You could go a while without spotting anything untoward about him. But sooner or later you become aware that James McConnel has an affliction. At one point, we are sitting in the composer’s study, talking about Mozart. Out of the blue he does a strange facial twitch – a little one.

As I pass him the peas at lunch, he has a full-blown twitch, jabs with his finger, brushes his face with his other hand, and lets out a snort. None of his family – least of all his wife, cartoonist Annie Tempest – seems to notice.

Mr. McConnel looks and sounds like a hyperactive version of Hugh Grant. He has a background in musicals but also composes for television and film. He is also about to host a British documentary called “What Made Mozart Tic?,” in which he suggests that Mozart had Tourette’s Syndrome. It’s not a new theory. But it is one that Mr. McConnel is in a unique position to argue because he himself is a sufferer.

The only time Mr. McConnel doesn’t twitch is when he’s composing. In the documentary he argues that Mozart, too, “self-medicated” by writing music. “The self-medicating theory is that music is a replacement for the twitching. With me it was subconscious. It wasn’t until I was about 25 that someone pointed out that I wasn’t twitching when I was at the piano.

“I suspect Mozart didn’t have physical jerks as much as me. But there is definite evidence of his grimacing and feet-tapping. We also know a lot about his inability to rein in impulses, the sudden boredom, his sense of mischief and his scatalogical obsession, which all point to Tourette’s. He even had a morbid fear of the trumpet until he was 9. Seriously! He would lie down and scream if he heard one.”

The letters Mozart wrote provide a useful starting point for Mr. McConnel. “There’s a very rare condition in Tourette’s called coprographia – the need to write down filth. We Touretters have filthy minds!” Mr. McConnel says.

“My sense of humor is the same. I never know when to stop.”

Because he suffers from obsessive compulsive disorder, Mr. McConnel takes half an hour to go through his bedtime rituals. “I can’t not do it and feel comfortable,” he says.

The same was apparently true for Mozart. “When he was a child he had to do his bedtime rituals with his father, Leopold, and if they weren’t right he would do them all over again.”

Although Tourette’s wasn’t recognized in Mozart’s day, much has been made of his childhood illnesses. These, apparently, could have triggered pediatric autoimmune neuropsychiatric disorder, which can lead to Tourette’s. Mr. McConnel himself wasn’t diagnosed with Tourette’s until 12 years ago – he’s now 46. He can, however, pinpoint the onset.

“I can remember the very instant I started twitching. It was during the third verse of ‘Hark the Herald Angels Sing’ at my school carol service when I was 6. It was a sniff and I didn’t have a cold. I started sniffing, which became permanent.”

Mr. McConnel believes the root of Mozart’s Tourette’s is to be found in the music itself – something Mr. McConnel knows from his own experience.

“Tourette’s is a constant battle between having a compulsion and trying to control it, and that can translate into music. Mozart let his music run off in chaotic directions but then always brought it back under control. His interest in counterpoint and fugue – its unfashionable complexity – appealed to his Touretty side. The music for me has just so much happiness, for want of a better word. There’s no residual misery.”

Scientific tests have proved the efficacy of Mozart in music therapy: It is deeply calming. Not that this stops his fan going about the house trilling and twitching like a dervish. The family, however, regard the condition with a sense of humor; Mr. McConnel’s e-mail name is McTwitch and there is a plate in the kitchen inscribed: “Warning: mad twitcher on the loose.”