Chapter 31: Neurolaryngology

NEUROLARYNGOLOGY: INTRODUCTION

The term neurolaryngology, first introduced in the late twentieth century, encompasses the study of neurologic disorders of the larynx and pharynx. This subdiscipline of laryngology was borne out of collaboration between laryngologists with neurologists who recognized that a wide variety of voice and swallowing disorders previously considered idiopathic or psychogenic were, in fact, a result of neurologic dysfunction. It is no coincidence that this field developed when flexible fiber optic endoscopy became widely available, which for the first time allowed for a dynamic assessment of the laryngopharynx during a variety of tasks, as opposed to that which could be observed by transoral mirror laryngoscopy alone.

NEUROANATOMY AND NEUROPHYSIOLOGY

Viewed phylogenetically, the larynx evolved as a sphincter to protect against aspiration as species migrated from the aquatic to the terrestrial environment. Secondary and tertiary functions of respiratory modulation and phonation developed only later. The precise and intricate neural control required to coordinate these, at times, competing functions makes this organ system particularly vulnerable to neurological derangement.

The laryngeal neural system is composed of afferent neural activity triggered by chemical or mechanical stimuli and efferent activity emanating from brainstem laryngeal motor neurons that have direct connections with the cerebral cortex. Afferent input from laryngeal mucosa is carried to the nucleus tractus solitarius via the nodose ganglion. Laryngeal motor neurons are topographically organized within the nucleus ambiguous with the abductor neurons grouped ventromedially. Subcortical coordination of constant laryngeal motor activity necessary for respiration, deglutition, and phonation occurs within the basal ganglia.

The discussion of laryngeal innervation in otolaryngology is often limited to peripheral afferent and efferent components up to the level of the brainstem. Beyond this schema exists a laryngeal motor cortex (LMC) that is structurally connected to numerous cortical and subcortical brain regions. Unique to humans is a direct monosynaptic projection from LMC to the nucleus ambiguous, which facilitates the production of complex voluntary learned laryngeal movements during speaking and singing. The cerebellum also seems to have a modulatory role in the motor control for speech production while the basal ganglia regulate motor control from these inputs as another layer of control over laryngeal medullary nuclei.

Clinical Assessment