The anterior approach to the cervical spine has become a popular method of treating a wide variety of cervical pathologies. This approach provides excellent access to the vertebral body and interbody space and provides an effective approach for decompression of the spinal cord and nerve roots. Although this approach is generally welltolerated, certain approach-related complications occur (1,2,3,4,5). Postoperative dysphagia is a well-documented complication following anterior cervical spinal surgery (6,7,8,9,10,11). Although dysphagia is relatively common, the severity of this condition varies widely (6,7,12,13,14). Usually, postoperative dysphagia is mild and improves with time, but in rare cases, it can be severe enough to require a long-term, alternate feeding strategy. Esophageal injury is a lesscommon, potentially devastating complication that, if unrecognized, can lead to life-threatening mediastinitis, purulent spondylitis, meningitis, or septicemia (15,16). In this chapter, we will discuss swallowing and the diagnosis and treatment of dysphagia following anterior cervical surgery. In addition, we will discuss the prevention, recognition, diagnosis, and management of intraoperative esophageal injuries.

The key swallowing structures in the oral cavity include the large muscles of mastication, the soft palate, and the muscular tongue. The pharynx is larger in its upper portion and narrows toward the esophageal junction. The walls of the pharynx are thin but contain an important posterior and lateral muscle layer. Anteriorly, the pharynx communicates with the larynx. The perilaryngeal tissues are richly innervated by several nerves, including the superior laryngeal nerve, and participate in active protection of the airway during swallowing.

The lower region of the pharynx is called the hypopharynx and extends from the hyoid bone to the inferior border of the cricoid cartilage, where the pharynx blends with the cervical portion of the esophagus. At the junction of the pharynx and esophagus lies the cricopharyngeal muscle, which forms the upper esophageal sphincter. This muscle contracts tonically to prevent the entry of air into the esophagus with respiration. During swallowing, the cricopharyngeus muscle must relax in a coordinated fashion to allow the passage of the food bolus. All intrinsic laryngeal muscles, except the cricothyroid, are innervated by the recurrent laryngeal nerve. The wall of the esophagus contains smooth muscle that reacts to bolus distension by peristaltic contraction, moving the food bolus toward the stomach.

Normal swallowing is a complex process involving the coordinated contraction of about 50 paired muscles. The swallowing process is normally completed in approximately 1.5 seconds. Neural control of swallowing resides in the brain stem adjacent to the respiratory centers. Here, a pattern generator controls the coordinated activity of the swallowing muscles. Sensory afferent information enters the swallowing center carried by the 5th, 7th, 9th, and
10th cranial nerves, while the motor efferent output to the swallowing muscles is carried primarily by the 5th, 7th, 10th, and 12th cranial nerves.

The act of swallowing has been divided into four phases: (a) the oral preparatory phase, (b) the oral phase, (c) the pharyngeal phase, and (d) the esophageal phase. The oral preparatory phase serves primarily to grind and moisten the food substances and form a bolus in preparation for swallowing. The oral phase transports the food from the mouth to the pharynx. The pharyngeal phase begins as the food bolus passes the faucial pillars to enter the pharynx and ends when the bolus passes the cricopharyngeus muscle to enter the esophagus. During this phase, airway protection is necessary during passage of the food bolus. Once in the esophagus, the bolus is transported by reflexic peristalsis to the stomach.

During the oral preparatory phase, the perioral muscles must seal the oral cavity, while the teeth and tongue work in coordination to break down the food and mix it with saliva to form a bolus. During this phase, the soft palate budges downward, sealing off the pharynx and nasal airway, allowing nasal breathing during the chewing process. The tongue plays a crucial role in pushing food laterally onto the surfaces of the teeth and in forming a food bolus on the floor of the mouth.

The oral phase of swallowing is initiated by placing the tip of the tongue against the maxillary incisors and depressing the midportion of the tongue to open the pharyngeal passage. Then, coordinated peristaltic contractions of the intrinsic tongue muscles propel the food bolus backward into the pharynx. As swallowing is initiated, respiration is paused momentarily to prevent aspiration of food into the larynx. In addition, the larynx moves anteriorly and superiorly, while the intrinsic laryngeal muscles contract to appose the arytenoids and vocal folds.

