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Expands sides of pharynx (dilates pharynx)

      The Hypopharynx

This is the area of the airway that is mostly unsupported and is subject to narrowing or collapse and that may be associated with the SRBD. The pharyngeal constrictor muscles function mainly during swallowing (Table 3.4).

      There are other muscles that need to be considered that involve mandibular function as well as the cervical spine. These muscles play a role in positioning of the mandible and the head and assist with maintaining the airway in the awake state and during sleep. The muscles that need to be considered are the Masseters, the Temporalis, and the Pterygoids, both lateral and medical. The muscles of the neck, both anterior and posterior, also need to be considered because they control head and neck position, and this in turn may have an impact on the airway. As an example, consider the principles of CPR as it relates to head position.

The two main muscles of the neck that are important, particularly with respiration, are the SCMs and the scalenes [11]. The SCMs and scalenes are considered accessory muscles to respiration. The primary purpose of the SCMs is to rotate and tilt the head as well as to cause flexion of the head. They also act as accessory muscles of respiration by elevating the sternum. The scalenes cause lateral flexion of the neck, and the anterior scalene specifically elevates the first rib and with respiration stabilizes the upper ribs.

      The primary muscles of respiration during inspiration are the diaphragm and the external intercostals. During expiration the abdominal muscles and the internal intercostals are active.

      In Summary

      All of the muscles that have been discussed in some way play a role in maintaining the airway during respiration. During inspiration if the airway becomes compromised, then inspiratory pressures will elevate, and this will result in an increased negative pressure in the airway that will lead to further airway compromise as well as collapse. Many times the muscles that are intended to support the airway as well as tongue position can no longer maintain the airway, having become more relaxed, and further collapse is inevitable.

      Nerve Supply to the Muscles of the Airway

To supplement the understanding of the airway it is also important to understand the nerves that supply the muscles of the upper airway (Table 3.5).

      To best facilitate this, the specific nerve that supplies the muscle is listed. It should be recognized that the muscles that are involved with respiration either are supplied by either cranial nerves or are nerves that originate from the cervical spine. For that reason, it is equally important that the evaluation of patients with the SRBD also has an evaluation of their posture, the cervical spine, and when indicated a cranial nerve screening evaluation.

      An area of interest and concern is that there is also a neurologic basis related to SRBD. Neurologic considerations related to SRBD involve what is termed the upper airway reflex along with various neurologic conditions and neurogenic remodeling of the upper airway associated with repetitive mechanical trauma (snoring and apneas). The upper airway reflex impacts the tone of the airway musculature to limit or prevent collapse [12]. If this reflex does not adequately maintain the airway, then the possibility for airway collapse and for apnea to occur is more likely. This reflex may be both motor and sensory. In addition, neurogenic remodeling or changes associated with apnea may also occur [13]. These changes have been attributed to inflammation that occurs as well as vibrations associated mainly with snoring and hypoxia. These changes have been found to involve the hypoglossal nerve and the genioglossus muscle.

      Previously a condition termed progressive snorers disease also contributed to neurogenic changes in the upper airway [14]. These changes were found to be present based on biopsies of the musculature in the palatopharyngeal area and were attributed to mechanical trauma associated with snoring, hence the recommendation to manage snoring as a preventive action. It has been found that there is an increased amount of inflammation associated with vibration in the airway associated with snoring [15], and this may result in carotid artery atherosclerosis [16].

Table 3.5 Nerve supply to the muscles of the upper airway.

      Source: Adapted from Norton [7].

Nerve	Supplies and innervates
Trigeminal CN V	Tensor veli palatini
	Mylohyoid
	Anterior belly of digastric
	Masseter
	Medial/lateral pterygoids
	Temporalis
Facial CN VII	Stylohyoid
	Posterior belly of digastric
Glossopharyngeal CN IX	Stylopharyngeus
Pharyngeal plexus from vagus CN X	Musculus uvulae
	Palatoglossus
	Palatopharyngeus
Cranial part of accessory CN XI	Palatopharyngeus (possibly)
	Levator veli palatini
	Salpingopharyngeus
	Superior constrictor
	Middle constrictor
Spinal accessory CN XI	Sternocleidomastoid (SCM)
	Trapezius (along with C‐3 and C‐4 branches)
Pharyngeal plexus – external laryngeal of vagus and recurrent	Inferior constrictor laryngeal of vagus
Hypoglossal CN XII	Genioglossus
	Hyoglossus
	Styloglossus
	Superior longitudinal
	Inferior longitudinal
	Transverse
	Vertical
C‐1 follows CN XII	Thyrohyoid
	Geniohyoid
Ansa cervicalis	Omohyoid
Motor component of cervical	Sternohyoid
Plexus (C‐1 to C‐4)	Sternothyroid
Ventral rami
C‐4 to C‐6	Anterior scalenes
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