The exact underlying cause of Frey syndrome is not completely understood. The most widely held theory is that Frey syndrome results from simultaneous damage to sympathetic and parasympathetic nerves in the region of the face or neck near the parotid glands. Parasympathetic nerves are part of the autonomic nervous system, which is the portion of the nerve system that controls or regulates involuntary body functions (i.e., those functions that occur without instruction from the conscious mind). One function of parasympathetic nerves is to regulate the activity of glands including the parotid glands, but not the sweat glands. Sweat glands and blood vessels throughout the body are controlled by sympathetic fibers.
In Frey syndrome, researchers believe that the parasympathetic and sympathetic nerves near the parotid glands are cut, especially tiny branches originating from the auriculotemporal nerve. The auriculotemporal nerve supplies nerves (innervates) to certain structures in the face including the parotid glands.
Normally, damaged nerve fiber(s) eventually heal themselves (regenerate). In Frey syndrome, it is believed that damaged nerve fibers regenerate abnormally by growing along the sympathetic fiber pathways, ultimately connecting to the miniscule sweat glands found along the skin. Therefore, the parasympathetic nerves that normally tell the parotid glands to produce saliva in response to tasting food now respond by instructing the sweat glands to produce sweat and the blood vessels to widen (dilate). The cumulative result is excessive sweating and flushing when eating certain foods.
Damage to the nerves in the parotid gland region of the face may occur for several different reasons including as a complication of surgery or blunt trauma to the side of the face. In older reports, infections of the parotid glands were suspected, but a detailed examination always points to a surgical drainage of a parotid abscess. The most common reported cause of Frey syndrome is a surgical procedure called a parotidectomy (the surgical removal of a parotid gland). Although the exact percentage is not agreed upon in the medical literature, some sources suggest that more than half of all individuals who undergo a parotidectomy eventually develop Frey syndrome. A recent meta-analysis concluded that the interposition of tissue after parotidectomy might decrease the incidence of Frey syndrome after parotidectomy.
Another rarely described cause (etiology) of Frey syndrome is damage to the main sympathetic nerve chain in the neck.
In extremely rare cases, Frey syndrome has been described in newborns, possibly following trauma due to delivery with forceps. Actual careful examination reveals that the principal symptom is flushing which might be physiologic at a younger age. The key symptom of facial sweating is not emphasized in newborns rising doubts about the correctness of these observations.