Weekly question

PMR 4 MSK

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    Easy one for the week:


    45 yo female with numbness in the left hand palmar thumb and index finger. NCS:

    L Median motor onset 3.8 ms (<4.2) Amplitude 4.1 (>5) NCS forearm 56 (>50)
    R Median motor onset 3.6, amplitude 6.7, NSC 58

    L Ulnar motor onset 3.1 (< 3.2) Amp 4.4 (>3) NCS forearm 58, across elbow 62 (>50)

    L median sensory orthodromic peak 2.2 (<2.5), ampltude 25 (>10)
    L ulnar sensory orthodromic peak 2.2 (<2.5), amplitude 18 (>10)
    R median sensory orthodromic peak 2.1 (<2.5), ampltude 32 (>10)

    Needle exam of left muscles deltoid, biceps, triceps, PT, FDIM, APB all normal.


    Explain the abnormality.
     

    Taus

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      pgy2 w/ limited emg experience....but I'll take a stab at it....

      Ok... so only abnormality is decreased amplitude of the left median nerve at the wrist.... normal velocities/latencies/sensory/needle studies. Was the L median n. checked at the elbow? Antidromic median motor across the wrist? Was recruitment decreased at all? Was there true sensory loss on exam? only palmar side? How long has this been going on?

      If this is relatively acute, ie < 10 days, this could be a median neuropathy with axonal loss at the wrist (ie CTS) that was caught in that 2-3 day period after the CMAP dropped out, but before the SNAP dropped out (though odd that the velocities were fine). Or it could be a normal exam w/ a Martin Gruber to explain the decreased distal amplitude..







      Easy one for the week:


      45 yo female with numbness in the left hand palmar thumb and index finger. NCS:

      L Median motor onset 3.8 ms (<4.2) Amplitude 4.1 (>5) NCS forearm 56 (>50)
      R Median motor onset 3.6, amplitude 6.7, NSC 58

      L Ulnar motor onset 3.1 (< 3.2) Amp 4.4 (>3) NCS forearm 58, across elbow 62 (>50)

      L median sensory orthodromic peak 2.2 (<2.5), ampltude 25 (>10)
      L ulnar sensory orthodromic peak 2.2 (<2.5), amplitude 18 (>10)
      R median sensory orthodromic peak 2.1 (<2.5), ampltude 32 (>10)

      Needle exam of left muscles deltoid, biceps, triceps, PT, FDIM, APB all normal.


      Explain the abnormality.
       
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      PMR 4 MSK

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        pgy2 w/ limited emg experience....but I'll take a stab at it....

        Ok... so only abnormality is decreased amplitude of the left median nerve at the wrist.... normal velocities/latencies/sensory/needle studies. Was the L median n. checked at the elbow? Antidromic median motor across the wrist? Was recruitment decreased at all? Was there true sensory loss on exam? only palmar side? How long has this been going on?

        If this is relatively acute, ie < 10 days, this could be a median neuropathy with axonal loss at the wrist (ie CTS) that was caught in that 2-3 day period after the CMAP dropped out, but before the SNAP dropped out (though odd that the velocities were fine). Or it could be a normal exam w/ a Martin Gruber to explain the decreased distal amplitude..

        Awesome thinking! This kind of thought processing is what I am looking for. You are on the right track. +3 internets to Taus for the cohones to answer and for the rational thinking.

        I deliberately left out the elbow median motor amplitude to see if anyone would think of MGA. It had to have been tested since there is a NCV. If it was indeed significantly higher than the wrist amplitude, MGA definately comes to mind. However, the amplitude at the wrist should still be normal, except in the case of CTS with MGA. In that case there would often be a positive deflection of the motor response at the elbow, confusing the onset marker.

        In this case, the amplitude at the elbow was the same as the wrist and there was no positive deflection.

        No antidromic motor performed. No decreased recruitment. There was only paresthesias in the hand, no discriminatory loss.

        This has been going on for 2 months.

