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INTRODUCTION

Pediatric disorders of the neuromuscular junction (NMJ), a group of disorders that affect the structure, function, or both the structure and function of the NMJ, can be acquired or congenital. Patients with NMJ disorders typically present with weakness that fluctuates throughout the day and is fatigable in nature. Classically, fluctuating weakness affects extraocular musculature early in the disease course, causing symptoms such as fluctuating ptosis or diplopia. Bulbar weakness is also commonly described in NMJ disorders, which can present as difficulty swallowing, choking, gagging, weak crying, nasal-sounding voice, or neck flexion or extension weakness.

This chapter discusses juvenile myasthenia gravis, other acquired NMJ disorders, and congenital myasthenic syndromes.

JUVENILE MYASTHENIA GRAVIS AND OTHER ACQUIRED NEUROMUSCULAR JUNCTION DISORDERS

ESSENTIALS OF DIAGNOSIS AND TYPICAL FEATURES

  • Juvenile myasthenia gravis is an autoimmune disease in which neuromuscular transmission is affected by antibodies that bind to the acetylcholine receptor or to related proteins on the NMJ’s postsynaptic membrane.

  • Clinically, juvenile myasthenia gravis is characterized by either isolated extraocular muscle weakness presenting with ptosis or diplopia or more generalized bulbar and limb weakness that is classically fluctuating and fatigable in nature.

  • In the setting of relevant signs and symptoms, serum autoantibody testing can be diagnostic. In seronegative patients with a clinical picture consistent with myasthenia gravis, electrodiagnostic studies including repetitive stimulation can help make the diagnosis.

  • Other less common acquired disorders of the NMJ include neonatal myasthenia gravis, botulism, and pediatric Lambert-Eaton syndrome.

Juvenile myasthenia gravis (JMG), in which antibodies to components of the postsynaptic NMJ cause impaired synaptic transmission in patients under the age of 18, is a rare condition that more commonly occurs in children over the age of 10 years. The clinician must recognize JMG as a potential cause of fatigable weakness and institute proper antibody testing and immunosuppressive treatment as warranted by the clinical severity of the case. In the case of JMG, a high suspicion for progression to myasthenic crisis must be maintained.

Clinical Findings

Symptoms and Signs

Clinically, patients with JMG will classically present with fluctuating, fatigable weakness. It is more commonly seen in adolescents than in young children, although it can occur at any age. This weakness can either exclusively affect the extraocular muscles (pure ocular myasthenia gravis) or have more generalized skeletal muscle involvement. All symptoms should be least noticeable after periods of rest and worsen with prolonged activity. Patients most commonly present first with ocular symptoms of asymmetric ptosis and binocular diplopia. Patients with bulbar involvement may report nasal speech that worsens with prolonged speaking. They may also report difficulty drinking through a straw, escape of liquids through the nose when drinking, difficulty chewing, or the sensation of food getting stuck when swallowing, all of which may cause meals to take a long time to eat. In the case of generalized ...

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