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Why is vibration the first sensation to be lost ???

Discussion in 'Neurology' started by cardio32, Oct 15, 2008.

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  1. cardio32

    cardio32

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    In a standard neuro exam. in a patient with a high suspicion of nerve involvement ... it is usually said that vibration sense is the first to be lost ... so if suspicious of nerve involvement then vibration is to be tested ... why is it that vibration is first lost ...??? kindly let me know
  2. Nerdoscience

    Nerdoscience Senior Member

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    As you imply, vibratory sense is lost in demyelinating neuropathies distally very early on. This is because you will affect posterior column somatosensation (position, vibration) before spinothalamic somatosensation (pain, temperature) because posterior column modalities are carried by large fibers. These are the ones that are myelinated. Pain and temperature are mostly (but not all) carried by small fibers, which are not myelinated. Thus, your statement is NOT true for axonal neuropathies.

    When you have diffuse demyelinating injury, such as in diabetes, the first place to suffer is where the most slowing of conduction from the demyelination has accumulated, meaning where the path is longest. That is why the pattern is distal, and worst in the toes.

    As for why vibration is lost before position sense and light touch, I'm not completely sure. For light touch, there are many different modalities of sensation involved, such as hair follicle position and nociception, some of which is carried by the spinothalamic tracts and unmyelinated fibers. Maybe the reason that position sense goes later is that it involves more area of sensation and larger amplitudes of stimuli.
  3. neurolddoc

    neurolddoc Lifetime Donor

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    In my experience either large fiber (vibratory) or small fiber (temp, pin) may be lost first. It depends on what the underlying cause is. I have even seen dissociation of vibratory and position sensation loss which are carried over similar peripheral fibers and are both carried in the lemniscal tracts centrally.
  4. cardio32

    cardio32

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    thanks for the help
  5. Phantom Spike

    Phantom Spike

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    Diabetic neuropathy is predominantly axonal, not demyelinating.
  6. Nerdoscience

    Nerdoscience Senior Member

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    Diabetes and alcohol, two very common causes of neuropathy, clearly have both axonal and demyelinating components. That is why modalities affected and EMG/NCS often do not rule them in or out.

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