Neuromuscular Disorders of the Larynx

Chapter 20


Neuromuscular Disorders of the Larynx


David S. Younger



The cardinal functions of the larynx include phonation and breathing, as well as airway protection. Laryngeal paralysis results when some or all of the intrinsic laryngeal muscles fail to contract, causing decreased or absent motion and abnormal positions of the vocal cords. Neuromuscular diseases that lead to weakness of laryngeal muscles and paralysis of the vocal cords notably include disorders of the neuromuscular junction, peripheral nerve, and motor neuron disease. Myasthenia gravis (MG), botulism, Lambert-Eaton myasthenic syndrome (LEMS), and drugs that block neuromuscular transmission can result in laryngeal paralysis. Laryngeal nerve injury can result from nerve compression, trauma, tumor metastasis, local tumor extension, and connective tissue disorders. Bulbar amyotrophic lateral sclerosis (ALS), poliomyelitis and postpolio syndrome, syringomyelia, and brainstem encephalitis can all result in laryngeal dysfunction. Primary muscle disease rarely affects the larynx. The chapter reviews the anatomy, examination, laboratory evaluation, etiopathogenesis, differential diagnosis, and treatment of neuromuscular disorders of the larynx.


Laryngeal Anatomy


Contraction of the lateral cricoarytenoid and interarytenoid muscles adduct and appose the vocal cords. Contraction of the posterior cricoarytenoid muscle abducts and separates the cords. Contraction of the thyroarytenoid, vocalis, and cricothyroid muscles tighten and change vocal cord shape. Motor innervation of the intrinsic laryngeal muscles originates in the nucleus ambiguus of the brainstem and in corresponding visceral efferent fibers of the vagus nerve, the principal branches of which include the superior and inferior laryngeal or recurrent nerve. The course of the trunk of the vagus nerve in the neck and anterior mediastinum makes it particularly susceptible to surgical and traumatic injury, as well as compression by mass lesions. The nerve descends in the sheath common to the internal carotid artery and internal jugular vein in the neck. The left nerve is displaced ventrally onto the anterior surface of the esophagus. The inferior laryngeal or recurrent nerve ascends in the mediastinum on the right, posteriorly under the right subclavian artery, and, on the left, under the aortic arch. The recurrent nerves ascend in the tracheoesophageal sulcus and divide into anterior and posterior rami that supply all laryngeal muscles except for the cricothyroid. The superior laryngeal nerve innervates the cricothyroid muscle by an external ramus; an internal terminal ramus pierces the thyrohyoid membrane and carries sensory fibers from the larynx.


Laryngeal Function


The larynx normally closes during swallowing, thereby preventing epitracheal aspiration of secretions or food. Aberrant exaggerated reflex activity is observed in laryngospasm, which may be precipitated by endotracheal intubation, presence of a foreign body, or manipulation of the larynx during surgery. During normal breathing, the glottic opening widens by active abduction of the vocal cords during inspiration. Cough and clearing of the airway is produced by tight glottic closure, increasing subglottic pressure, sudden glottic release, and a burst of airflow clearing the airway. Speech results from the combined action of the larynx, tongue, and palate. Sound is generated by isotonic tension and vibration of the vocal cords. The shape and positioning of the vocal cords during normal speech result from the combined action of the cricothyroid and thyroarytenoid muscles, which are involved in fine tuning of the voice, thereby affecting pitch and volume.


Patient Evaluation


Clues to the cause of laryngeal weakness can be ascertained by analysis of a careful history that may include recent neck or mediastinal surgery, local trauma, tracheostomy, recent stroke, or transient ischemic attack. The neurologic examination includes assessment of language, cranial nerve function, muscle strength, cerebellar function, sensation, and tendon reflexes. Direct examination of the larynx should determine if the patient has normal sensation and reflex responses. In particular, the clinician should observe the pattern of breathing, the quality and character of phonation, and the active swallowing of liquids and solids.


