What condition is suggested by a decreased M-wave and a normal SNAP?

The presence of a decreased M-wave alongside a normal sensory nerve action potential (SNAP) strongly suggests the involvement of a myopathic condition. The M-wave reflects the electrical activity of the muscle fibers in response to stimulation of the motor nerves, while the SNAP indicates sensory nerve function.

In myopathy, there is often a reduction in the amplitude of the M-wave due to muscle fiber dysfunction or damage, resulting in the decrease. This condition specifically impacts the muscle tissue, leading to impaired muscle contraction and thus a lower M-wave response. Meanwhile, since myopathy does not directly affect sensory neurons, the SNAP can remain normal, demonstrating that sensory nerve function is intact, which aligns with the findings in this scenario.

In contrast, other conditions would present differently. Peripheral neuropathy generally affects both motor and sensory nerves, leading to deductions in both the M-wave and the SNAP. Cervical radiculopathy can cause changes in M-wave responses due to nerve root involvement but usually presents with corresponding sensory changes as well. Multiple sclerosis may display abnormal findings in both M-waves and SNAPs, as it typically disrupts both motor and sensory conduction pathways.