Amyotrophic lateral sclerosis, or Lou Gehrig’s disease, is characterized in the documentary “I Am Breathing” as the last truly incurable disease that still exists today. Those diagnosed suffer from gradual breakdown of the nerves connecting the brain to the muscles, slowly losing their motor abilities. First a finger goes, then a foot, then a leg and then even a lung. Eventually, the body stops running entirely and everything shuts down.

The late Scottish architect Neil Platt was one of the unlucky suffering from the disease, eventually passing away in 2009. As shown in “I Am Breathing,” Platt regressed from being a healthy 30-year-old husband and father to being a paralyzed shell in need of an oxygen machine over the course of a year. There’s little overt struggling, as he and his family learn to find a new normal. But the truth of the disease still remains. As Platt says in the film, having to conduct your own funeral arrangements is an interesting experience.

Even though “I Am Breathing” essentially chronicles one man’s slide into death, Platt and his family remain a spirited, lively bunch. Platt devotes his remaining time to spreading awareness of ALS through a blog and the film itself. More importantly, he is trying to piece together something for his one-year old son, Oscar, to remember him by. It proves a convenient excuse for the documentary to delve into Platt’s past, as well as the first signs of his eventual diagnosis. But selecting what to save for Oscar is difficult for Platt. “How can I anticipate things he would find useful to know about me … 15, 20 years from now,” Platt asks the crew.

Despite the humor in many of Platt’s interactions, directors Emma Davie and Morag McKinnon frequently wrap the documentary in morbidity. There are a lot of silent sequences set to gloomy ambiences. Were this a fiction film, it would be distasteful, but the reality of Platt’s decline is truly affecting. We don’t get inside his head through “I Am Breathing,” but we do learn about who he is. And the end, when it comes, is handled by the filmmakers with subtlety and respect.

It’s difficult to say how effective the documentary is at spreading the word about ALS, but “I Am Breathing” succeeds as a tribute to Platt. In ways, the film is less a documentary in the proper sense than it is a poetic grave marker, or an urn on the mantelpiece — a beautiful reminder of what once was.