Unilateral upper motor neuron dysarthria is a motor speech disorder caused by damage to nerve pathways on one side of the brain. The term breaks down into “unilateral” (one side), “upper motor neuron” (nerve pathways from the brain), and “dysarthria” (difficulty with speech). These pathways, part of the corticobulbar tracts, control voluntary speech movements. Damage to them on one side of the brain leads to weakness in the face and tongue muscles on the opposite side of the body, causing speech difficulties.
Underlying Neurological Causes
The most frequent cause of this dysarthria is a stroke, a disruption of blood flow to the brain that can be either ischemic (caused by a blockage) or hemorrhagic (caused by bleeding). Small, localized strokes called lacunar infarcts are often associated with this condition.
Traumatic brain injury (TBI) is another significant cause. A focal injury to one side of the brain from an accident or surgical procedure can damage the upper motor neuron pathways.
Brain tumors can also lead to this speech disorder. A tumor growing in or near the motor regions of the brain can compress or destroy the upper motor neuron fibers on one side. Infections or inflammatory conditions that affect the brain, such as encephalitis or meningitis, are less common but recognized causes.
Key Symptoms and Speech Patterns
The most common symptom of unilateral upper motor neuron dysarthria is imprecise consonant production, often described by listeners as “slurred,” “thick,” or “clumsy” speech. A harsh or strained-strangled voice quality is another frequent characteristic, which can be accompanied by breathiness or reduced loudness. This vocal change is thought to result from incomplete or weakened vocal fold closure on the affected side.
Changes to the rate and rhythm of speech are also common. Individuals often exhibit a slow rate of speech, which may be a direct result of muscle weakness or a compensatory strategy to improve clarity. The prosody, or the melody of speech, can be affected, leading to a monopitch or monoloudness where the voice lacks normal variation.
A primary diagnostic feature is the presence of physical signs, specifically weakness in the lower part of the face and on one side of the tongue. Difficulties with chewing and swallowing, known as dysphagia, can also co-occur, as the same muscles are involved.
The Diagnostic Process
Diagnosis is performed by a Speech-Language Pathologist (SLP) and begins with a review of the patient’s case history. The SLP gathers information about the onset and progression of the speech difficulties and the patient’s medical history. Understanding the nature of the neurological event, such as a recent stroke confirmed by imaging like a CT scan or MRI, provides context for the speech symptoms.
An oral motor examination is a part of the assessment. During this exam, the SLP evaluates the structure and function of the speech muscles, assessing the strength, speed, range, and coordination of the lips, tongue, and jaw. This examination is designed to identify the characteristic one-sided weakness of the lower face and tongue.
The diagnostic process involves a detailed speech assessment. The SLP listens to the individual perform various speech tasks to identify specific patterns of impairment. These tasks may include reading a passage, engaging in conversation, and producing sustained vowel sounds. Diadochokinetic rates, which involve rapidly repeating syllables like “puh-tuh-kuh,” are also assessed to evaluate the speed and regularity of articulatory movements.
Treatment and Management Strategies
Therapy focuses on improving articulatory precision to make speech clearer. Exercises may be used to strengthen the muscles of the tongue and lips. SLPs use drills that focus on exaggerating consonant sounds, a technique known as overarticulation, to help the speaker make sounds more distinct. Another method, phonetic placement, involves the therapist instructing the individual on the correct positioning of the tongue and lips to produce specific sounds.
Managing the speed of speech is another strategy. Because a slow rate can improve intelligibility, SLPs may teach pacing techniques. This can involve using a pacing board, where the individual touches a series of dots as they say each word or syllable, or tapping a finger for each word. These external cues help the person slow down and allow more time to articulate sounds accurately.
For individuals with a harsh or strained vocal quality, voice therapy can be beneficial. Treatment may involve exercises aimed at achieving a more relaxed and efficient voice production. This could include techniques to improve breath support for speech or to initiate voicing more gently, reducing strain on the vocal folds.
Compensatory strategies are also taught to both the speaker and their communication partners to improve communication success. The speaker might be encouraged to use shorter sentences, ensure they have the listener’s attention before speaking, and use gestures to supplement their message. Communication partners are advised to reduce background noise, maintain face-to-face contact, and ask for clarification when a message is not understood.