This condition is not really coccydynia, as the coccyx is not involved. But it can be a cause of pain when sitting, and so can be confused with coccyx pain.
The pudendal nerve carries senations from the external genitals, the lower rectum, and the perineum (between the genitals and the anus). Neuropathy is disease of or damage to nerves, so pudendal neuropathy can cause symptoms in any of these areas. Some people have mostly rectal pain, sometimes with defecation problems. Others have mostly pain in the perineum or genitals. The symptoms may include stabbing, twisting or burning pain, pins and needles, numbness or hypersensitivity. Usually the symptoms are made worse by sitting, and better by either standing or lying down.
Damage to the pudendal nerve can occur suddenly as a result of trauma, such as surgery in the pelvic region, falls, bicycle accidents or childbirth and sometimes even from severe constipation. It can also occur from sustained trauma over time, such as from bicycle riding or aggressive weightlifting that strains the pelvic muscles. It can be caused by diseases such as diabetes or multiple sclerosis.
Trauma may cause stretching or compression of the nerve directly, or by causing fibrosis which can pinch the nerve.
Injection of a local anesthetic into the area suspected of causing the pain can help with diagnosis. If the injection stops the pain temporarily, then the area where the injection took place is the cause of the problem. The most common diagnosis is pudendal nerve entrapment.
Nerve blocks to stop the pain rarely have a permanent effect. Injections of steroids give permanent relief to a minority of patients. Botox (botulinum toxin) injections may also help some people. Surgery to decompress the nerve has a 60-70% success rate.
There is a very helpful website, The Health Organization for Pudendal Education. There is also a website run by a doctor, www.pudendal.com, which has a lot of information on the subject.
A US doctor recommends that sufferers should contact Professor Roger Robert, c/o Dr. Maurice Bensignor, Centre Catherine de Sienne, 2 Rue Eric Tabarly, 44202 NANTES CEDEX, France.
Thanks are due to an anonymous sufferer, who collected most of this information from newsgroups and other sources on the internet. I have reorganised it.