OMG and AchR-Ab

Q: Patients with Ocular Myasthenia Gravis (OMG) are more likely to be seronegative for acetylcholine receptor antibodies (AChR-Ab) than patients with Generalized Myasthenia Gravis (GMG).

A) True

B) False

Answer: A

Reasons are not fully understood why ocular muscles are distinctly involved in MG? And why do about 15 percent of patients only develop OMG and never progress to GMG? 

There are four proposed mechanisms for this:

1. The most likely reason is the fact that levator palpebrae superioris is under constant activation (eye-opening) and is susceptible to fatigue. 

2. The junctional folds of muscle endplates are sparse in the extraocular and levator muscles. This leads to a lower safety factor for neuromuscular transmission.

3. The complement regulatory genes are expressed differently in extraocular muscles. This reduces the protective mechanisms to complement-mediated tissue injury.

4. Patients with OMG are more likely to be seronegative for acetylcholine receptor antibodies (AChR-Ab) than patients with GMG. This may be the reason for the development of only OMG, but not GMG.

#neurology

References:

1. Kaminski HJ, Maas E, Spiegel P, Ruff RL. Why are eye muscles frequently involved in myasthenia gravis? Neurology 1990; 40:1663.

2. Kaminski HJ, Li Z, Richmonds C, et al. Susceptibility of ocular tissues to autoimmune diseases. Ann N Y Acad Sci 2003; 998:362.

3. Sommer N, Melms A, Weller M, Dichgans J. Ocular myasthenia gravis. A critical review of clinical and pathophysiological aspects. Doc Ophthalmol 1993; 84:309.