Radiation and the metallic taste phantom.

A story on Enenews describes the ordeal of Navy sailor Lindsay Cooper, on the USS Ronald Reagan, when the ship was in the radiation plume of Fukushima, soon after the catastrophe. She said,

As soon as you step foot on the flight deck and went outside you had this taste of like aluminum foil… We thought that we had felt a plume because there was kind of this warm air that went past the ship and you could kind of tell the differences between jet exhaust — we didn’t have any jets going around at the time. It was like 20 degrees outside and you could feel this warm air and you kind of enjoyed it at first and then you’re like, ‘Is that aluminum foil that I taste?’

A metallic taste (metal mouth) is a common symptom after radiotherapy, and also radioactive contamination of all kinds. It also occurs with certain diseases and after some surgeries.

I am very familiar with this personally. On April 10, 2011 I mowed the lawn, and started coughing. Then I developed a metal taste in my mouth. I was drinking bottled spring water from California at the time… whenever I drank it, the metal taste got stronger. It was like I had a mouthful of pennies. Whenever I ate Pacific fish, I would start tasting it. The metal taste usually appeared 15 minutes – 2 hours after I consumed these substances. It got so bad that I was having metal mouth continuously, and I started drooling. I kept a rag on my desk to catch the drool… I didn’t want to go out in public with this embarrassing problem.

This lasted until the end of April. But after that, foods got more contaminated. I have described this journey in Adventures with radiation in food. This was going on until January 2013, when I started taking medication to counter a pituitary tumor.

This article by Logan et al. describes the metallic taste (and other similar tastes) as a taste phantom. That is, it is not that we are actually tasting something, it is brought on by nerve damage.

Radiotherapy used as an adjunctive therapy in head and neck cancer increases the incidence of chronic pain [8]. Radiation toxicity is divided into early and late effects. Early or acute effects including nausea, skin reactions, diarrhea, and neutropenia are self-limited. Late effects including connective tissue fibrosis, neural damage, secondary malignancies, and pain can occur long after completion of radiotherapy. Neural damage from radiotherapy is well documented, as is the presence of taste phantoms (taste sensations in the absence of stimulation), particularly metallic [23, 33]. Phantom taste is believed to be a proxy for oral structure damage (e.g., neural). We chose to assess metallic taste phantom because we reasoned that it would be most recognizable and salient to survivors. Metallic taste phantoms are associated with taste damage produced by conditions ranging from tonsillectomy to third-molar extraction [10, 18, 46, 48].

So this taste is not exclusively brought on by radiation, but also by other procedures that affect nerves which affect taste.

The chorda tympani is a nerve that originates from the taste buds in the front of the tongue, runs through the middle ear, and carries taste messages to the brain. It joins the facial nerve (cranial nerve VII) inside the facial canal, at the level where the facial nerve exits the skull via the stylomastoid foramen.

The chorda tympani is part of one of three cranial nerves that are involved in taste. The taste system involves a complicated feedback loop, with each nerve acting to inhibit the signals of other nerves. The chorda tympani appears to exert a particularly strong inhibitory influence on other taste nerves, as well as on pain fibers in the tongue. When the chorda tympani is damaged, its inhibitory function is disrupted, leading to less inhibited activity in the other nerves. (link)

Another article by Bartoshuk et al. goes into more detail.

The chorda tympani nerve is accessible for anesthesia at two sites. First, the chorda tympani leaves the tongue with the lingual nerve (CN V) and the two travel through the pterygomandibular space. The inferior alveolar nerve, which conveys pain from the lower teeth, passes through the same space; thus dental anesthesia abolishes taste and touch as well as pain. Secondly, the chorda tympani passes through the middle ear after separating from the lingual nerve, so injection of an anesthetic just under the skin near the ear drum anesthetizes taste but not touch. Using both procedures, we showed that anesthesia of the chorda tympani intensifies tastes evoked from the contralateral rear of the tongue, the area innervated by the glossopharyngeal nerve (Lehman et al., 1995; Yanagisawa et al., 1998). This finding supports the earlier evidence of Halpern and Nelson (1965) for central inhibitory connections between the chorda tympani and glossopharyngeal nerves. This inhibition acts as a constancy mechanism: when one nerve is damaged, its input to the central nervous system (CNS) is reduced, releasing inhibition on other taste structures and thus compensating for the loss of input from the damage.

So the chorda tympani mainly inhibits or blocks tastes, in a complex relationship with other nerves. When this nerve suffers damage (from radiation exposure), the other nerves increase their output, in a constancy or homeostatic mechanism. These tastes are also predictors of oral pain and burning mouth syndrome (BMS). So the taste is an indicator of something worse going on inside the mouth. The taste may also have gastrointenstinal consequences.

Bartoshuk also mentions that GABA agonists reduce or abolish the taste. This means there may be damage to the GABA system, the main inhibitory system of the brain. This sort of syndrome is also associated with fibromyalgia. Glutamate, prolactin, and TSH are known to disrupt GABA. TSH is thyroid stimulating hormone, and is elevated in primary hypothyroidism. Prolactin is countered by dopamine agonists, which accounts for the disappearance of this taste in my mouth after I started taking them.