Lipofilling: This is a relatively new experimental treatment of pudendal neuralgia. Venturi first described this technique in fifteen female patients. It requires an autologous injection of adipose tissue along with stem cells into Alcock's canal. Ten patients showed significantly decreased pain and a better quality of life at the end of six months.[54] Since this was a single study with a very small sample size without a control group, further research into this therapy is needed before it can be recommended outside a clinical trial.(B2)

(For a more detailed and comprehensive description of all the various therapies, medications, and procedures for pudendal nerve entrapment, please see our companion article on Pudendal Neuralgia.)[38]

Differential Diagnosis

Since there is no confirmatory diagnostic test, pudendal neuralgia is a diagnosis of exclusion. Other conditions merit consideration before making a final diagnosis.[55]

1. Compression by an external source, including a benign or malignant tumor or metastasis
2. Superficial infections of the skin in the dermatomes covered by the pudendal nerve
3. Neuropathy of the sacral region is caused by damage to the sacral nerve plexus
4. Childbirth trauma that causes a stretching of the perineum
5. Complex regional pain syndrome is a chronic pain condition that causes pain in one of the limbs and usually occurs after an injury
6. Chronic prostatitis
7. Prostatodynia
8. Vulvodynia
9. Vulvar vestibulitis
10. Chronic pelvic pain syndrome
11. Coccygodynia
12. Sacroiliac joint dysfunction
13. Piriformis syndrome
14. Ischia bursitis
15. Interstitial cystitis

Prognosis

Pudendal neuralgia due to pudendal nerve entrapment can immensely affect the quality of life, but it does not affect life expectancy. The overall response to pudendal nerve blocks in properly selected patients is roughly 80%, but the relief typically lasts only about 30 days in most patients. A repeat injection or an alternative treatment would then need to be utilized. The best long-term cures are from decompressive surgery, where the response rates are 60% to 80%.[43] Patients who fail decompressive surgery, possibly up to 80%, can still obtain relief from sacral neuromodulation.[15][48]

Complications

The complications associated with pudendal nerve blocks are rare. These complications include:

1. Laceration of the vaginal mucosa
2. Accidental intravascular injection of a local anesthetic can cause cardiovascular and CNS toxicity. The patient can present with palpitations, hypotension, bradycardia, dysarthria, tinnitus, drowsiness, confusion, loss of consciousness, and convulsions.
3. Hematoma from injury to the pudendal artery or surrounding vessels
4. Infection and/or pain at the injection site
5. Abscess formation

Pudendal decompression surgery may also produce complications, although these are also uncommon. There can be an injury to a small branch of the nerve during surgery. A microsurgical repair of the injured nerve can be performed if necessary. An injury or transection of the sacrotuberous ligament can occur during surgery. This would cause an unstable pelvis. (Using a midline vertical incision would help avoid this rare complication.)

Relief from surgery is rarely immediate. Rather, the pain slowly decreases over a period of months. This is not a complication of the surgery but rather just the expected healing process.