Laryngeal Electromyography (LEMG)

Definition

• Cummings Otolaryngology, p. 1058

Indications

1. Vocal fold immobility - to differentiate neurogenic paralysis from mechanical (cricoarytenoid joint) fixation
2. Vocal fold paresis - confirms residual nerve function; paralysis implies complete denervation (no motor activity on EMG); paresis implies some residual nerve function
3. Prognosis - predicts recovery after RLN injury
4. Spasmodic dysphonia - differentiates adductor/abductor type; guides Botulinum toxin injection
5. Essential tremor of the larynx
6. Myasthenia gravis, motor neuron disease, myopathy
7. Guidance of Botulinum toxin (BotoxR) injection into laryngeal muscles
8. To differentiate vagus nerve vs. RLN injury (cricothyroid function is spared in isolated RLN injury)

Muscles Tested

• Cummings Otolaryngology, p. 1059

Procedure

1. Patient positioned supine or semi-recumbent
2. Skin cleaned; local anesthetic may be applied to skin only
3. Monopolar or concentric needle electrode inserted percutaneously into target muscle
4. Needle placement confirmed by:
   • Asking patient to phonate (TA activation)
   • Glide to falsetto pitch (CT activation)
   • Sniff maneuver (PCA activation)
5. Four parameters assessed:
   • Insertional activity (on needle insertion)
   • Spontaneous activity (at rest)
   • Volitional/recruitment activity (during phonation/tasks)
   • Motor unit potential (MUP) morphology

EMG Signal Analysis and Interpretation

1. Insertional Activity

• Brief burst when needle is inserted
• Increased insertional activity suggests membrane irritability (denervation or myopathy)
• Little standalone diagnostic utility

2. Spontaneous Activity (at rest)

• Normal: Electrically silent at rest
• Abnormal:
  • Fibrillation potentials - single muscle fiber discharges; indicate active denervation; poor prognostic sign
  • Positive sharp waves - also indicate denervation; poor prognostic sign
  • Appear 3 weeks after nerve injury

3. Volitional (Recruitment) Activity

• Contraction generates a Motor Unit Potential (MUP) - compound action potential from all muscle fibers in a motor unit
• With increasing effort, more MUPs fire and overlap creating an interference pattern
• Normal: Full interference pattern (dense, cannot discern individual MUPs)
• Neuropathic (denervation): Reduced recruitment - "picket fence" pattern - individual MUPs visible, reduced amplitude and density
• Absent MUPs: Complete denervation (nerve fully severed)

4. MUP Morphology

• Polyphasic potentials: Indicate reinnervation (but reinnervation does NOT necessarily mean return of vocal fold motion)
• Synkinesis: Aberrant reinnervation where nerve fibers reconnect with wrong muscles (e.g., TA activates on sniff instead of phonation) - downgrades prognosis even if recruitment appears good

Prognostic Value

• LEMG has a positive predictive value of ~90.9% for poor recovery (patients predicted poor prognosis indeed had poor recovery)
• LEMG is better at predicting when a vocal fold will NOT recover than when it will
• Optimum timing: 6 weeks to 6 months after onset of paralysis

Differentiating Vagus vs. RLN Injury

• In isolated RLN injury: cricothyroid (CT) muscle is normal on EMG (because CT is supplied by the external branch of SLN)
• In high vagus nerve injury: CT muscle also shows denervation changes
• EMG can thus localize the level of the lesion

Contraindications

• Coagulopathy or anticoagulant therapy (relative)
• Patient inability to cooperate (children, psychiatric disorder)
• Local infection at needle site

Limitations

1. Primarily a qualitative test; lacks standardized quantitative interpretation
2. Highly dependent on examiner experience
3. Early EMG (within 3 weeks) may miss fibrillation potentials (they take time to develop)
4. Reinnervation (polyphasic MUPs) does not guarantee return of motion (synkinesis can occur)
5. Cannot directly visualize structural joint pathology

Clinical Importance in ENT Practice

• Helps decide timing of medialization procedures in unilateral vocal fold paralysis (avoid early irreversible surgery if nerve recovery is expected)
• Essential for Botulinum toxin guidance in spasmodic dysphonia
• Distinguishes neurogenic from mechanical/arytenoid fixation causes of immobility
• Guides management in post-thyroidectomy/post-neck surgery vocal fold palsy