Pudendal nerve entrapment syndrome

Pudendal nerve entrapment (PNE) syndrome is a rare and under-diagnosed condition associated with chronic pain, sexual dysfunction and impaired sphincter control due to compression of the pudendal nerve.

Anatomy

The pudendal nerve arises from S2-S4 roots of the sacral plexus, carrying both sensory and motor fibers. It traverses inferiorly between the piriformis and coccygeus muscles, exiting the pelvic cavity via the greater sciatic foramen. It then crosses underneath the sacrospinous ligament, re-entering the pelvic cavity through the lesser sciatic foramen. The pudendal nerve then travels alongside the pudendal artery and vein through a canal formed by the obturator fascia, known as pudendal canal or Alcock’s canal. In the pudendal canal, the nerve gives rise to three terminal branches: the inferior rectal nerve, the perineal nerve and the dorsal sensory nerve of the penis or clitoris.

Four types of PNE syndromes have been identified based on the site of entrapment ¹:

• type I – as the nerve exits the greater sciatic foramen
• type II – at the level of the ischial spine and lesser sciatic foramen
• type III – at the entrance of the pudendal canal (often associated with obturator internus spasm)
• type IV – entrapment of terminal branches

Etiology

Pudendal nerve entrapment syndrome arises from the mechanical compression of the pudendal nerve, excluding external compression from a mass or tumor.

Causes include:

• childbirth with vaginal delivery
• chronic constipation
• pelvic surgery
• repetitive minor trauma over several months or years, such as cycling or horse riding 

Diagnosis

Pudendal nerve entrapment syndrome is a condition of clinical diagnosis, with no specific diagnostic investigations.

Imaging is important to identify any mass or lesion that may be compressing the pudendal nerve, as well as to characterize the site of entrapment.

An improvement in pain symptoms in response to a pudendal nerve block may confirm the diagnosis.

The Nantes criteria ³ was published in 2008 to facilitate the diagnosis of pudendal nerve entrapment and includes a set of essential criteria of which all must be present, as well as complementary criteria and exclusion criteria.

Nantes criteria

Essential criteria:

• pain in the territory of the pudendal nerve
• pain primarily occurs in seated position
• pain does not wake the patient while sleeping at night
• no objective loss of sensation
• improvement in pain after pudendal nerve block

Complementary criteria:

• neuropathic character of pain – burning, shooting or stabbing
• presence of allodynia or hyperpathia
• sensation of a foreign body in the rectum or vagina
• pain increases throughout the day
• unilateral pain
• pain triggered after defecation
• tenderness on palpation of ischial spine
• positive findings on neurophysiological testing in men and nulliparous women

Exclusion criteria:

• pain exclusively in regions outside of the territory of the pudendal nerve
• pruritis
• exclusively paroxysmal pain
• pain can be attributed to abnormal findings on imaging

Clinical presentation

PNE commonly manifests as neuropathic pain in the genitals, perineum or anus. The pain is typically worse when seated for long periods and may be associated with a sensation of swelling in the affected region.

PNE can also cause sexual dysfunction including dyspareunia, persistent sexual arousal, erectile dysfunction or premature ejaculation ². Other manifestations include sphincter dysfunction such as urinary frequency or hesitancy, fecal incontinence and dyschezia.

Radiographic features

MRI

Inflammation of a compressed pudendal nerve may be demonstrated on T2 weighted MRI as edematous swelling and hyperintensity, as well as a kinking or a change in the thickness of the nerve ⁴.

MRI may also help characterize the location of pudendal nerve entrapment or identify any masses or lesions causing external compression of the pudendal nerve.

Ultrasound

High-resolution ultrasound may identify an increased cross-sectional area of the entrapped nerve compared to that of a healthy pudendal nerve, however this is an evolving application of ultrasound ⁵.

Treatment and prognosis

Conservative measures

• avoidance of exacerbating activities such as prolonged sitting, cycling, horse riding, etc.
• analgesia such as acetaminophen and NSAIDs, or neuroactive medications including amitriptyline, carbamazepine, gabapentin or pregabalin
• management of bladder and bowel function to avoid pelvic straining

Physiotherapy

• exercises to achieve relaxation of the pelvic floor muscles that may be overcontracted and compressing the pudendal nerve
• transcutaneous nerve stimulation (TENS) in combination with physiotherapy has been shown to improve pain and reduce analgesia requirements. ⁶

Pudendal nerve block 

• injection of a local anesthetic agent, often under the guidance of computed tomography, ultrasound or fluoroscopy to target the pudendal nerve at the lesser sciatic foramen. ⁷ 
• clinical improvement in response to a pudendal nerve block may confirm a diagnosis of pudendal neuralgia as the cause of symptoms

Surgical decompression

• surgery through the transperineal, transischiorectal, transgluteal or laparoscopic approach can address the cause of pudendal nerve entrapment.
• surgical decompression may be of less benefit to patients with irreversible nerve damage due to prolonged compression

Radiofrequency 

• minimally invasive application of an electrical current into the site of the pudendal nerve to achieve either nerve ablation (continuous radiofrequency) or neuromodulation (pulsed radiofrequency) ⁸

Differential diagnosis

• vulvodynia and vaginismus
• chronic prostatitis
• chronic pelvic pain syndrome
• complex regional pain syndrome
• shingles or superficial skin infections
• external compression by tumor or mass
• persistent genital arousal disorder or priapism