Juvenile myasthenia gravis

Juvenile myasthenia gravis is a rare disorder acquired in childhood, representing 10% to 15% of all cases of myasthenia gravis. Like the adult form, it is generally characterized by an autoimmune attack on acetylcholine receptors at the neuromuscular junction. Most patients present with ptosis, diplopia, and fatigability. More advanced cases may also have bulbar problems and limb weakness and may progress to paralysis of the respiratory muscles.


DISCUSSION
Autoimmune JMG is an uncommon disorder in the pediatric population, characterized by fatigable weakness due to antibodymediated destruction of the AChR at the neuromuscular junction.Children typically present with ocular symptoms (ptosis, diplopia, ophthalmoplegia) but can also present with generalized weakness or bulbar symptoms (facial weakness, voice change, diffi culty in chewing or swallowing).About 50-69% of JMG patients are seropositive (AChR antibodies), compared to 80% of adult patients.In both age groups, generalized MG has a higher seropositivity rate than pure ocular MG. [4] The differential diagnosis of JMG includes congenital myopathies, congenital myasthenic syndromes, toxins, hypothyroidism, mitochondrial myopathies, multiple sclerosis and brainstem tumors. [5]The diagnosis is based upon clinical signs and symptoms, with laboratory and electrophysiological studies used for confi rmation.Although thymoma in children is rare, the thymus must be imaged (usually by CT) once JMG has been diagnosed.Most mediastinal tumors in the pediatric population are either neurogenic in origin (33%) or lymphomas (41%).Primary thymic lesions (such as thymic cysts, thymolipomas, and thymic hyperplasia) represent only 2.5% of mediastinal tumors, while thymomas comprise about 1%. [6]The Myasthenia Gravis Foundation of America (MGFA) clinical classifi cation divides MG into fi ve main classes and several subclasses, which is designed to identify subgroups of patients who share distinct clinical features or severity of disease that may indicate different prognoses or responses to therapy. [7]

Class I
Any ocular muscle weakness.May have weakness of eye closure.All other muscle strength is normal.

Class II
Mild weakness affecting other than ocular muscles.May also have ocular muscle weakness of any severity.
IIa: Predominantly affecting limb, axial muscles, or both.May also have lesser involvement of oropharyngeal muscles.
May also have lesser or equal involvement of limb, axial muscles, or both.

Class III
Moderate weakness affecting other than ocular muscles.May also have ocular muscle weakness of any severity.
May also have lesser involvement of oropharyngeal muscles.
IIIb: Predominantly affecting oropharyngeal, respiratory muscles, or both.May also have lesser or equal involvement of limb, axial muscles, or both.

Class IV
Severe weakness affecting other than ocular muscles.May also have ocular muscle weakness of any severity.
IVa: Predominantly affecting limb and or axial muscles.May also have lesser involvement of oropharyngeal muscles.
May also have lesser or equal involvement of limb, axial muscles, or both.

Class V
Defi ned by intubation, with or without mechanical ventilation, except when employed during routine postoperative management.The use of feeding tube without intubation places the patient in class IVb.
This patient fi ts into class IIa MGFA classifi cation.
Treatment consists of anticholinesterase drugs like pyridostigmine (30-90 mg every 6 hourly) to be given fi rst with oral corticosteroids  (prednisone 15-20 mg/day).When long-term immunosuppression is necessary, azathioprine is recommended to allow tapering the steroids to the lowest possible dose whilst maintaining azathioprine.Cyclosporine A, mycophenolate mofetil and cyclophosphamide are used for severe cases.Plasma exchange is recommended in severe cases to induce remission and in preparation for surgery.Intravenous immune globulin and plasma exchange are effective for the treatment of MG exacerbations.For patients with non-thymomatous MG, thymectomy is recommended as an option to increase the probability of remission or improvement.It is considered an appropriate procedure for many patients with generalized MG between puberty and 55 years of age.If possible thymectomy should be postponed until puberty because of the importance of the gland in the development of the immune system, but JMG is also quite responsive.The remission rate after thymectomy is approximately 35% provided it is done in the fi rst year or two after the onset of the disease and another 50% will improve to some extent. [8]Once thymoma is diagnosed, thymectomy is indicated irrespective of MG severity.The course of the illness is extremely variable.The long-term outlook for children with myasthenia is better than it is for adults.
International Journal of Medicine and Public Health | Oct-Dec 2014 | Vol 4 | Issue 4

Figure 2 :
Figure 2: Thymus gland scientigraphy: Spect tomographic images in different views reveal physiological tracer concentration in myocardium and moderate increased tracer concentration in retrosternal region