Examination of the neck is necessary for detection of masses in the lymphatics or thyroid, and assessing the mobility of the laryngeal framework. Sudden interruption of the sensory supply to the larynx may cause difficulty with swallowing and aspiration. This is usually due to a lesion of the internal ramus of the superior laryngeal nerve. The syndrome may result from surgical trauma, such as radical neck dissection, or operations on the supraglottic portion of the larynx. If the cricothyroid muscle is affected or if signs implicate other cranial nerves or medullary pathways, the lesion is proximal to the bifurcation of the superior laryngeal nerve. When paralysis of the larynx is unilateral, symptoms are minimal and compensation is rapid. Absence of reflex response to palpation of the larynx may be an indication of sensory paralysis, but may also be seen in psychogenic disorders.


Two procedures, laryngeal electromyography (EMG) and direct laryngoscopic examination, are essential in the evaluation of laryngeal paralysis. Laryngeal EMG can assist in the differentiation of mechanical fixation from true weakness and paralysis. Direct laryngoscopic evaluation is essential in the identification of normal anatomy, vocal cord weakness, paralysis, and associated mass lesions such as laryngeal cancer.


Differential Diagnosis


Laryngeal or vocal cord paralysis may be classified by either the site of the lesion, whether supranuclear, bulbar, peripheral nerve, or muscle, or by the nature of the disorder, including inflammatory, neoplastic, traumatic, postsurgical, or idiopathic. Although in most instances, the causative lesion lies between the jugular foramen and the port of entry into the larynx, the disorder may also be caused by an intralaryngeal and intramedullary lesion. Lesions of the cerebral cortex and supranuclear corticobulbar pathways, usually evidenced by corticospinal tract signs with bilaterally overactive reflexes, Babinski signs, pseudobulbar palsy, and frontal release signs, can be the cause of spastic vocal cord paralysis, the underlying causes of which include possible cerebral concussion, encephalitis, multiinfarct disease of the hemispheres, and basilar artery insufficiency.


The causative lesions of bilateral vocal cord paralysis are similar to those that cause unilateral lesions. The clinical consequences of bilateral paralysis usually depend on the position of the cords; in general, the prognosis is more serious in cases of sudden onset. Bilateral paralysis after surgical injury may demand urgent relief by tracheostomy, but compression by tumor or aneurysm may cause only exertional dyspnea and stridor. In general, the nearer the cords to the midline, the greater the risk for respiratory insufficiency and more normal the voice. Conversely, the more the cords separate from the midline, the better the laryngeal airway, but the voice is weaker.


Isolated paralysis of the superior laryngeal nerve is less common because it has a short course. Respiration is usually unaffected, but the voice may have a lower pitch and tire easily. Combined paralysis of the superior and recurrent laryngeal nerve may be caused by separate lesions in the respective nerves or an isolated vagal nerve lesion. Combined paralysis can also result from lesions in the upper part of the neck or in the region of the jugular foramen. In bilateral cases, however, the lesion is usually in the medulla and, hence, other cranial nerves are often affected. In unilateral cases, the voice is hoarse, weak, and easily fatigued. Respiration is not affected and the glottic reflex is preserved by the bilateral sensory innervation. In bilateral cases, however, the voice is almost nonexistent, with a monotonous tone and lack of pitch change. Expectoration of secretions is difficult and stridor may be present. Aspiration is common.


Idiopathic vocal cord paralysis is diagnosed by exclusion. Complete evaluation includes indirect laryngoscopy, neurologic examination, and complete serum blood tests for diabetes and thyroid disease. Chest radiograph and chest computed tomography (CT) or magnetic resonance imaging (MRI) should be performed in patients with left-sided paralysis to identify causative lesions before they enlarge enough to be palpable. Skull x-ray, head CT, and MRI evaluate the jugular foramen for mass erosion. Endoscopy should include the nasopharynx as well as the air and food passages.


Central Nervous System Disorders


Sporadic and Familial Amyotrophic Lateral Sclerosis


Motor neuron disease (MND) comprises a group of disorders characterized by progressive lower motor neuron (LMN) signs including weakness, wasting, and fasciculation, often with superimposed upper motor neuron (UMN) signs, including hyperreflexia, the Hoffman and Babinski signs, and clonus, the combination of which makes the diagnosis of ALS inescapable.1 About 10% of ALS cases are inherited as an autosomal dominant (AD) trait in adults, the majority of whom have bulbar oropharyngeal and laryngeal involvement. Onefifth of familial ALS (FALS-AD) cases are associated with mutations in the copper/zinc-dependent superoxide dismutase (SOD1) gene located on chromosome 21 in ALS1. Bilateral vocal cord paralysis was the presenting manifestation of ALS in one patient with ALS1 due to missense mutation resulting.2 Paralysis of the abductor muscles of the vocal cords was reported in non-FALS.3


