Joan of Arc's claim to fame — the mysterious voices she heard and visions she saw during the Hundred Years' War — may actually have been due to a form of epilepsy, Italian researchers suggest.

Dr. Guiseppe d'Orsi, a neurologist at the University of Foggia in Italy, and Paola Tinuper, an associate professor of biomedical and neuromotor sciences at the University of Bologna, also in Italy, described their hypothesis in a letter to the editor, published in May in the journal Epilepsy & Behavior.

Joan of Arc may have had a type of epilepsy that affects the part of the brain responsible for hearing, or "idiopathic partial epilepsy with auditory features (IPEAF)," d'Orsi and Tinuper wrote. ("Idiopathic" means that the epilepsy likely has a genetic cause, and "partial" means that the epilepsy affects only one area of the brain.) [Senses and Non-Sense: 7 Odd Hallucinations]

When a person has epilepsy, they experience chronic, unprovoked seizures. A seizure occurs when electrical signals in the brain misfire. Where these misfired signals occur in the brain determines how a person acts during a seizure. For example, a seizure may cause a person's muscles to jerk, or cause them to become dazed and confused, or hear voices.

D'Orsi and Tinuper said they first came up with their hypothesis 10 years ago, when examining documentation of Joan during her Trial of Condemnation, during which she was accused of being a heretic and a witch, and was sentenced to be burned at the stake.

Historical clues

Several aspects of Joan's symptoms, which have been detailed in historical accounts, help support this diagnosis, d'Orsi and Tinuper wrote.

For example, Joan reported hearing voices and sometimes reported seeing different saints, such as St. Catherine and St. Margaret. These auditory hallucinations and occasional visual hallucinations are symptoms of this type of epilepsy, according to the researchers.

Joan is reported to have said that the "sound of bells" sometimes triggered the voices. Hearing certain sounds can be a trigger for seizures, d'Orsi and Tinuper said.

During a public examination on Feb. 22, 1431, Joan said, "It said to me two or three times a week," referring to how often she heard the voices. Several days later, on March 1, 1431, she is reported to have said, "There is never a day that I do not hear them [the voices]," according to the researchers.

But the frequency with which Joan reported hearing voices is not entirely consistent with the researchers' diagnosis. Other researchers have pointed out that patients with this type of epilepsy have a low frequency of seizures, they wrote. Patients may have seizures infrequently at the beginning, and may also have seizures if they are withdrawing from certain drugs, they wrote. It's unclear if Joan of Arc had taken any substances that may have affected whether she had seizures, they added.

Joan of Arc also sometimes experienced seizures during sleep. "…I was asleep: the Voice woke me… It awoke me without touching me," Joan was reported to have said during a private examination on March 12, 1431, the researchers wrote. According to d'Orsi and Tinuper, 40 percent of people with this type of epilepsy have seizures during sleep. [Top 10 Spooky Sleep Disorders]

Of course, diagnosing a medical condition in a woman who lived in the 1400s is not exactly easy. Indeed, the researchers noted that "after six hundred years from Joan's death, we reaffirm the impossibility to arrive at a final conclusion."

However, there may be one remaining opportunity to find an answer, and it lies in a missing strand of Joan's hair.

History suggests that Joan of Arc sealed letters with red wax that had "the imprint of a finger and a hair" in order to prove her identity, the researchers wrote. If historians can locate these letters, scientists may have the opportunity to test the DNA of the hair.

Indeed, in recent studies, scientists have discovered genes that are linked to the specific type of epilepsy that d'Orsi and Tinuper suspect Joan of Arc may have had.

But efforts to find the letters and the hair have come up short so far. "After ten years from our first hypothesis, we are still looking for this hair…" d'Orsi and Tinuper wrote.

Originally published on Live Science.