6.2 Assessment of Motor Speech Disorders

6.2 Assessment of Motor Speech Disorders

Motor speech disorders are neurogenic speech impairments resulting from neurological damage affecting the planning, programming, control, or execution of speech production. The two major diagnostic categories are Dysarthria and Apraxia of Speech (AOS). Precise assessment requires differentiating neuromuscular subsystem failure from motor planning disruption, localizing the underlying neurological lesion based on Mayo Clinic (Darley, Aronson, and Brown [DAB]) perceptual taxonomy, and quantifying speech intelligibility and acoustic parameters.

---

Differential Framework: Dysarthria vs. Apraxia of Speech

Understanding the fundamental distinction between execution and planning deficits is the foundation of motor speech evaluation:

• Dysarthria: A group of neurogenic speech disorders characterized by abnormalities in muscle strength, velocity, range, tone, steadiness, or accuracy. It affects one or more of the five speech subsystems: respiration, phonation, resonance, articulation, and prosody. Speech errors are consistent, predictable, and present across both automatic and volitional speech tasks.
• Apraxia of Speech (AOS): A neurogenic speech disorder characterized by impairment in the capacity to select, sequence, and program sensorimotor commands for the volitional positioning and movement of speech articulators. It occurs in the absence of muscle weakness, paralysis, or tone abnormalities. Automatic and reactive speech (e.g., swearing, counting, singing) is frequently preserved, whereas volitional speech exhibits articulatory groping, inconsistent errors, and prosodic disruption.

---

Mayo Clinic Classification of Dysarthria Subtypes

+---------------------------------------------------------------------------------------------------------------------+ 
|                                    DYSARTHRIA SUBTYPES & NEURO-LOCALIZATION                                        | 
+---------------------------------------------------------------------------------------------------------------------+ 
| Subtype       | Neurological Site         | Common Etiologies        | Dominant Perceptual & Acoustic Features      | 
+---------------+---------------------------+--------------------------+----------------------------------------------+ 
| Flaccid       | Lower Motor Neuron (LMN)  | ALS, Myasthenia Gravis,  | Hypernasality, nasal emission, breathiness,  | 
|               | (CN V, VII, IX, X, XII)   | Wallenberg syndrome      | fasciculations, soft/breathy voice, diplophonia| 
+---------------+---------------------------+--------------------------+----------------------------------------------+ 
| Spastic       | Bilateral Upper Motor     | Pseudobulbar palsy, TBI, | Strained-strangled voice, harshness, low     | 
|               | Neuron (UMN)              | bilateral strokes        | pitch, slow rate, hyperactive gag reflex     | 
+---------------+---------------------------+--------------------------+----------------------------------------------+ 
| Ataxic        | Cerebellum (cerebellar    | Cerebellar stroke, MS,   | Irregular articulatory breakdown, dysmetria, | 
|               | control circuit)          | Friedreich's ataxia      | scanning prosody, equal/excess stress        | 
+---------------+---------------------------+--------------------------+----------------------------------------------+ 
| Hypokinetic   | Basal Ganglia control     | Parkinson's disease,     | Monopitch, monoloudness, reduced loudness,   | 
|               | circuit (substantia nigra)| Progressive Supranuclear | short rushes of speech, rapid blurry AMRs    | 
+---------------+---------------------------+--------------------------+----------------------------------------------+ 
| Hyperkinetic  | Basal Ganglia control     | Huntington's disease,    | Involuntary movements interrupting speech,   | 
|               | circuit (striatum)        | Tardive dyskinesia       | voice stoppages, sudden pitch breaks         | 
+---------------+---------------------------+--------------------------+----------------------------------------------+ 
| Unilateral    | Unilateral UMN            | Single hemisphere stroke | Mild imprecise consonants, lower facial      | 
| UMN (UUMN)    |                           |                          | weakness, transient dysprosody               | 
+---------------+---------------------------+--------------------------+----------------------------------------------+ 
| Mixed         | Multiple motor systems    | ALS (Flaccid-Spastic),   | Combination of perceptual characteristics    | 
|               |                           | MS (Spastic-Ataxic)      | from two or more single dysarthria types     | 
+---------------------------------------------------------------------------------------------------------------------+ 

---

Oral Mechanism & Cranial Nerve Examination Protocol

A rigorous cranial nerve examination isolates lower motor neuron pathology and structural asymmetry affecting the speech mechanism:

1. CN V (Trigeminal Nerve - Motor & Sensory)

• Motor Assessment: Inspect jaw at rest (deviation toward side of lesion). Assess jaw opening/closing strength against resistance. Evaluate masseter and temporalis muscle palpation.
• Speech Impact: Inability to elevate mandible impairs labial and lingual articulation.