X-Linked Spinal Bulbar Atrophy


X-linked spinal bulbar atrophy presents with slowly progressive lower motor neuropathy in men, which, unlike ALS, is confined to the LMN, with a time course slower than in ALS, in association with gynecomastia and testicular atrophy. Affected patients can manifest laryngeal involvement. The molecular defect is an expansion of a CAG repeat in the first exon of the androgen receptor gene, which expands a polyglutamine tract within the receptor. As the length of the tract of CAG increases, the illness becomes more severe; however, it is not clear how the molecular lesion causes MND. One affected patient developed bilateral abductor vocal cord paralysis.4 Cranial MRI of the brainstem was normal. Direct laryngoscopy showed a complete paralysis of both vocal cords in the paramedian position. Respiratory distress improved after tracheostomy. Most experts agree that it is clinically important to perform a careful assessment of vocal cord function in patients with X-linked spinal bulbar atrophy, as in other similar patients because when found, vocal cord paralysis can shorten life expectancy.


Spinal Muscular Atrophy


Childhood spinal muscular atrophy (SMA) presents with significant bulbar involvement and vocal cord paralysis.58 Children with SMA type 1 or Werdnig-Hoffmann disease, present at birth or in the first few months of life with hypotonia, and LMN signs in the limbs and oropharynx, leading to death by 2 years of age from recurrent aspiration due to laryngeal paralysis and respiratory failure. Those with type 2 SMA achieve normal milestones up to approximately 8 months of age, including sitting, without support in spite of hypotonia, but they fail to walk normally and can display variable oropharyngeal involvement. Many affected children can survive into the third or fourth decade. Individuals with SMA type 3 or Wohlfart-Kugelberg-Welander syndrome, have an onset any time after 18 months, typically in late childhood and adolescence as a proximal neurogenic muscular atrophy that may be confused with limb girdle muscular dystrophy. There may be an elevated serum creatine kinase (CK). Such patients present with a waddling gait, lumbar lordosis, genu recurvatum, and protuberant abdomen, or may appear thin like a stick man. Childhood SMA manifests autosomal recessive (AR) inheritance and linkage to chromosome 5q11.2–13.3, in two genes, one for neuronal apoptosis inhibitory protein (NAIP) accounting for up to 67% of cases, and another, in the survival motor neuron (SMN) gene, which was found to contain greater than 98% of deletions. The protein product of SMN is known to interact with RNA binding proteins and may actually be a spliceosome. Distal hereditary motor neuronopathy type VII is an AD disorder characterized by distal muscular atrophy and vocal cord paralysis, linkage of which was established to chromosome 2q14 in a large Welsh pedigree.9 The management of the oropharyngeal, laryngeal, and respiratory symptoms in the various syndromes of MND includes airway protection and voice, which may be improved with local therapy of the vocal cords, cricopharyngeal muscle myotomy, tracheostomy, feeding gastrostomy, parasympatholytic drugs, and if necessary, laryngeal diversion, the primary disadvantage of which is the complete loss of phonation.10


Poliomyelitis


Acute poliomyelitis, now a rare cause of acute lower motor neuron paralysis in industrialized countries, is due to poliovirus infection of spinal anterior horn cells and other motor neurons. A minor illness occurs 1 to 3 days before onset of paralysis, with gastrointestinal complaints of nausea and vomiting, abdominal cramps, pain, diarrhea, and the systemic manifestations of sore throat, fever, malaise, and headache. The major illness, which includes all forms of central nervous system (CNS) disease caused by the poliovirus, including aseptic meningitis or nonparalytic polio, polioencephalitis, bulbar polio, and paralysis, follows the minor illness by 3 to 4 days. Such patients can present with stiff neck, back pain, photophobia, headache, tremulousness, obtundation, agitation, myalgia, cramps, fasciculation, and radicular pain. Bulbar poliomyelitis, which occurs in 10 to 15% of paralytic patients, involves cranial nerves VII, IX, and X and the medullary reticular formation, resulting in facial weakness, difficulty swallowing, and phonation, as well as variable respiratory difficulty, ataxic breathing, lethargy, obtundation, hypotension, hypertension, and arrhythmias. One reported patient with spinobulbar poliomyelitis had residual dysfunction of cranial nerves IX and X, producing bilateral vocal cord paralysis and recurrent aspiration.11 Critical glottic stenosis developed 28 years after the initial episode of poliomyelitis, which appeared to be related to fibrosis of the intrinsic laryngeal muscles and ankylosis of the right cricoarytenoid joint. Accordingly, significant upper airway obstruction may develop as a late complication in those with stable neurologic deficits and chronic immobility of the vocal cords.


