Predilection For Eye Muscles
The precise reasons for the preferential involvement of eye muscles in MG is not well understood, but there are several lines of thought.
Functional hypotheses propose that although multiple muscles may be affected, a deficit may be more readily apparent in the eyes for several reasons. Slight weakness in a limb may be tolerated, but slight weakness in the extraocular muscles would lead to misalignment of the two eyes, even a small degree of which could lead to diplopia. Eyes may also be less able to adapt to variable weakness, because extraocular muscles use visual rather than proprioceptive (body position-sensing) cues for fine-tuning.
Immunologic hypotheses proposes that there may be differences in the antibodies in ocular MG versus generalized MG that may favor the muscles responsible for eye movement and eyelid elevation.
Physiologic hypotheses propose that it is the unique structure and function of extraocular muscles that predispose them to weakness in MG. Compared to extremity muscles, extraocular muscles are smaller, served by more nerve fibers, and are among the fastest contracting muscles in the body. This higher level of activity may predispose them to fatigue in MG. Additionally, some reports indicate that there may be fewer acetylcholine receptors in extraocular muscles versus limb muscles.
Read more about this topic: Ocular Myasthenia
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