What leads you to suspect that a foot drop is secondary to a myopathy rather than a neuropathic process?  

Factors suggesting that a foot drop is due to a myopathy include: Clinical factors (slow progression (myopathy but also seen in CMT) versus acute or sub-acute onset (usually neurogenic), absence of sensory findings, absence of pes cavus, signs of facial or shoulder girdle weakness (FSHD can cause...

Distal myopathies are much less common than distal neuropathies but I have seen patients followed for years as CMT when they had an hereditary distal myopathy. The red flag was the lack of any sensory symptoms, signs. However, some of these myopathies can also have sensory signs due to an associated...