When the superior laryngeal nerve (SLN) is damaged, it primarily affects an individual's ability to control vocal pitch and can lead to a range of voice-related symptoms, often significantly impacting vocal performance and daily communication.
What Happens When the Superior Laryngeal Nerve is Damaged?
Damage to the superior laryngeal nerve can lead to ipsilateral cricothyroid muscle paralysis, causing a characteristic set of vocal and laryngeal symptoms, most notably impacting pitch control and vocal endurance.
The Role of the Superior Laryngeal Nerve
The superior laryngeal nerve is a crucial branch of the vagus nerve (cranial nerve X), which plays a vital role in both sensory perception and motor control of the larynx. It typically divides into two main branches:
- Internal Laryngeal Nerve: This branch is primarily sensory, providing sensation to the mucous membranes of the larynx above the vocal folds, including the epiglottis. Damage here can impair the cough reflex, increasing the risk of aspiration (food or liquid entering the airway).
- External Laryngeal Nerve: This is the motor branch, responsible for innervating the cricothyroid muscle. The cricothyroid muscle is essential for increasing tension on the vocal cords, which lengthens them and raises vocal pitch.
Clinical Manifestations of SLN Damage
When the external branch of the superior laryngeal nerve is damaged, the cricothyroid muscle on the affected side becomes paralyzed. This results in the inability to properly tense and lengthen the vocal cord, leading to the following common symptoms:
| Symptom | Explanation |
|---|---|
| Ipsilateral Cricothyroid Muscle Paralysis | The cricothyroid muscle on the same side as the nerve injury loses its function. |
| Hoarse, Breathy Voice | Inability to properly tense the vocal cord leads to incomplete closure during phonation, causing air leakage. |
| Frequent Throat Clearing | A compensatory mechanism due to perceived vocal effort or discomfort. |
| Vocal Fatigue | The unaffected vocal cord and other laryngeal muscles work harder to compensate, leading to exhaustion. |
| Diminished Vocal Frequency Range | Especially noticeable when trying to raise pitch, as the vocal cords cannot be adequately lengthened. |
Impact on Voice and Vocal Performance
The most prominent effect of superior laryngeal nerve damage is on pitch control. Individuals may find it difficult or impossible to sing high notes, to achieve a full vocal range, or even to vary pitch during normal speech. This can be particularly devastating for professional voice users, such as singers or public speakers, as their ability to perform their craft is severely compromised. The voice may sound monotone, strained, or simply unable to reach higher registers.
If the internal branch is affected, sensory impairment can lead to a reduced cough reflex and difficulty detecting the presence of foreign substances in the upper larynx, potentially increasing the risk of silent aspiration.
Causes of Superior Laryngeal Nerve Injury
Damage to the superior laryngeal nerve can occur due to various reasons, including:
- Surgical Complications: It is a known risk during neck surgeries, such as thyroidectomy, carotid endarterectomy, or anterior cervical spine surgery.
- Trauma: Direct injury to the neck.
- Viral Infections: Certain viral infections can cause inflammation and temporary or permanent nerve damage.
- Tumor Compression: Growth or mass compressing the nerve.
Diagnosis and Management
Diagnosing superior laryngeal nerve damage typically involves a thorough clinical history, vocal assessment, and laryngeal examination, often with a procedure called stroboscopy or laryngoscopy to visualize vocal cord movement. Laryngeal electromyography (LEMG) of the cricothyroid muscle can confirm the paralysis and assess the extent of nerve injury.
Management often includes voice therapy to help compensate for the loss of function, and in some cases, surgical interventions may be considered to improve vocal cord positioning or tension.
