Why do patients with Amyotrophic Lateral Sclerosis (ALS) experience speech difficulties later in disease progression?

Speech Difficulties in ALS Disease Progression

Patients with ALS experience speech difficulties later in disease progression primarily due to progressive weakness and atrophy of muscles involved in the oral and pharyngeal phases of speech production, resulting from both upper and lower motor neuron degeneration. 1

Pathophysiology of Speech Deterioration in ALS

Muscle Weakness Progression

• Oral Phase Impairments:

  • Poor lip seal leading to drooling and trapped food particles
  • Weakness of masticatory muscles causing poor chewing
  • Tongue muscle weakness impairing the ability to propel food and form speech sounds 1

• Pharyngeal Phase Impairments:

  • Reduced soft-palate closure causing hypernasality
  • Altered pharyngeal peristalsis increasing aspiration risk
  • Incomplete epiglottic closure 1

Timeline of Speech Deterioration

• Speech typically remains functional for approximately 18 months from the first bulbar symptom 2
• Speech deterioration occurs more rapidly in patients with bulbar-onset ALS compared to spinal-onset 3
• Time to speech loss in bulbar-onset ALS is approximately:
  • 23 months based on speaking rate <120 words/minute
  • 32 months based on speech intelligibility <85% 3

Progression of Speech Changes

Five Stages of Swallowing/Speech Changes

1. Normal eating/speaking habits
2. Early problems (difficulty chewing, mild articulation issues)
3. Dietary consistency changes/speech modifications
4. Need for tube feeding/augmentative communication
5. Nothing by mouth/loss of natural speech 1

Specific Speech Deterioration Patterns

• Initially, patients with mild-moderate dysarthria may show decreased tongue and lip movement variability (possibly as a compensatory mechanism) 4
• As disease progresses to severe dysarthria, patients show increased movement variability indicating loss of motor control 4
• Speech rate slows progressively, with articulation rate and intelligibility declining in nearly all patients 2

Neurophysiological Basis

• ALS affects both upper motor neurons (UMNs) and lower motor neurons (LMNs) 5
• UMN involvement alters the rate and pattern of firing of motor neurons in the nucleus ambiguus in the medulla that control laryngeal muscles 1
• LMN degeneration leads to muscle weakness, atrophy, and fasciculations in the bulbar region 1
• The combined UMN and LMN involvement creates a mixed dysarthria pattern with both spastic and flaccid components 5

Clinical Implications

• Bulbar-onset ALS patients require more urgent evaluation for augmentative and alternative communication (AAC) methods 2
• 60% of ALS patients will eventually require AAC methods as speech deteriorates to poor functionality 2
• Instrumental evaluation with videofluoroscopy (VFS) or endoscopic evaluation of swallowing (FEES) should be used to confirm diagnosis and assess function 5
• Postural maneuvers (particularly chin-tuck posture) can help protect airways during swallowing in patients with moderate dysphagia 1

Caveat and Pitfalls

• Speech deterioration may be overlooked or underestimated due to focus on other symptoms
• Language impairment (not just speech motor problems) may co-occur with ALS, especially in bulbar-onset cases 6
• Evaluation of writing samples is important to identify language problems that may be masked by dysarthria 6
• Slow speech rate may actually increase articulatory variability and potentially worsen intelligibility 4
• Monitoring for signs of respiratory insufficiency is crucial, as respiratory function decline affects speech production and is associated with shorter survival 5

Speech deterioration in ALS reflects the progressive nature of the disease and significantly impacts quality of life, necessitating timely intervention with communication strategies and eventually AAC methods.