Superior laryngeal nerve: identification-preservation

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Superior laryngeal nerve: identification-preservation

F. Tamariz, A. Rafiq, R.C. Merrell
Tehnici chirurgicale, no. 6, 2005
* Virginia Commonwealth University, Medical Informatics and Technology Applications Consortium Richmon
* Medical Informatics and Technology Applications Consortium

An average 60,000 thyroidectomies are performed each year in the United States (1). Although the incidence of injury to the RLN is said to be 0.5% the incidence of injury to the external branch of the superior laryngeal nerve is not well known. Highly sensitive stroboscopic analysis and acoustic analysis shows some at least temporary change of the voice or vocal apparatus in as many as 30% of cases (2). Concern about complication of thyroidectomy with regard to voice is high for clinical reasons and medicolegal issues although the incidence of litigation is remarkably low at only 30 total in recent years (3-4). Thyroidectomy continues to be fraught with a great deal of concern largely focused upon injury to the recurrent laryngeal nerve or parathyroid glands. However, the external branch of the superior laryngeal nerve is of concern as well although injury has limited impact of the spoken voice.
However, damage of the superior laryngeal nerve (SLN) (paralysis or paresis) may affect force in singing when sustained high pitch is needed. The famous index case is that of soprano Amelita Galli-Curci, whose career was purportedly ended after injury to the SLN after thyroid surgery. Due to compressing goiter Galli-Curci was operated in 1935. After surgery her return to the stage was a failure so at the age of 53 she retired. Although it was said that the alteration of her voice was a consequence of the surgery, many people noticed dramatic changes in her voice long before the operation (5). Her failure to hold sustained high notes implicates to external branch of the SLN. Myth or reality, this story has challenged thyroid surgeons to prevent damage to the SLN. In 1997, the Broadway star Julie Andrews underwent surgery to remove non-cancerous nodules from her throat, after the procedure the actress and singer lost her high range voice, although she began to have voice problems back in 1990s. Although clinical consequences of SLN injury (paralysis or paresis) are still unclear, meticulous preservation of the nerve seems appropriate.
The SLN is a very small structure with a quite variable course related to the superior pole of the thyroid gland (6-7). A recent report claims 65% visualization of the EBSLN using video assisted thyroidectomy (7). In this report we describe a method that allows identification of the EBSLN in standard thyroidectomy 80% of the time in over 250 thyroidectomies over the past 10 years. Recent work makes possible documentation by digital image.

Anatomy Review
A full understanding of the anatomy of the SLN and its branches, could prepare the surgeon to reduce this risk of damage.
The superior laryngeal nerve (SLN) is a branch of the vagus nerve that arises from the middle of the inferior nodosal ganglion of the vagus. About 36mm caudal to the jugular foramen of the skull (8-9) the SLN leaves the vagus nerve and receives a sympathetic autonomic branch from the superior cervical ganglion. The SLN descends, divides into two branches, external and internal at the level of the hyoid bone (9-10). The internal branch (ramus internus) (IBSLN) travels parallel and medial to the superior laryngeal artery, passes beneath the thyrohyoid muscle towards the thyrohyoid membrane, pierces it, and is distributed to the mucous membrane of the larynx (gives sensation to the pyriform fossa and laryngeal mucosa above the vocal cords) (11-14).
The external branch (ramus externus) (EBSLN), the smaller branch with an approximated diameter of 0.2 mm continues caudal after the departure of the internal branch (7). The tiny filament descends on the larynx lying on the inferior pharyngeal constrictor muscle, beneath the sterno-thyroid, to supply the cricothyroid muscle. It communicates with the superior cardiac nerve behind the common carotid artery (8, 14, 15). The relationship of the EBSLN with the superior thyroid artery (STA) and the upper pole of the
thyroid gland is variable; this condition is what makes identification of the nerve not always possible in the routine dissection of the superior pole of the thyroid. The variability is defined based upon how far above the superior pole the external branch leaves the pedicle to lie upon the larynx at the inferior pharyngeal sphincter. When the nerve leaves the vessels 1 cm or more above the upper pole of the gland the variant is termed Type 1. In variant Type 2a the nerve leaves vascular pedicle closer to the upper pole of the gland (less than 1cm above it); In variant Type 2b the nerve is virtually on top of the upper pole of the gland when it crosses to the larynx. Sometimes the EBSLN approaches the larynx quite high without ever joining with the vascular pedicle (8, 14, 16, 17). The anatomical variations is not related by ethnicity, height or side of the neck. The EBSLN is slightly larger in men than in women (10).

