Sorry to hijack thread but can anyone with more EMG experience explain what can cause an isolated low proximal median CMAP but with normal sensory? Assuming no evidence on needle EMG for radic
Essentially a normal median sensory response and low median motor amplitude? I would definitely need more information like presenting symptoms, time course of symptoms, etc.
Just with the information provided, there are a host of things:
-Median neuropathy with a conduction block - SNAP normal. Distal median CMAP normal; low proximal median CMAP (typically >50% drop in amplitude). Needle EMG would show reduced recruitment, but the MUP should be normal.
-Marinacci anastomosis (reverse Martin-Gruber) - SNAP normal. Distal median CMAP would be normal; low proximal median CMAP. This is rare, but occurs from an ulnar-to-median nerve anastomosis, so the distal CMAP would be much higher than proximal CMAP.
-Median neuropathy with isolated motor fascicular involvement - SNAP normal. CMAP low amplitude. If entrapped, typically has demyelinating features (such as prolonged distal latency or slowed conduction velocity).
-C8-T1 radiculopathy - SNAP would be normal since radiculopathies are preganglionic injuries. If the CMAP amplitude is low, you should see abnormalities with needle EMG
-Motor neuron disease - SNAP are normal as it is a disorder of only the lower motor neurons and their axons. CMAP amplitude is low. You should see abnormalities (fibrillation potentials, fasciculations, reduced recruitment, large MUP) on needle EMG.
-Myopathies - SNAP are normal; CMAP amplitude may be low. Many myopathies are proximal; however, there are distal myopathies that recording from the APB muscle on the hand can show low amplitude CMAP. Needle EMG should show early recruitment of small MUP (+/- fibrillation potentials if inflammatory)
-Neuromuscular junction disorders (particularly Lambert-Eaton myasthenic syndrome) - SNAP are normal. CMAP amplitude may be low. Repetitive nerve stimulation after 10-second exercise should show increase in CMAP amplitude by like 200%. Needle EMG shows variable unstable MUP.
-AMAN (acute motor axonal neuropathy, a variant of Guillain-Barre Syndrome) - SNAP are normal. CMAP amplitudes are low. Acute-onset. Needle EMG should show abnormalities.