The external branch of the superior laryngeal nerve is a nerve that plays a specific part in voice production. It originates from the vagus nerve, a major nerve extending from the brainstem to the abdomen. In the upper neck, the superior laryngeal nerve emerges from the vagus nerve and divides into an internal and an external branch. The internal branch is responsible for sensation inside the larynx, or voice box, while the external branch carries motor signals to a muscle in the neck.
Anatomy and Pathway of the Nerve
The external branch of the superior laryngeal nerve descends from high in the neck alongside the pharynx. As it travels downward, it maintains a close relationship with the superior thyroid artery, the main blood vessel supplying the thyroid gland.
The nerve continues its descent on the larynx, situated beneath the sternothyroid muscle. Its path leads it to the cricothyroid muscle, which is located on the external surface of the larynx between the cricoid and thyroid cartilages. The nerve pierces the inferior pharyngeal constrictor muscle just before it innervates the cricothyroid muscle.
The point where the external branch crosses the superior thyroid artery can vary among individuals. In some people, this crossing occurs high in the neck, far from the thyroid gland. In others, it happens much lower, placing the nerve in immediate contact with the upper part of the gland. This anatomical variability is a factor considered during neck surgeries.
Function in Voice Production
The primary role of the external branch of the superior laryngeal nerve is to innervate the cricothyroid muscle. When this muscle receives a signal from the nerve, it contracts, pulling the thyroid cartilage forward and downward. This action tenses and lengthens the vocal cords, which modulates the pitch of the voice.
This process is comparable to tightening a guitar string. A tighter string produces a higher note, and similarly, tensed vocal cords vibrate more rapidly as air passes through them, resulting in a higher-pitched sound. The nerve’s function is therefore directly linked to the ability to produce high-frequency sounds, such as during singing.
Beyond pitch control, the tension created by the cricothyroid muscle contributes to vocal stability and projection. By ensuring the vocal cords are properly tensed, the nerve helps produce a strong, clear sound and prevents the voice from breaking. This also allows for increases in vocal volume and helps sustain notes without fatigue.
Common Causes of Nerve Injury
Damage to the external branch of the superior laryngeal nerve is most frequently iatrogenic, meaning it is an inadvertent consequence of a medical procedure. During a thyroidectomy or parathyroidectomy, the nerve is at high risk of injury as the surgeon works near the superior thyroid vessels. The nerve can be stretched, trapped, or severed, with injury rates reported as high as 58% in these operations.
A nerve that runs very close to the superior thyroid blood vessels low in the neck is more difficult for a surgeon to protect. Other factors that elevate the risk include a large goiter, a short neck, or repeat neck surgery where scar tissue obscures landmarks. Thermal damage from electrocautery tools, which are used to control bleeding, is another potential cause of injury.
While surgery is the leading cause, other events can lead to nerve damage. External trauma to the front of the neck can crush or sever the nerve. In rarer cases, tumors or inflammation from infections can disrupt its function. These non-surgical causes are much less common than iatrogenic injury.
Symptoms and Diagnostic Process
Damage to the external branch of the superior laryngeal nerve causes paralysis of the cricothyroid muscle, leading to distinct vocal symptoms. Patients report a voice that feels weak or tires easily, a condition known as vocal fatigue. They may find it difficult to project their voice or be heard in noisy environments. A common complaint is the loss of vocal range, specifically an inability to produce high-pitched sounds, which can be distressing for singers.
The voice may also take on a monotone quality because the ability to modulate pitch is diminished. In some cases, the voice can sound breathy if the vocal cords do not tense properly to close completely during speech. These symptoms can be subtle, which sometimes delays diagnosis.
To diagnose an injury, a specialist performs a physical examination and takes a detailed history of the symptoms. A diagnostic tool is laryngoscopy, where a scope is used to visualize the larynx for subtle signs like vocal cord asymmetry. A definitive diagnosis is made with laryngeal electromyography (LEMG). This test involves inserting a fine needle electrode into the cricothyroid muscle to measure its electrical activity, directly confirming whether the nerve is sending proper signals.
Management and Treatment Strategies
Managing an injury depends on its cause and severity. For many patients, the primary treatment is voice therapy with a speech-language pathologist. Therapy focuses on compensatory strategies, such as exercises to strengthen other laryngeal muscles and optimize breath support. The goal is to improve vocal range and projection, reduce vocal fatigue, and make the voice more efficient to counteract the paralysis.
If the nerve has been severed or if symptoms are severe and do not improve with therapy, surgery may be considered. One option is a nerve reinnervation procedure, where a healthy nerve is rerouted to the cricothyroid muscle to restore its electrical supply. These surgical options are reserved for cases with permanent and significant vocal impairment. The decision to proceed with surgery is made after careful consideration of the potential benefits and risks.