The complex pharyngeal phase of swallowing is triggered by sensory input from the 9th cranial nerve, which is stimulated as the bolus passes into the upper pharynx. In the pharyngeal phase of swallowing, the soft palate is elevated to seal off the nasal cavity. During the pharyngeal phase, five specific activities must be coordinated: (a) tightening of the muscles that seal off the nasopharynx (to prevent retropulsion of food into the nose); (b) retraction of the base of the tongue to allow food to be propelled into the pharynx; (c) contraction of muscles which pull the hyoid bone and larynx anteriorly and superiorly to protect the airway and widen the pharynx; (d) pharyngeal contraction to move the food through the pharynx; and (e) relaxation of the cricopharyngeus (upper esophageal sphincter) to allow bolus passage into the esophagus (22,23). In the latter portions of the pharyngeal phase, after the bolus is past the laryngeal opening, the strap muscles pull the larynx downward to its resting position, to assist with the emptying of the pharynx.

Aspiration is defined as the entry of material into the airway below the true vocal cords. Patients with swallowing dysfunction are at risk for aspiration and often experience frequent lower-airway infections and/or inadequate oral intake leading to weight loss and dehydration.

In dysphagia resulting from reduced laryngeal elevation (RLED), the larynx is not synchronously lifted anteriorly and superiorly (24). This results in a narrow pharynx which inhibits successful bolus passage and increases the risk of aspiration. Another type of swallowing dysfunction results from failure of the cricopharyngeus muscle (upper esophageal sphincter) to relax, resulting in bolus obstruction and accumulation in the pharynx (25,26,27,28,29). As food builds up in the pharynx, the intrabolus pressure increases and the risk of laryngeal penetration rises (30). Overflow aspiration may occur if excessive bolus accumulation remains and is inhaled into the larynx when respiration is resumed.

In patients with mild to moderate swallowing dysfunction, several factors may contribute to the success of the swallowing process. These factors include the consistency of the bolus, bolus temperature, bolus acidity, and the position of the neck. Swallowing is facilitated by neck flexion, which shortens critical swallowing muscles and facilitates upward and forward movement of the larynx during the early pharyngeal phase. In contrast, forced neck extension (for instance, by a rigid cervical collar or halo) increases the difficulty of swallowing (6).

The consistency of the food bolus also affects the difficulty of the swallowing task. A thickened liquid consistency (e.g., pudding) is generally easiest to swallow, while large solids (i.e., chunks of food) are the most difficult. Patients who have lost protective sensation to the perilaryngeal region (i.e., superior laryngeal nerve injury) often have difficulty with swallowing liquids that may enter the airway due to the absence of the protective laryngeal reflexes. Foods with a neutral pH and temperature close to body temperature are generally easiest to swallow and should be used for patients with difficulty swallowing.

The exact incidence of dysphagia is difficult to estimate because subtle dysphagia is often overlooked and underreported. Most of the clinical literature on dysphagia following neck surgery is retrospective with variable criteria used to detect and define dysphagia. Not surprisingly, the reported incidence of postoperative dysphagia varies widely, from 2% to 80% (6,7,31,32,33,34). Most studies fail to account for preoperative swallowing function and fail to rely on objective criteria to define the swallowing function.

Cloward et al. reported transient, mild dysphagia in approximately 80% of patients following anterior cervical fusion (34). In contrast, Stewart et al. reported transient dysphagia in 45% of patients and noted persistent symptoms at the 6-month time point in 27% of patients (6).

The proposed risk factors for dysphagia include prolonged retraction of the trachea and esophagus, multiple-level
surgery, older patient age, surgery involving the upper cervical region, postoperative edema or hematoma, prominent hardware or bone grafts, and revision surgery.

Proposed mechanisms of dysphagia include deinnervation of the pharyngeal or esophageal muscles, direct muscle trauma to the pharynx or esophagus, pharyngeal fibrosis, edema or hematoma, and postoperative pain or kinking of the esophagus due to prominent hardware or bone grafts. In terms of time course, the incidence of dysphagia is highest in the early postoperative period and decreases with time. Generally, by the 6-month time point, any residual symptoms are mild. Fortunately, for those patients with symptoms lasting this long, continued improvement beyond the 6-month time point is still possible (11).