Superior Laryngeal Nerve (SLN) Paralysis

Background

The larynx is innervated by four primary nerves:

• Right and left recurrent laryngeal nerves (RLNs): supply all intrinsic laryngeal muscles that support vocal fold movement and internal tension—thyroarytenoid, lateral cricoarytenoid, posterior cricoarytenoid, and interarytenoid.
• Superior laryngeal nerves (SLNs): the external branch supplies the cricothyroid muscles, sometimes called the “singer’s muscles.” These muscles lengthen and externally tense the vocal folds by rotating the cricothyroid joint, closing the cricothyroid space. The effect is critical for higher-range phonation, even though all laryngeal muscles participate in singing.
• Sensory fibers: the SLN also gives off an internal branch that provides sensation to the supraglottic larynx (vocal folds and above).

Clinical Manifestations

• Motor function

  • The cricothyroid muscles adjust vocal fold length, thinness of edge, and overall tautness.
  • Their action enables the upper range of voice and vocal projection.
  • When impaired, the two most consistent symptoms are:
    • Loss of the upper voice (an octave or more)
    • Reduced projection—an inability to “throw the voice across the street” rather than just loss of volume.

• Sensory function

  • Each SLN internal branch supplies supraglottic sensation.
  • Loss of supraglottic sensation is usually clinically silent, with patients compensating as long as swallowing mechanics remain intact.

Causes

• Idiopathic (often presumed post-viral);
• Postsurgical—since the nerve is in close proximity to necessary surgical “pathways” for cervical spine or carotid surgery, or in the way for removal of large thyroid tumors.
• Special note: Sometimes we conceptualize injury or dysfunction to the “superior laryngeal system.” That is because nerve damage, cricothyroid muscle damage (say due to removal of a large thyroid tumor that involves the muscle), or cricothyroid joint fixation (arthritic, or post-traumatic) would all manifest with the same voice and laryngeal examination findings. 

Diagnosis

Diagnosis rests primarily on vocal capability testing and laryngoscopic observation.

History (Part I of the Integrative Diagnostic Model)

• The patient describes loss of upper range and projection.

Vocal Capability Battery (Part II)

• Speaking voice is not notably abnormal when approached informally.
• Examiner elicits high-voice and projection tasks (e.g., upward siren).
• The diagnosis is confirmed by recognizing dramatic loss of expected upper range, when comparing the patient’s response to the expected capability for age and sex.

Laryngoscopic Findings (Part III)

• Unilateral SLN paralysis: posterior commissure may deviate toward the paralyzed side; epiglottis may pull toward the intact side during high-pitch elicitation. The affected fold may appear slightly shorter, lower, or more bowed—though these findings are subtle.
• Bilateral SLN paralysis: diminished anterior movement of the epiglottis on upward siren; vocal folds fail to lengthen appropriately at higher pitches.
• Electromyography (EMG) can confirm paralysis, but in practice it is rarely necessary and often less helpful than careful history, vocal capability testing, and laryngoscopic examination.

Management Options

1. Observation

   • Allow up to one year for possible spontaneous recovery.

2. Voice therapy and active use

   • Encourage continued use of the voice, sometimes even “goading” it—what we call asking the voice to do what it should be able to do. Frequent, brief up-sirens that stretch the upper range, for example.
   • Even without recovery, this can yield modest gains in upper range and projection.

3. Laryngeal framework surgery

   • Performed under local anesthesia with sedation, only for highly motivated patients with realistic expectations.
   • Technique: permanent sutures approximate the inferior thyroid cartilage border to the superior cricoid cartilage border, statically closing the cricothyroid space to mimic cricothyroid contraction.
   • Results vary, but in selected patients, benefits can be substantial.

Summary

SLN paralysis is less conspicuous than RLN paralysis but can be profoundly disabling for professional voice users or those who need to project voice as for teaching large groups of students, public speaking, etc. Its hallmarks are loss of the upper voice and loss of projection, best verified through elicitation tasks in the Vocal Capability Battery. Physical exam findings are often subtle. Management ranges from watchful waiting to surgery, with outcomes tailored to patient goals and expectations.