        There are 2 other important possible explanations for these findings.
         

        webtek

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          Kudos to Taus for the thinking of MGA....

          A couple of other things come to mind when looking at the stated case. For one I would like more information, such as the comparative studies including median-radial and median-ulnar. Using comparative studies will increase the sensitivity of diagnosing CTS with EMG into the 90th percentile.

          The other thought that I had was of the Superficial branch of the Radial nerve, which has innervation of a portion of the proximal palmar thumb region. Could this be the cause with some referring symptoms. Definitely has to be part of the differential.
           

          PMR 4 MSK

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            Dont overthink it too much. I said it was an easy one. At least to me. :D

            Low amplitude median nerve at the wrist, you first think of CTS severe enough to cause axonal loss. But the sensory is normal, so that votes against acute CTS. Not impossible, but unlikely.

            Then you should think "error." If you are just off a little bit on the placement of the actiuve electrode for the motor point for the APB (and many muscles), the amplitude can drop significantly. In my expereience, this is probably the most common explanation for a low amplitude motor response with maximal stimulation. Sometimes, just moving it a mm or two will fix the problem.

            The second thing to think about in this case is a pt s/p carpal tunnel release, with incomplete healing. If the patient had significant axonal loss prior to CTR, they might not get complete axonal regrowth. The fact that the onset of the motor and peak of the sensory are normal show that remylination occured.

            In this case, the pt had a CTR 10 years ago, with incomplete healing. Depsite my fishing around for a better motor point, this was the max median motor amplitude I could get, at the wrist and the elbow.

            So when you have a low amplitude median motor response with normal latencies, NCV and needle exam, think error, s/p CTR and MGA. Certainly there are other potential explanations, but these are what you are most likely going to encounter in clinical practice.
             

            Taus

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              Interesting, thank you.. I definitely learned something. A few Q's:

              1) re imperfect placement: wouldn't you know that also by a poor takeoff or near point/initial positive deflection? Can you be off the motor point enough to have a falsely lowered cmap and not have a poor takeoff/near point?

              2) re old axonal loss: no wide duration/inc amplitude on emg?

              Thanks




              Dont overthink it too much. I said it was an easy one. At least to me. :D

              Low amplitude median nerve at the wrist, you first think of CTS severe enough to cause axonal loss. But the sensory is normal, so that votes against acute CTS. Not impossible, but unlikely.

              Then you should think "error." If you are just off a little bit on the placement of the actiuve electrode for the motor point for the APB (and many muscles), the amplitude can drop significantly. In my expereience, this is probably the most common explanation for a low amplitude motor response with maximal stimulation. Sometimes, just moving it a mm or two will fix the problem.

              The second thing to think about in this case is a pt s/p carpal tunnel release, with incomplete healing. If the patient had significant axonal loss prior to CTR, they might not get complete axonal regrowth. The fact that the onset of the motor and peak of the sensory are normal show that remylination occured.

              In this case, the pt had a CTR 10 years ago, with incomplete healing. Depsite my fishing around for a better motor point, this was the max median motor amplitude I could get, at the wrist and the elbow.

              So when you have a low amplitude median motor response with normal latencies, NCV and needle exam, think error, s/p CTR and MGA. Certainly there are other potential explanations, but these are what you are most likely going to encounter in clinical practice.
               

              RUOkie

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                Interesting, thank you.. I definitely learned something. A few Q's:

                1) re imperfect placement: wouldn't you know that also by a poor takeoff or near point/initial positive deflection? Can you be off the motor point enough to have a falsely lowered cmap and not have a poor takeoff/near point?

                2) re old axonal loss: no wide duration/inc amplitude on emg?

                Thanks
                You are a PGY 2? Good thought processes!

                1) yes you can definitely see a false low CMAP without a positive initial deflection. Anytime you have a low amplitude CMAP, take the time to change your electrode placement.

                2) After 10 years, especially in small muscles like APB, FDI, and the intrinsic foot muscles, you can have atrophy without large MUAPs.
                 
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