Autoimmune Myasthenia Gravis
This disorder is similar to the autoimmune myasthenia gravis observed in adults. The onset is often insidious, but at times, patients may present with acute respiratory difficulties.
Patients usually present with variable degrees of ophthalmoparesis and ptosis. In addition, patients may exhibit facial weakness, swallowing difficulties, speech problems, and weakness of the neck, trunk, and limbs. Proximal muscles are more affected than distal, and the upper limits are more affected than the lower.Fluctuation in the disease course with relapse and remission is common. Patients often complain of fatigue and diplopia, as well as progressive difficulty with chewing or swallowing. Patients are often worse with fatigue towards the end of the day. Thymoma, which occurs in about 10% of adult cases, is not a feature of the childhood-onset disease.
Serum AChR antibodies are an important diagnostic screening tool. Anti-AChR antibodies can be detected in the serum in about 85% to 90% of patients with generalized myasthenia gravis and greater than 50% of those with ocular myasthenia. The most common antibodies detected are AChR binding, followed by AChR modulating and then striational AChR antibodies. Muscle-specific kinase (MUSK) antibodies are an additional marker present in some seronegative patients and many patients with ocular myasthenia.
Diagnosis may also be confirmed by clinical response to an anticholinesterase drug such as edrophonium (Tensilon) Alternatively, neostigmine, a longer-acting agent, can be used. Repetitive nerve stimulation studies show a characteristic decrement in the compound muscle action potential with slow stimulation rates (2-5 Hz) over a train of four to five stimuli. A decrement greater than 12% to 15% is often noted. Electrophysiologic studies may be more sensitive with proximal muscle groups such as the accessory nerve to the trapezius or study of the facial nerve. Abnormal repetitive nerve stimulation studies may also be seen in Lambert-Eaton syndrome, botulism, and congenital myasthenic syndromes. Singlefiber EMG is usually impractical in children; however, stimulated single-fiber EMG may be performed under anesthesia. Management may include treatment with anticholinesterase drugs, such as pyridostigmine, corticosteroids (prednisone), intravenous (IV) immunoglobulin, immunosuppressants (azathioprine, cyclosporine, mycophenolate mofetil, or cyclophosphamide), plasma exchange, or thymectomy.