347 - Absent late EMG responses due to anesthesia or CIDP?

Case Diagnosis: Probable anesthesia effect on late nerve conduction responses.

Case Description: A 16-year-old non-verbal male with PMH of autism spectrum disorder, cognitive impairment, and nutritional deficiencies presented with six-month history of bilateral lower extremity (BLE) distal weakness. Chronic Inflammatory Demyelinating Polyneuropathy (CIDP) or other treatable neuropathies were in the differential diagnoses.

Nerve conduction studies/electromyography (NCS/EMG) of the left upper extremity (LUE) and BLE were completed under general anesthesia due to poor tolerance and normal MRI. Sedation included fentanyl, versed, sevoflurane, and propofol. Left ulnar and median NCS/EMG were normal, however ulnar and median F-waves were absent. BLE distal compound motor action potentials and bilateral sural sensory nerve action potentials were absent. More proximal fibular and tibial motor responses were present; tibial H-reflexes were absent. Tibial F-waves to the soleus/gastrocnemius on the left were absent and on the right had reduced penetrance and normal latency. LUE and bilateral rectus femoris needle EMG were normal. Fibrillations were noted in bilateral distal leg muscles.

Discussions: Although altered late responses in all limbs tested suggests a proximal demyelinating process such as CIDP, this was inconsistent with physical exam, NCS, and needle exam. This led to a diagnostic dilemma. We hypothesized that the anesthetic necessary for study completion confounded late response waveforms. Published research investigating the effect of anesthesia on EMG late responses is surprisingly sparse. There is at least one report of propofol altering F-wave appearance and penetrance. Central and peripheral muscle relaxants may also affect F-wave reliability.

Conclusions: In cases with sedation, anesthesia may confound NCS/EMG findings, including absent or poor penetrance of late responses (F-waves and H-reflexes). In this case study, propofol, or a combination of agents may have caused these changes. Further research investigating specific anesthetics and their effect on EMG data is needed. Absent late responses should be interpreted with caution in anesthetized patients.