Pudendal nerve entrapment syndrome

Pudendal nerve entrapment syndrome is also called “Alcock’s syndrome”.

What is pudendal nerve entrapment syndrome?

Pudendal nerve entrapment syndrome is an unusual condition which arises from compression of the pudendal nerve, which cause chronic pain in the perineal, perianal and genital areas, including one type of vulvodynia.

What causes pudendal nerve entrapment syndrome?

Pudendal nerve entrapment syndrome is caused by compression of the pudendal nerve as it leaves or enters the pelvis in various tunnels created by adjacent muscles, tendons or bony and ligamentous tissues.

In this condition the nerve is most commonly compressed at:

• The space between sacrotuberous and sacrospinous ligaments (~70% cases)
• Within the pudendal canal of Alcock (~20% cases)
• While straddling of the falciform process of the sacro-tuberal ligament by the pudendal nerve and its branches
• Anywhere along the course of the pudendal nerve or its branches

It is thought that changes in the shape and position of the ischial spine occur in young cyclists. This predisposes them to pudendal nerve entrapment in later years especially if they continue to cycle for prolonged periods.

The most common causes for pudendal nerve entrapment syndrome include:

• Repeated mechanical injury (e.g. sitting on bicycle seats for prolonged periods over many years or months)
• Trauma to the pelvic area
• Damage to the nerve during surgical procedures in the pelvic or perineal regions
• Compression from lesions or tumours arising in the pelvis
• Any cause for the development of peripheral neuropathy (e.g. diabetes or vasculitis).

What are the symptoms of pudendal nerve entrapment syndrome?

The symptoms of pudendal nerve entrapment syndrome arise from changes in nerve function and structural changes in the nerve that arise from the mechanical effects of compression. These changes give rise to so-called “neuropathic” pain in the perineum, genital and ano-rectal areas.

Neuropathic pain has many manifestations, most commonly spontaneous or evoked burning pain (also called “dysaesthesia”) with or without a component of severe lancinating (sudden, ‘electric shock-like’) pain. Other manifestations of “neuropathic pain” include a deep aching pain/sensation, increased appreciation of a sensation to any physical stimulus (“hyperaesthesia”), exaggerated sensation of pain for a given stimulus (“hyperalgesia”), pain sensation occurring with stimulation which doesn’t normally cause pain (“allodynia”) or an unpleasant, exaggerated prolonged pain response (“hyperpathia”).

The characteristic feature of pudendal nerve entrapment syndrome is aggravation of symptoms with assuming a sitting position, often after a short duration of sitting. Symptoms are typically relieved by standing and are usually absent when lying down or sitting on a toilet seat.

Various other symptoms may occur in some cases, for example urinary hesitancy (difficulty starting the flow of urine), frequency (frequent need to pass urine), urgency (sudden sensation to pass urine), constipation/painful bowel movements, reduced awareness of defecation (the process of passing bowel motions), sexual dysfunction, recurrent numbness of the penis and/or scrotum after prolonged cycling, altered sensation of ejaculation and impotence in men.

What investigations can be done to diagnose the syndrome?

Pudendal nerve entrapment syndrome is mainly a clinical diagnosis based on:

• History
• Characteristic symptoms and aggravating or relieving factors
• Typical location of symptoms.

The so called “skin rolling test” can be a helpful clinical sign. In this test, a thick roll (or fold) of skin just below and lateral to the anus is pinched and then rolled forwards. If pain is elicited, then this suggests the pudendal nerve is compressed.

It is important to exclude lesions in the pelvis which might compress the nerve by an ultrasound or computed tomography scan. Sometimes special nerve studies (electrophysiological studies) can be helpful. Local anaesthetic nerve blocks of the pudendal nerve may be helpful to confirm the diagnosis in some cases if it demonstrates complete abolition of symptoms after a nerve block.

What is the treatment for pudendal nerve entrapment syndrome?

The condition may be amenable to treatment in a number of ways. It is important to avoid prolonged periods of sitting, particularly in cyclists who have this condition.

Various medical treatments may be tried to alleviate neuropathic pain including nerve stabilising agents (for example, tricyclic antidepressants such as amitriptyline, anti-convulsants such as carbemazapine and sodium valproate, or nerve stabilisers such as gabapentin).

Where medical treatments are not successful in relieving symptoms, surgical treatments may be tried. Surgical treatments include local anaesthetic nerve blocks, injections of corticosteroids (to try to reduce swelling and inflammation which may cause nerve compression) and also surgical decompression of the pudendal nerve.

Surgical decompression of the nerve can be variably effective. Surgery may not be completely effective in all cases for various reasons, for example, irreversible damage to the nerve due to the effects of prolonged or severe nerve compression, processes which irreversible affect nerve function (such as longstanding poorly controlled diabetes mellitus), inadequate surgical decompression, and surgical decompression of the incorrect site.

Draft 25 November 2008