We have described an isolated plexiform neurofibroma that developed in a man with myasthenia gravis but no stigmata of von Recklinghausen's disease. Plexiform neurofibromas consisting of a heterogeneous mix of various cell types, including Schwann cells and fibroblasts6,7
, are reported only in patients with von Recklinghausen's disease, and are considered to be pathognomic for NF11,2
. Enzinger and Weiss8
, while acknowledging the extremely high association with von Recklinghausen's disease, were somewhat more dubious about this relationship when the neurofibromas involved small nerves in the subcutis.
Moreover, several cases of isolated plexiform neurofibroma have recently been observed in otherwise healthy patients with no family history of NF or other anomalies, suggesting that the presence of a plexiform neurofibroma does not always indicate associated NF or structural anomalies9-11
. In our case, the tumor presented on the right 4th finger tip as a small rubbery nodule that was located superficially on the dermis. It involved a small superficial nerve, and was similar to recently described cases in which an isolated plexiform neurofibroma occurred as a superficial form involving small nerves in the dermis10,11
. The patient had no personal or family history of other abnormalities suggestive of NF, but had been treated for myasthenia gravis since 1989.
Myasthenia gravis is a relatively rare autoimmune disorder of peripheral nerves in which antibodies form against acetylcholine nicotinic postsynaptic receptors at the myoneural junction12
. It is characterized by weakness and rapid fatigue of the muscles under voluntary control. It may cause double vision, drooping eyelids, difficulties with speech, chewing, swallowing and breathing, as well as limb weakness. Myasthenia gravis is associated with other autoimmune diseases, such as thyroiditis, rheumatoid arthritis, systemic lupus erythematosus and thymoma3,13
. It is also associated with skin diseases such as pemphigus, alopecia areata, vitiligo and psoriasis3-5,14
There has been one other reported case of neurofibroma associated with myasthenia gravis. Waugh15
reported the development of myasthenia gravis in a case of multiple NF. However, no neurofibroma developing in a case of myasthenia gravis has been reported. The relationship between the diseases may be coincidental, but is worthy of further investigation due to the fact both developed from peripheral nerves.
In myasthenia gravis, autoantibodies block the postsynaptic receptor at the myoneural junction, preventing signal transduction which results in inflammation around the junction. This inflammation tends to be worse in areas of the body subject to frequent trauma. This inflammatory process enhances cell proliferation and, ultimately, the proliferation of nerve cells occurs. Based on this hypothesis, a clear mechanism for the development of the neurofibroma on our patient's fingertip is evident.
This case seems to confirm the premise that the superficial form of plexiform neurofibroma, involving small nerves in the dermis or subcutis, is not necessarily pathognomic for von Recklinghausen's disease, as previously reported10,11
. Furthermore, we have reported this case because of the rare occurrence of plexiform neurofibroma in myasthenia gravis.