Muller, 72, has suffered with the illness for nine years and has been confined to a nursing home since 2015 as his condition continues to deteriorate.
The former Bayern Munich striker, known as Der Bomber, is often hailed as the greatest German footballer of all time and was a World Cup winner in 1974.
But he now resides in a care home outside Munich.
His wife Uschi, who visits every day, said: “Even though he is restricted in his possibilities, I still reach him. He’s happy when I come. It’s a pleasure to spend time with him. But he’s not sad when I go back.”
Muller scored 525 goals in 572 appearances for Bayern Munich between 1965 and 1979, a longstanding record tally with a single club in Europe’s top five leagues.
Earlier this month, the 40-year-old record was finally equalled by Barcelona superstar Lionel Messi in his side’s 2-0 win against Villarreal.
Muller won the Golden Boot at the 1970 World Cup after netting 10 goals.
He also scored the winner in the 1974 final, as West Germany defeated Holland 2-1 in Munich.
He is currently receiving Bobath therapy in his battle with dementia, in which the whole body is stretched and the nervous system stimulated.