Function and Dysfunction
The superior laryngeal nerve (SLN) innervates the cricothyroid muscle which by its normal performance elongates and tenses the vocal cords, this action is accomplished by pulling the arch of the cricoid cartilage upward around its articulation with the inferior horn of the thyroid cartilage, as a result the arythenoids cartilages are forced backward stretching the vocal cords. This muscle originates on the lateral surface of the cricoid cartilage, runs upward and backward and then inserts into the inferior horn of the
thyroid cartilage; (9, 11, 14, 18) it also innervates the inferior pharyngeal constrictor muscle that along with the superior and middle pharyngeal constrictor muscles participate in the process of driving a bolus of food down into the esophagus; it is believed that the inferior pharyngeal constrictor muscle also works as a sphincter, preventing the entrance of air into the esophagus at rest. Clinical signs or symptoms of EBSLN dysfunction are not completely understood (18). They are not specific for EBSLN dysfunction but may be shared with other voice disorders. For example, changes like altered singing voice, variation of pitch or volume range, and fatigue are seen occasionally after thyroidectomy despite a careful
procedure that identifies and preserves the EBSLN (2). Quantitative diagnosis is by electromyogram, stroboscopy, or acoustic testing. EBSLN palsy with failure of cord tension can be identified by stroboscopy. The video assessment allows real time visualization of larynx, to reveal pathological glottal rotation, bowing and vertical mismatch of the vocal cords (2). EMG is an invasive procedure less and less used. Acoustic signature is probably the most sensitive test but the source of the change is less readily assigned to EBSLN damage.

Identification of the EBSLN is greatly facilitated by gentle lateral traction of the pedicle to demonstrate the tiny fiber tethered to the larynx as the pedicle bows laterally. In order to accomplish this lateral traction advantage is taken of the avascular plane between the upper pole and the larynx. This plane can be opened by blunt dissection with a kitner dissector which can be used to circumscribe the pole in an almost atraumatic manner. With the pole circumscribed the cotton dissector is removed from the clamp and replaced with an umbilical tape. The tape is drawn through the space between the upper pole and larynx and tied around the upper pole. Gentle lateral traction will demonstrate the tiny fiber Type 2a and 2b. Type 1 variants may be much higher in the field of dissection. Of course, in that circumstance the nerve is not at risk when the vascular pedicle is secured. With nerve in view or clearly above the pedicle the vessels are clamped preserving the nerve (Fig. 1 -3).
A bare Stryker laparoscope camera (without the laser) issued to capture and document the dissection of the EBSLN. A light cord attached to the camera, and the camera holder is held by a flexible arm (Figure 4). The integrity of the the EBSLN is sought and demonstrated using the Stryker monitor and recorded with digital disk. This arrangement can be for not only documentation but also for teaching.
Figura 1
Figura 2
Figura 3
Figura 4
It is generally possible to identify the EBSLN during thyroid surgery. Injury to the nerve can generally be avoided by meticulous division of individual branches of superior pole vessels at the thyroid capsule of the upper lobe. This procedure is more common in Europe and is similar to a method used to preserve the RLN by avoiding the vascular pedicle. Identification of the nerve prior to securing the upper pole vessels or the use of nerve stimulator or nerve monitor are routines also reported (16, 18). However, the avascular plane between the larynx and the upper pole allows blunt dissection and lateral retraction of the upper pole and with a high likelihood of either seeing the nerve or reassuring the surgeon that the nerve is safely above the dissection and its pedicle. The EBSLN is the only structure entering the larynx from the pedicle and is readily apparent upon retraction in 80% of cases. When not seen it must leave the pedicle even higher and therefore is not at any risk, if the pedicle is clamped and tied.
Identification of the EBSLN or its absence from view allows one to clamping and ligature of the upper pole vessels with confidence. The use of standard camera regiment in the OR allows documentation of efforts to preserve the nerve and providing the opportunity to demonstrate pertinent anatomy for pedagogical purposes.

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