Robert F. Kennedy Jr. appeared on the Fox News show Tucker Carlson Tonight to talk about vaccines, but for many viewers what struck a cord, if you will, was the sound of his voice.

During Kennedy’s segment, the environmental attorney and author, and the son of the assassinated Sen. Robert F. Kennedy, responded to criticism from comedian John Oliver over his comments about preservatives in childhood vaccines. He also elaborated on his views about vaccines, saying that they are largely untested for their safety and potential interactions with other vaccines and suggesting corruption within the health industry.

Under other circumstances, Kennedy’s comments might have been the sole focus of viewer reactions, as certain groups in the U.S. are deeply divided over vaccines and their use. The World Health Organization just released a statement that reported yet another child had died of measles, a viral infection for which there is a vaccine, bringing the European total to 35 measles deaths in the last 12 months along with thousands of infections. Vaccination rates in some places, including the U.S., are low compared to what they used to be — sometimes seeing a drop due to people believing in debunked claims that vaccines cause autism.

But what many people took away from the interview was the trembling of Kennedy’s voice, which made him sound like he was on the verge of tears.

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He reportedly has a condition called spasmodic dysphonia, a neurological condition that the U.S. National Institutes of Health say affects muscles in the larynx, also known as the voice box.

“When we speak, air from the lungs is pushed between two elastic structures — called vocal folds or vocal cords — with sufficient pressure to cause them to vibrate, producing voice,” the organization says. “In spasmodic dysphonia, the muscles inside the vocal folds experience sudden involuntary movements — called spasms — which interfere with the ability of the folds to vibrate and produce voice.” Sometimes those spasms occur on every other word.

People with the condition may sound like their voices are hoarse, shaky or otherwise strained, perhaps mimicking the sound of a person choking back tears while speaking.

According to the NIH, the disorder is rare and usually starts appearing when a person is between 30 and 50 years old. “Symptoms of spasmodic dysphonia generally develop gradually and with no obvious explanation.”

There is no cure for the neurological disorder, but it can be controlled somewhat through a botox injection into the voice muscles or through voice therapy.

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Botox is a toxin officially called botulinum toxin that is produced by bacteria. It is known for tainting canned foods but in smaller doses has been connected to wrinkle-reducing cosmetic procedures. It also has medical applications, however. In the case of spasmodic dysphonia, it works because it weakens the voice muscles and makes the vocal cords shut less forcefully, the American Speech-Language-Hearing Association explains.

The National Spasmodic Dysphonia Association estimates that about 50,000 people in North America have the neurological disorder.