Unintelligibility

Unintelligibility is the condition in which spoken language is not understandable to a listener. It is a perceptual judgment about how clearly someone speaks, rather than a specific disease. Intelligibility can vary with the speaker’s language or dialect and the listener’s experience.

Causes and contributing factors include a range of conditions and circumstances. Neurological disorders such as stroke, traumatic brain injury, and neurodegenerative diseases (for example, amyotrophic lateral sclerosis and Parkinson’s disease) can affect articulation, voice, or prosody. Developmental speech disorders such as apraxia of speech or dysarthria, hearing loss, and cognitive impairment also contribute. Environmental factors—background noise, distance, and listener familiarity—can further reduce intelligibility. Articulation errors, altered resonance, slurred or excessively rapid speech, and prosodic abnormalities are common perceptual indicators.

Assessment of intelligibility in clinical settings typically involves listening to connected speech or isolated words. Standardized measures, such as the Assessment of Intelligibility of Dysarthric Speech (AIDS) or the Sentence Intelligibility Test (SIT), may be used, along with perceptual rating scales. Evaluations may report the proportion of words correctly identified or a qualitative level of intelligibility, and may consider context effects on meaning.

Implications of unintelligibility include communication breakdown, social withdrawal, and reduced quality of life. Management usually centers on speech-language pathology approaches to improve articulation, resonance, rate, and prosody, with compensatory strategies or augmentative and alternative communication (AAC) as needed. Prognosis depends on the underlying condition, onset, and response to therapy.