Very good and very rare question and occurrence.
Unilateral means of one side only.
Pectoral muscle- the one which makes the anterior part of the chest and gives maximum movements of the arm over shoulder joint in association with other muscles.
Atrophy means reduction in the volume of the muscle mass.
The causes can be:
Injury or affection of the nerves supplying this particular muscle.
Fixation of the shoulder joint due to any of the reasons can cause of disuse atrophy.
To get a confirmation, to find the cause and to take a treatment you should consult an Orthopedic Surgeon and carry on.
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