Working Diagnosis:
Myasthenia gravis

Treatment:
Neurology started the patient on pyridostigmine 1 mg/kg/day, an acetylcholinesterase inhibitor.

Outcome:
The patient demonstrated significant improvement in his strength after starting pyridostigmine. However, he continued to have easy fatigability and could not exert himself for more than 5 minutes without become fatigued, which kept him from returning to sports. His pyridostigmine dose was increased, but he still was not able to return to competitive play. Prednisone was then added to his medication regimen and he was referred to surgery by neurology for a possible thymectomy.

Author's Comments:
Myasthenia gravis is an autoimmune disorder of the neuromuscular junction characterized by muscle weakness and fatigability. The majority of patient’s develop autoantibodies against the acetylcholine receptor (AChR) and have thymic hyperplasia or a thymoma. Consideration for myasthenia gravis should arise in those with fluctuating weakness and fatigability worsened with exertion and repetitive motions. A unique characteristic of this case is that the patient had lower extremity symptoms, whereas upper extremity involvement is much more common.

Editor's Comments:
Myasthenia gravis encompasses autoimmune diseases of the postsynaptic motor endplates caused by autoantibody mediated damage to the acetylcholine receptors at the neuromuscular junction resulting in skeletal muscle weakness and fatigue. There are neonatal, juvenile and adult-onset variants of the disease. Juvenile myasthenia gravis (JMG) commonly presents with oculomotor muscle involvement with or without generalized involvement. Pubertal status tends to affect the clinical presentation. Incidence between males and females is approximately equal during prepubertal years, during puberty and after 40 years-old whereas women are affected nearly 3 times more often than men during early adulthood. Patients with prepubertal symptom onset seem to present with exclusively ocular symptoms whereas patients with postpubertal onset tend to have more generalized symptoms. Our patient was 13 years-old and no growth chart or tanner staging was available to determine where he fell in regards to his pubertal status, but his symptoms seem more consistent with the postpubertal form of the disease.

Upon workup, our patient was positive for anti-AChR antibodies. The rate of prepubertal JMG patients that test positive for these antibodies is generally lower than in the postpubertal and adult population with only about 50% of prepubertal patients testing positive compared to approximately 80% of generalized adult patients.

While the juvenile and adult onset forms share many pathophysiological and symptomatological aspects, there are slight differences between these two forms that need to be considered when determining treatment. Most notably, spontaneous remission rates are higher in children compared to adults, especially in children who undergo thymectomy close to presentation. The mainstay of treatment includes acetylcholinesterase inhibitors (anti-AChEs), medical immunomodulation and/or surgical immunomodulation in the form of a thymectomy. Anti-AChEs, especially pyridostigmine, are the first line therapy, and regimens are often adjusted depending on response. Our patient was started on this medication with the addition of steroids to try and further minimize symptoms. The most common thymic abnormality found in patients with JMG is hyperplastic thymus compared with thymomas in adult patients. The patient was also referred to surgery for possible thymectomy, however it is unknown if the patient underwent surgical intervention.

References:
Castro, Diana, Derisavifard, Samir, Anderson, Mariam, et al. Juvenile Myasthenia Gravis: A Twenty-Year Experience. Journal of Clinical Neuromuscular Disease. 2013;14(3):95-102. doi:10.1097/CND.0b013e318253a48e.
Chiang, Laura M, Basil T Darras, and Peter B Kang. “Juvenile Myasthenia Gravis.” Muscle & nerve 39.4 (2009): 423–431. Web.
Jayam Trouth A, Dabi A, Solieman N, Kurukumbi M, Kalyanam J. Masythenia gravis: a review. Autoimmune Dis. 2012; 2012:874680. doi:10.1155/2012/874680

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