http://libweb.allencc.edu/Netters/NettersPlate184.html
In the above link it show the sympathetic trunk traveling down to the coccyx but in the link below near the coccyx area they are describing the ganglion impar which an unlisted code 64999. Also I questioned 64520 due to the fact the descriptor for 64520 states thoracic or lumbar only and does not mentioned below that.
http://en.wikipedia.org/wiki/Ganglion_impar
"The pelvic portion of each sympathetic trunk is situated in front of the sacrum, medial to the anterior sacral foramina. It consists of four or five small sacral ganglia, connected together by interganglionic cords, and continuous above with the abdominal portion. Below, the two pelvic sympathetic trunks converge, and end on the front of the coccyx in a small ganglion, the ganglion impar (or ganglion of Walther)."
From Pain Management Answer Book they describe the sympathetic nerves in thoracic-lumbar area in terms of how they contribute to feelings of pain
Pain Management Answer Book Effective Date 01/10/2005 Publish Date February 2005 Subject Sympathetic Nerve Blocks and Nerve Destruction SYMPATHETIC NERVE BLOCKS AND NERVE DESTRUCTION The nerves of the body constitute a critical communications system. Afferent nerves transmit important sensory information from the skin and organs to the central nervous system while efferent signals are instructions sent by the central nervous system back to muscles and internal organs. Sympathetic nerves, located in the thoracic-lumbar area, are a subset of afferent nerves. They allow the body to monitor and control autonomic body functions (e.g., heart rate, respiration, digestive functions, etc.) in cases of emergency. Sympathetic nerves are not normally a cause of pain. When sympathetic nerves are damaged or diseased, painful symptoms can result. Because damaged sympathetic nerves often result in a number of inter*related symptoms, pain specialists refer to the resulting condition as Complex Regional Pain Syndrome (CRPS). There is Complex Regional Pain Syndrome type I, which replaces the formerly used term, Reflex Sympathetic Dystrophy (RSD), and Complex Regional Pain Syndrome type II, which replaces the formerly used term causalgia. CRPS is most commonly associated with limb disorders. The most typical example involves the arm. A patient with CRPS may experience a burning sensation in the hand or arm, a hypersensitivity to touch, stiffness, discoloration of the skin and general weakness. Of these symptoms, the most obvious evidence of sympathetic nerve damage is intense, burning sensation in the limb where there is specific nerve damage, known as CRPS II.
Below I found a definition of the coccygeal plexus but via the internet there was not a lot information of this:
a small plexus formed by the fifth sacral and coccygeal nerves
I agree I have seen 64450 mentioned for coccygeal nerve block without really an in depth look at what the exact approach is and final needle placement, which makes this code sometimes questionable. You are mentioning the coccygeal plexus block which I had not seen before and agree with your previous response that 64999 might the most appropriate code.