2. CN VII (Facial Nerve - Motor)

• Assessment: Observe facial symmetry at rest (nasolabial fold flattening, eye droop). Test volitional non-speech movements: lip puckering, smiling, puffing cheeks against resistance, wrinkling forehead.
• LMN vs. UMN Differential: LMN lesion (facial nerve nucleus/nerve) causes ipsilateral weakness of both upper and lower face. UMN lesion causes contralateral weakness of the lower face only (due to bilateral UMN innervation to upper facial motor nucleus).
• Speech Impact: Imprecise bilabial (/p, b, m/) and labiodental (/f, v/) consonants.

3. CN IX & X (Glossopharyngeal & Vagus Nerves)

• Velopharyngeal Assessment (Pharyngeal Branch of CN X): Observe soft palate elevation at rest and during phonation of /a/. Symmetric elevation indicates intact innervation; unilateral lesion causes palate to pull toward the intact side with uvula deviation. Evaluate hypernasality and nasal emission using a mirror or nasometer.
• Laryngeal Assessment (Superior & Recurrent Laryngeal Nerves of CN X): Assess vocal fold adduction via cough and glottal coup strength. Weak cough with normal coup suggests respiratory weakness; weak cough AND weak glottal coup indicates laryngeal adductor weakness. Evaluate pitch gliding (Superior Laryngeal Nerve innervating cricothyroid) and breathiness/stridor (Recurrent Laryngeal Nerve).

4. CN XII (Hypoglossal Nerve - Motor)

• Assessment: Inspect tongue at rest inside oral cavity for fasciculations and atrophy. Assess tongue protrusion (tongue deviates toward the side of lesion/weakness). Evaluate lateralization, elevation to alveolar ridge, and force against cheek resistance.
• Speech Impact: Imprecise lingual consonants (alveolar /t, d, n, s/, palatal /ʃ, ʒ/, velar /k, g/).

---

Diadochokinetic (DDK) Rate Analysis

Diadochokinetic tasks assess the speed, regularity, and sequencing capability of the motor speech architecture:

+-----------------------------------------------------------------------------------+
|                              DIADOCHOKINETIC NORMS & PATTERNS                      |
+-----------------------------------------------------------------------------------+
| Task Type                         | Clinical Purpose & Norms                      |
+-----------------------------------+-----------------------------------------------+
| Alternate Motion Rates (AMRs)     | Assesses speed & rhythmicity of single        |
| (/pa-pa-pa/, /ta-ta-ta/, /ka-ka/) | articulatory posture. Norm: 5-7 reps/sec.     |
|                                   | - Spastic: Slow, regular                      |
|                                   | - Ataxic: Slow, irregular rhythm & pitch      |
|                                   | - Hypokinetic: Rapid, blurred, accelerating   |
|                                   | - Flaccid: Slow, breathy                      |
+-----------------------------------+-----------------------------------------------+
| Sequential Motion Rates (SMRs)    | Assesses motor sequencing across shifting     |
| (/pa-ta-ka/)                      | articulatory postures. Norm: 5-7 reps/sec.    |
|                                   | - Apraxia of Speech: Severe breakdown,        |
|                                   |   phoneme substitutions, groping, inability   |
|                                   |   to sequence phonemes compared to AMRs.      |
+-----------------------------------------------------------------------------------+

---

Diagnostic Markers of Apraxia of Speech (AOS)

When evaluating suspected AOS, the clinician must confirm the presence of primary diagnostic features while ruling out dysarthric muscle weakness or aphasic phonemic paraphasias:

Primary Clinical Features of AOS

1. Phonetic & Prosodic Abnormalities: Slow overall rate of speech, extended intra-segmental pauses between syllables and words, equalized stress across syllables (syllable segregation).
2. Articulatory Groping & Effortful Speech: Visible or audible trial-and-error placement of articulators prior to or during phonation.
3. Inconsistent Error Patterns: Variable articulatory errors on repeated productions of identical words.
4. Initiation Difficulties: False starts, restarts, and audible/silent hesitation at the beginning of utterances.
5. Spared Automatic Speech: Marked discrepancy between fluent automatic/reactive speech (e.g., counting 1-10, cursing) and severely impaired volitional speech.

Formal Assessment Batteries & Intelligibility Testing

• Apraxia Battery for Adults - Second Edition (ABA-2): Quantifies AOS severity across limb/oral apraxia, latency/utterance time, repeated trial inconsistency, and inventory of articulatory errors.
• Assessment of Intelligibility of Dysarthric Speech (AIDS): Standardized metric measuring single-word intelligibility %, sentence-level intelligibility %, and rate of intelligible speech per minute (IWPM).
• Frenchay Dysarthria Assessment (FDA-2): Standardized rating scale evaluating 8 speech subsystems/components.