Neurologic Evaluation of the Larynx and Pharynx

Normal Function

Phylogenetically, the most important function of the larynx is protective; the larynx prevents ingested food and drink from entering the lungs. To fulfill this role in humans, the larynx should be open during breathing and tightly closed during swallowing. Phonation is a more advanced evolutionary function. Glottic closure during forced exhalation results in an effective cough to clean the lungs and prevent atelectasis. Laryngeal closure is required to generate positive intrathoracic pressure for defecation, childbirth, and lifting heavy objects and for the stabilization of the thorax. The larynx also seems to play a sophisticated role in controlling airflow and pressure during breathing. The vocal folds open just before respiration and then close gradually during exhalation; thus, they act as a brake on expiratory airflow and thereby influence the rate of breathing. The sensory feedback loops that control these respiratory functions of the larynx are not well understood, but it is clear that feedback from a variety of receptors is involved.

Swallowing is a deceptively simple function. Food prepared in the mouth is ejected into the pharynx and then rapidly propelled downward around the glottis, through the piriform sinus, and into the esophagus. During the swallow, the same pharynx that remains patent during breathing is responsible for constricting tightly in an organized sequence from top to bottom. Regurgitation into the nose or reflux into the mouth is prevented by actions of the soft palate, tonsillar pillars, and base of the tongue. The larynx is pulled upward and forward, away from the flow of the bolus; this action also decompresses the sphincter between the pharynx and esophagus, and flexion of the epiglottis and closure of the glottis prevent the entry of any errant material into the airway. After this complex and coordinated activity, when the bolus reaches the caudal end of the pharynx, the cricopharyngeal muscle should be relaxed to permit its passage into the esophagus; if pharyngeal peristalsis is inadequate, or if the upper esophageal sphincter is insufficiently opened, residual material in the pharynx could penetrate the airway during the next inhalation. The unique anatomic configuration in humans after infancy renders swallowing more difficult. In all other mammals, the feeding and breathing channels are offered a degree of separation by interdigitation of the epiglottis and uvula. In humans, these structures are separated during early development by the descent of the larynx relative to the palate.

Speech is an audible communication that results from phonation, resonance, and articulation. The production of sound, or phonation, requires several conditions: expired airflow and pressure should be sufficient to induce oscillation of the vocal folds, which should be appropriately approximated; if the folds are closed tightly, excessive expiratory force is required and results in a strained, harsh voice or in complete aphonia; if the vocal folds are too far apart, increased expiratory airflow volume is required, such that the voice becomes weaker and breathier or even fades to a whisper. The three-dimensional shape of the vocal fold is also important in imparting favorable aerodynamic features to the glottis. Atrophy of the vocal fold causes concavity in the axial and coronal planes; concavity in the axial plane results in incomplete glottal closure, even during tight approximation of the vocal processes, whereas concavity in the coronal plane results in a convergent airflow tract. Control of length and tension is required to produce normal inflections in pitch and tone. In the absence of such control, the voice may be flat and expressionless, or it may be distorted by uncontrolled pitch breaks. The mucosa of the vocal folds should be supple to permit free vibration. All but the last of these requirements for phonation are susceptible to derangement by neurologic dysfunction.

Resonance is the modification of phonation to produce voice. Pure phonated sound as produced by the vocal folds does not sound like a human voice, rather it is a strident and unpleasant noise. This sound is modified by resonance of the head, neck, and chest so that component frequencies are selectively amplified or dampened. Vocal resonance is largely determined by anatomy, but it is also significantly modulated by motor activity of the pharynx, soft palate, and oral cavity. This principle may be used in speech therapy to achieve stronger voices in patients with impaired function. Certain neurologic disorders such as ALS, stroke, and Guillain-Barré syndrome result in altered vocal resonance. The characteristic sound of the voice in patients with these disorders may be a valuable physical sign to aid in early diagnosis.

Articulation is the shaping of voice into words by actions of the lips, tongue, palate, pharynx, and larynx, and it is highly susceptible to neurologic impairment. Whereas dysarthria in children is caused most often by a hearing deficit, acquired dysarthria in adults usually indicates neurologic impairment. Dysarthria may be a result of impaired motor output—such as weakness, paralysis, or incoordination—but it may also result from cognitive or language defects or from apraxia of speech.

Neurologic Examination of the Mouth, Larynx, and Pharynx

Listening to the patient is the first step of the examination in the patient with a voice complaint. It is often possible to make a working diagnosis within minutes on the basis of the sound of the voice and the patient's description of the problem. The patient should be questioned carefully about vocal fatigue, pain with speaking, increased effort required for speech, glottal tightness, pitch breaks, and tremor.

The following protocol is suggested as a systematic approach to the detection of neurologic dysfunction and has been summarized for quick reference in Table 56.1 . The objectives are to assess the integrity of the lower cranial nerves and to seek signs of central nervous system disorders.