Sir Terry, who was very vocal about the disease after being diagnosed in 2007, had posterior cortical atrophy (PCA), also known as Benson's syndrome.
The progressive degenerative condition involves the loss and dysfunction of brain cells, and is thought to be behind 5% of cases of Alzheimer's representing thousands of people in the UK.
But although both diseases involve the loss and dysfunction of brain cells, they affect different parts of the brain.
Typically Alzheimer's disease first impacts the sides of the brain - areas which play an important role in memory, which is why memory problems are usually the first symptom of Alzheimer's.
In PCA, the disease first affects the back (posterior) of the brain, known as the occipital lobe.
Professor Sebastian Crutch, an expert on Alzheimer's at University College London (UCL), said this part of the brain was responsible for vision, so people with PCA often initially experienced visual problems.
"Rather than being based on memory where people forget appointments and where they've put things, for people with PCA the first thing they notice is complex visual behaviour," he said.
"They might find they're clipping wing mirrors when they're driving or having problems parallel parking.
"When they're reading, words will jump around and a lot of people will have difficulty perceiving objects such as glass doors."
These are best described as a "brainsight" problem than an "eyesight" issue, he added.
The first symptoms of PCA tend to occur when people are in their late 40s, 50s or early 60s, so it has an earlier onset than Alzheimer's disease.
But the first signs are often subtle and so it may be some time before a formal diagnosis is made.
Prof Crutch said the number of people with PCA was hard to say because it was so difficult to establish.
As damage in the brain spreads and the disease progresses, people develop the typical symptoms of Alzheimer's disease such as memory loss and confusion.
There is no specific treatment for PCA but some people use medications used for Alzheimer's disease.
Sir Terry announced he had the condition in December 2007 when he was 59.
He had been told by doctors that previous summer that he had suffered a mini-stroke at some point in the previous few years, although he had been unaware of it.
After problems with his dexterity and hand-eye co-ordination, he had a series of scans and was diagnosed with PCA.
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Dr Clare Walton, research manager at the Alzheimer's Society, said PCA was the "most confusing and poorly understood types of dementia".
"The changes in the brain are very similar to those that happen in Alzheimer's," she said.
"The same changes in brain cells happen, the same proteins build up, and they have the same pathology, but we simply don't know why it affects different parts of the brain."
She said post-mortem examinations show that PCA sufferers' brains "have all the hallmarks of Alzheimer's".
Although it is possible to stabilise people's symptoms, as with Alzheimer's there is no cure.
"Until we have better treatments that stop it from progressing there's not a whole lot we can do," she added.
In a comment piece he wrote on assisted dying for the Guardian in February 2010, Sir Terry described PCA as giving him the "opposite of a superpower".
"Sometimes I cannot see what is there," he wrote.
"I see the teacup with my eyes, but my brain refuses to send me the teacup message. It's very Zen. First, there is no teacup and then, because I know there is a teacup, the teacup will appear the next time I look.
"I have little work-arounds to deal with this sort of thing - people with PCA live in a world of work-arounds."
He described how PCA "manifests itself through sight problems and difficulty with topological tasks, such as buttoning up a shirt".
"If you did not know there was -anything wrong with me, you would not know there is anything wrong with me," he wrote.
"The disease moves slowly, but you know it's there."
Sir Terry likened the condition to a "very, very slow-motion car crash".
"There's an occasional little bang, a crunch, a screw pops out and spins across the dashboard as if we're in Apollo 13. But the radio is still playing, the heater is on and it doesn't seem all that bad, except for the certain knowledge that sooner or later you will definitely be going head-first through the windscreen."