Postpolio Syndrome


Of the approximately 250,000 survivors of the polio epidemics, up to a quarter experience progressive muscle weakness known as postpolio syndrome. Myopathic symptoms are associated with, in order of decreasing occurrence, fatigue, arthralgia, myalgia, muscle atrophy, cold intolerance, respiratory insufficiency, and dysphagia. Nine patients with postpolio syndrome were evaluated for swallowing complaints with comprehensive history, physical examination, acoustic voice analysis, and laryngeal videostroboscopic endoscopy, including three with laryngeal EMG12 that revealed some degree of phonatory and laryngeal deficit, and in those with prominent dysphagia, vocal cord paralysis. Bilateral vocal cord paralysis was noted in one affected patient with bulbar and spinal involvement beginning at age 13, resulting in left facial, vocal cord, and hemidiaphragm paralysis that was treated by mechanical ventilation.13 Thirty-five years later, she noted inspiratory and expiratory stridor, leading to respiratory failure and tracheostomy. Laryngoscopic examination revealed bilateral vocal cord paralysis with near-midline fixation of the cords and pooling of secretions. The syndrome was attributed to progressive bulbar motor neuron weakness. Three patients with prior polio infection who presented with new complaints, including slowly progressive dyspnea, dysphagia, and hoarseness, were evaluated by videostroboscopy and EMG that showed vocal cord abductor and adductor weakness, and recurrent posterior glottic web in one patient, and laryngeal muscle denervation and reinnervation in two others.14 Treatment was directed at attempting to maintain the airway and optimize vocal quality. One patient benefited from tacheostomy, one from vocal cord medialization, and one from resection of interarytenoid scarring. Three additional patients with laryngeal changes in postpolio syndrome presented with dysphonia, vocal weakness, and fatigue. All three manifested abnormalities on videostroboscopic and laryngeal EMG.


Syringomyelia


Laryngeal weakness with vocal cord paralysis occurs in syringomyelia.1517 In one case there was progressive loss of the pharyngeal reflexes with pooling of secretions in the upper esophagus and lower pharynx, requiring endotracheal intubation and feeding jejunostomy.18 At cervical laminectomy, the cavity was drained and repaired with improvement of pharyngeal reflexes, but there was aspiration 2 days after cervical laminectomy. Presumably, the syrinx had extended into the brainstem to affect neurons of the nucleus ambiguus that normally control pharyngeal, laryngeal, and esophageal muscles. One 35-year-old patient who complained of noisy breathing for 15 years and had been treated for chronic asthma, developed acute breathing difficulty and was found to have stridor and bilateral abductor vocal fold palsy.19 A syrinx was found with associated type I Chiari malformation. Three other patients with communicating syringomyelia had acute presentations, including one with paraplegia, a second with acute respiratory distress secondary to bilateral vocal cord paralysis, and a third with symptoms of brainstem ischemia.20 Each had a communicating spinal cord syrinx associated with a posterior fossa and foramen magnum region anomaly, including a large posterior fossa arachnoid cyst in one, and Chiari malformations in the other two. Syringobulbia was found to be the cause of laryngeal stridor in an 11-year-old patient who had chronic symptoms since birth, and suddenly died after development of respiratory obstruction due to vocal cord paralysis.21 Postmortem examination showed extensive bilateral syringobulbia with the greatest involvement in the nucleus ambiguus. Other brainstem tracts and nuclei were partially affected.


