Some patients eventually become mute from primary progressive apraxia of speech, a disorder related to degenerative neurologic disease.
Because patients and even many medical professionals do not recognise apraxia of speech, treatment typically is sought in later stages of the disease, said Keith Josephs, a neurologist at Mayo Clinic in US.
As apraxia progresses, it frequently is misdiagnosed as Alzheimer's disease or amyotrophic lateral sclerosis. Apraxia of speech even has been diagnosed as mental illness.
When it is caused by a stroke, apraxia of speech typically does not worsen and may get better over time.
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However, apraxia of speech often is ignored as a distinct entity that can evolve into a neurologic disorder, causing difficulty with eye movement, using the limbs, walking and falling that worsens as time passes.
"It is a devastating disease, in some sense worse than Alzheimer's disease, which typically spares balance and walking until very late in the disease course," said Josephs.
The benefit to getting an early and correct diagnosis is that people can receive appropriate therapy.
While speech therapy does not reverse or halt the progression of apraxia, it can develop compensations for producing better sounds.
People with apraxia of speech also can use computers or texting for alternate means of communicating. Both the value and complexities of speech often are underappreciated.
Speech is a complex brain-body achievement, researchers said. It first requires selection of appropriate words, organising them into a coherent message.
A problem with speech programming - directing the muscles and structures that move - is apraxia.
The findings were published in the journals Brain, Neurology, the American Journal of Alzheimer's Disease and Other Dementias, and the Journal of Neurology.