Botox Chemical Denervation
Botox, aka Onabotulinumtoxin A, is an example of a chemical compound used in selective denervation. It is isolated from the naturally occurring neurotoxin Clostridium botulinum. The mechanism of its action is via prevention of the release of the neurotransmitter acetylcholine at the presynaptic nerve terminal. In this way it effectively blocks neurotransmission from nerve to muscle or gland. This state can last for months, until certain components of the nerve are able to regenerate and again release acetylcholine at the presynaptic terminal.
Selective denervation refers to the providers ability to select which neuron they would like to deem non functional. This may be done in cases of facial muscle asymmetry after facial paralysis, or more commonly with Botox injections for facial “wrinkles”, or rhytids. Two ways of performing selective denervation are with mechanical, or “cold knife” excision, and chemically with injectable material. Cold knife denervation consists of a surgeon isolating a nerve (commonly with the innervated muscle) and selectively severing it without damaging surrounding structures. This should deem the innervated muscle paralyzed.
Frey syndrome, also knowing gustatory sweating, is classically described in patients after parotidectomy surgery. It occurs as a result of aberrant reinnervation of the superficial skin sweat glands by the deeper parotid parasympathetic neurons. Generally, there is a proportional increase in occurrence and severity of gustatory sweating with increases in extent of parotid tissue excised. The result of this phenomenon is that the patient may sweat from the cheek when encountering certain foods or flavors.
Since Frey syndrome (gustatory sweating) includes aberrant reinnervation of superficial sweat glands with parasympathetic nerves of the parotid gland, selectively rendering these nerves inoperable will decrease the ill effects of this disorder. Botox is used to carry out this action by injecting specific amounts into the area surrounding the parotid gland in the cheek, or where the parotid gland used to be. The aberrant parasympathetic nerves that innervate the superficial sweat gland will therefore be unable to neurotrasmit, and the superficial sweat glands should no longer be stimulated by foods or flavors.