Arnold-Chiari Malformations


The classification of Arnold-Chiari malformations, now termed Chiari complex,22 is based on the relative position of the cerebellum and brainstem in relation to the foramen magnum and the upper cervical canal.23 Types I and II are degrees of a similar abnormality in which a conical deformity of the posterior midline cerebellum and the elongated brainstem that lies at or below the foramen magnum. Type I is divided into a classic and myelencephalic form; symptomatic patients are treated with occipitocervical surgical decompression. One MRI classification designated Chiari malformation in association with syringomyelia as type A, and those with evidence of frank herniation of the cerebellar tonsils below the foramen magnum alone as type B.24 Those with Chiari type II present with progressive hydrocephalus and typically require insertion of a cerebrospinal fluid (CSF) shunt. Chiari type III is associated with an occipitocervical cephalocele and severe CNS malformations. Chiari type IV designates patients with myelomeningocele and severe cerebellar hypoplasia, the most common of which is probably the Dandy-Walker malformation, in which a defect in the inferior vermis is congruent with a ventricuocele of the enlarged fourth ventricle. Adults and infants alike can present symptomatic laryngeal symptoms.


The Arnold-Chiari malformation, in association with myelomeningocele, was considered the cause of progressive choking, apnea, and aspiration of 42 infants.25 Shunts were already in place and were thought to be functioning normally; however, at posterior fossa craniotomy, all had compression of the upper spinal cord. Mortality was highest (71%) in infants with rapidly progressive, irreversible symptoms. The Arnold-Chiari malformation and shunted myelomeningocele presented with adductor vocal cord paralysis and loss of laryngeal sensation for 10 months.26 In that infant, consideration was not given to posterior fossa exploration; treatment was supportive, including gastrostomy and close observation for several years. Bilateral abductor vocal cord paralysis due to Arnold-Chiari formation should be suspected in neonates and infants who present with high-pitched inspiratory stridor and airway compromise.27 Unilateral vocal cord paralysis suspected in an infant or child with hoarse voice, low-pitched cry, and breathy cry or voice should be evaluated with direct laryngoscopy with flexible fiberoptic nasopharyngolaryngoscope and photodocumentation using a videocassette recorders.


Chiari malformation was suspected in a previously healthy 13-year-old boy without myelodysplasia who had mild scoliosis and complaints of nasal congestion, noisy nighttime breathing, and difficulty sleeping.28 Flattening of the inspiratory loop on the flow-volume curve was found on pulmonary function testing, suggesting a variable extrathoracic obstruction due to a laryngeal lesion. Bilateral abductor vocal cord paralysis and sleep apnea developed precipitously following general anesthesia. Chiari malformations were discerned among six children by T1- and T2-weighted MRI with syndromes of failure to thrive, velopharyngeal incompetence, gastroesophageal reflux, or vagal hypertonia, leading to laryngeal obstruction due to vocal cord paralysis, paradoxical vocal cord motion, or laryngomalacia,29 and treated symptomatically before decompressive surgery, which led to full functional recovery in five children. Adults likewise present with symptomatic vocal cord paralysis due to type I Chiari malformations.30 Similarly, stridor and bilateral abductor vocal fold palsy was noted as well in a 35-year-old with complaints of noisy breathing for 15 years previously treated for chronic asthma19; subsequent evaluation revealed syringomyelia with a Chiari type I malformation.


Linder and Lindholm31 evaluated children born with a Chiari II malformation during their first 18 months; four of the 22 children studied by flexible fiberscopes had disturbed breathing, and among those, two suffered from central apneic attacks as well as bilateral vocal fold motion impairment, one from apneic spells only, and one from bilateral vocal fold motion impairment. All four had dysphagia with aspiration and respiratory symptoms within the first 3 to 6 months of life. One infant with severe symptoms expired at age 3 months. The vocal fold paralysis, apneic spells, and swallowing difficulties of another infant resolved following neurosurgical intervention. Choi and coworkers32 studied airway abnormalities in 16 patients with type I and eight patients with type II Chiari malformation. Vocal cord impairment was noted in three patients with type II Chiari, including one each with unilateral paralysis, bilateral paresis, and paralysis, but in none of those with type I disease. Tracheostomy was necessary in three, all Chiari type II; Central sleep apnea was noted in five of six patients, and tracheostomy was needed in three patients, all with type II malformations, but in none of those with type I disease.


Stroke


Laryngeal dysfunction occurs with infarcts of the brain, especially of the medulla. Venketasubramanian and colleagues33

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Jun 4, 2016 | Posted by in NEUROLOGY | Comments Off on Neuromuscular Disorders of the Larynx
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