10.1 Introduction

Dysphagia, or impaired swallowing ability, is a very common disorder that can have adverse effects on quality of life and overall health. Although over 20% of the adult population experiences dysphagia several times a month, ¹ the impact on overall health and quality of life can range from minimal, in the case of mild dysphagia, to life threatening, in the case of severe dysphagia. Dysphagia may result from dysfunction of any of the components involved in the complex neuromuscular interaction that facilitates the swallowing mechanism. While there are three functional categorizations of dysphagia (oral, oropharyngeal, esophageal), the only two types that respond favorably to botulinum neurotoxin (BoNT) injection are oropharyngeal, which refers to difficulty in the passage of a food bolus from the oropharynx into the esophagus, and esophageal, which refers to disturbances of the passage of the food bolus within the esophagus itself. ² Hyperfunction of any of the muscles involved in swallowing is a frequent cause of dysphagia, although in most cases of moderate or severe dysphagia, there are multiple, significant deficits related to muscle strength, coordination, and sensation. With regard to isolated hyperfunctional dysphagia, failure of relaxation of the cricopharyngeus (CP) muscle is the most common cause of oropharyngeal dysphagia, and hypertonicity of the lower sphincter is associated with esophageal dyshagia. ^{3 ,​ 4}

Treatment of these disorders depends on the etiology of the dysphagia (stroke, neurodegenerative disease, congenital disorders, high vagal lesions, postlaryngectomy, etc.), and may include surgery, medications, or swallowing therapy, or some combination of them. When the main cause of dysphagia is hyperfunctional muscles, chemodenervation with BoNT is a reasonable treatment option. This chapter reviews the anatomy, physiology, and management of hyperfunctional swallowing disorders that may affect the pharyngoesophageal phase of swallowing.

10.2 Anatomy and Physiology

The esophagus forms a muscular conduit that connects the hypopharynx to the stomach. It is approximately 18 to 26 cm in length and topographically is divided into three regions: cervical, esophageal, and abdominal. The upper or cervical segment is composed of striated muscle, whereas the lower or abdominal part is composed of smooth muscle. The esophageal region is a mixed transition zone composed of both muscle types. The upper esophageal sphincter (UES) and lower esophageal sphincter (LES), in conjunction with the esophagus, are responsible for anterograde transport of nutrients and for preventing the retrograde transport of gastric contents. Hypertonicity, that is, decreased or absent relaxation, in these zones, leads to dysphagia.

10.2.1 Anatomy and Physiology of the Upper Esophageal Sphincter

At each end of the esophagus, there lie high-pressure zones. ^{5 ,​ 6} The UES is a 2- to 4-cm high-pressure zone at the junction of the hypopharynx and the esophagus (Fig. 10‑1, Fig. 10‑2).

The most prominent component of the UES is the CP muscle, which constitutes the lower third of the UES. The upper two-thirds of the UES comprise the lower portion of the inferior pharyngeal constrictor. The fibers of the CP muscle originate and insert in the dorsolateral aspect of the cricoid cartilage, which defines the anterior aspect of the UES. Although the innervation of the CP muscle remains a matter of debate, the most prominent contributions are from the vagus and glossopharyngeal nerves as well as the sympathetic nerve fibers. ^{7 ,​ 8 ,​ 9} Innervation of the CP muscle is ipsilateral; therefore, the two halves of the CP muscle function independently. Acetylcholine is the primary neurotransmitter, thereby making the UES an attractive target for BoNT therapy in cases of hypertonicity. The CP is unique among the muscles of the neck, in that it maintains a high intraluminal pressure during respiration and speech, relaxing only during deglutition.

10.2.2 Anatomy and Physiology of the Lower Esophageal Sphincter

The LES, also referred to as the gastroesophageal sphincter, cardiac sphincter, or esophageal sphincter, is located at the junction of the esophagus and the stomach. It has intrinsic (esophageal) and extrinsic (diaphragmatic) components (Fig. 10‑3).

Lower esophageal spasm, or achalasia, is a primary esophageal motility disorder characterized by failure of a hypertensive LES to relax coupled with the absence of esophageal peristalsis. These abnormalities cause a functional obstruction at the gastroesophageal junction, leading to esophageal dysphagia, regurgitation, and weight loss. LES pressure and relaxation are regulated by excitatory (e.g., acetylcholine, substance P) and inhibitory (e.g., nitric oxide, vasoactive intestinal peptide) neurotransmitters. Patients with LES achalasia lack nonadrenergic, noncholinergic, inhibitory ganglion cells, causing an imbalance in excitatory and inhibitory neurotransmission. ^{3 ,​ 4} The result is a hypertensive nonrelaxed esophageal sphincter.

10.3 Dysfunction of the Upper Esophageal Sphincter

Failure of the UES to relax, termed cricopharyngeal achalasia (CA), can cause severe dysphagia. CA is a characteristic symptom in a variety of neurologic disorders including lesions of the skull base and stroke. UES hypertonicity is also associated with distal esophageal reflux ¹⁰ ; so in mildly symptomatic patients, treatment with proton pump inhibitors may be effective. Clinical symptoms of CA include globus sensation and dysphagia. In newborns, an esophagram confirms the diagnosis. In adults, however, the diagnosis is best aided by manometry or electromyography (EMG). However, equivocal or negative findings with these testing modalities, which have significant technical aspects, do not rule out CA.

CA can be managed with a variety of treatments, including mechanical dilatation, BoNT injection, and CP myotomy. The definitive treatment of CA remains CP myotomy, via either an endoscopic or open transcervical approach. ¹¹ Injection of BoNT into the CP muscle, either percutaneously or endoscopically under general anesthesia, has shown benefit in 70 to 100% of patients in noncontrolled series. ¹² The effects of the injection lasted between 4 months and 1 year. Therefore, BoNT injection is not only an attractive nonsurgical alternative treatment for CA but also a useful tool to identify which patients could be expected to experience benefit from CP myotomy. An important exception is patients with fibrosis of the CP, who will not experience benefit from BoNT, but may benefit from dilation or myotomy. Ahsan et al ¹³ reported a series of five patients with CA and severe dysphagia. Prior to treatment, all patients were dependent on feeding tubes. After BoNT injection, all patients had improvement of their swallowing and four out of five patients were able to have their feeding tubes removed. Murry et al ¹⁴ showed in a series of 13 patients that BoNT injection of the CP in the office improved swallow safety and reduced the need for nonoral feeding. In laryngectomized patients, Lightbody et al ¹⁵ showed that BoNT injection into the pharyngoesophageal segment is effective in treating dysphagia, dysphonia, and tracheoesophageal prosthesis valve leak.

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