Hi everybody, my name is Dr. Mike Skolka and I'm here to talk to you today about the electrodiagnostic features of demyelinating neuropathies. Demyelinating neuropathies are neuropathies that are pathologically characterized by primarily or predominant loss of myelin with relative preservation of axons. In addition to pathology, nerve conduction studies and EMG are invaluable in establishing the diagnosis. These can primarily be divided into two main groups. The first would be acquired demyelinating neuropathies with evidence of focal or segmental demyelination, so a disease such as chronic inflammatory demyelinating polyradiculoneuropathy or CIDP would fit under this category. Or secondary would be hereditary demyelinating neuropathies with evidence of uniform or diffuse demyelination, so diseases like Charcot-Marie-Tooth or leukodystrophies would fit here. So we'll talk about the electrodiagnostic features that can be seen in both hereditary and acquired demyelinating neuropathies. So first is conduction velocity slowing. So we say slowing less than 70% of the lower limit of normal would be consistent with a demyelinating neuropathy. However, we have to remember to take into consideration the nerve response amplitude when evaluating conduction velocity slowing. So as loss of large, fast-conducting axons will produce a mild degree of slowing, as can occur in axonal neuropathies, if there is evidence of axonal loss, and we say a low amplitude response less than 50% of the lower limit of normal, then the degree of conduction velocity slowing to indicate primary demyelination should be less than 50% of the lower limit of normal. The second is prolonged distal latency, so latencies greater than 125 to 150% of the upper limit of normal would be consistent with demyelination. So let's look at some examples of this. So here's an example of a right ulnar motor nerve conduction study recording over the ADM muscle from a patient who was diagnosed with CMT type 1a. And here you can see that the distal latency is prolonged at 7.8, and the conduction velocity is very slow at 15. Now there is a mild reduction in amplitude at 4.2, however, this reduction is greater than 50% of the lower limit of normal, and also if it was less than 50% of the lower limit of normal, this conduction velocity is very slow and would still be consistent with a demyelinating neuropathy. So third, you can see prolonged F-wave latencies, so prolongation of F-wave latency greater than 20 to 50% of the upper limit of normal, or absence of F-waves that are typically present may also support proximal demyelination. Fourth would be prolonged blink reflex latencies, so a prolongation of R1 and or R2 responses can be seen in demyelinating neuropathies. And blink reflexes can be very helpful in severe demyelinating neuropathies where responses are completely absent in all other routine distal nerve conduction studies, so that's something to keep in mind. So here's an example of a prolonged fibular F-wave in a patient diagnosed with Guillain-Barre And so here you can see the F-wave, the shortest F-wave latency is 70.9 milliseconds, the F-estimate is approximately 59.3, and so this would be prolonged. Here's an example of a prolonged R1 response in a patient with CMT1A, and so you can see the R1 latency is 17.6 milliseconds, and in our lab, normal would be less than 13, so this would be consistent with prolonged R1 response. Now there are some electrodiagnostic features that typically indicate an acquired demyelinating neuropathy. So the first would be conduction block, which is a failure of nerve impulse propagation down an axon that can occur due to the focal or segmental demyelination that can occur in acquired demyelinating neuropathies. Now there are different criteria out there for conduction blocks, but I'll quote to you the Peripheral Nerve Society criteria, which is greater than or equal to 30% of the reduction of the proximal relative to distal negative peak CMAP amplitude, excluding the tibial nerve, and distal negative CMAP amplitude greater than or equal to 20% of the lower limit of normal. The second would be abnormal temporal dispersion, so this is a greater than 30% duration increase between proximal and distal negative CMAPs, and we say at least 100% in the tibial nerve. So we'll take a look at what this looks like. So this is an example of a conduction block in the fibular motor nerve recording over EDB in a patient diagnosed with CIDP. So here you can see stimulating at the ankle, you get a response, but when you move and stimulate proximally, this response amplitude decreases dramatically, you know, greater than 90% reduction. It's almost a complete conduction block, and you can also see a reduction in the area underneath the waveform between the ankle and then proximal sites, and so this is consistent with conduction block. And then here's an example of abnormal temporal dispersion in the right median motor study at APB in a patient with CIDP. And so here when you stimulate at the wrist, you get a relatively normal-looking waveform, and the duration is approximately 17.2 milliseconds. When you stimulate proximally, though, you get this dispersed waveform, and the duration increases dramatically to 30.2 milliseconds, and so this would be consistent with temporal dispersion. Now notably, these exact criteria may vary slightly between laboratories, however, standard consensus guidelines for demyelinating neuropathies, and really specifically in CIDP, have been developed. They were most recently updated in 2021. And here is that article, it's the European Academy of Neurology and Peripheral Nerve Society Guidelines on the Diagnosis and Treatment of CIDP, and I would encourage you to read through this article to learn more about demyelinating neuropathies. And here is a table from this article that lists the motor nerve conduction criteria that can be seen in demyelinating neuropathies. And as you can see, they're very similar to what we just talked about. Okay, and then here are my references in case you want to read or learn more about this information. Thank you for listening.

electrodiagnostic features

demyelinating neuropathies

nerve conduction studies

EMG

CIDP

ncs

Chronic Inflammatory Demyelinating Polyneuropathy

electromyography