Intermittent weakness

Exam usually normal between attacks
Proximal > distal weakness during attacks

Muscle biopsy defines specific defect

Repetitive nerve stimulation decrement

Hypokalemic PP (myotonia confined to eyelids)

Hyperkalemic PP Paramyotonia congenital

DNA test confirms diagnosis

End

Myoglobinuria

Yes

No

AChR AB positive
Acquired MG

AChR AB negative
Congenital MG
Acquired MG

Forearm exercise

Reduced lactic acid rise: Consider glycolytic defect

Normal lactic acid rise: Consider CPT deficiency or other fatty acid metabolism disorders

Variable weakness includes EOMs, ptosis, bulbar and limb muscles