Evoked potentials

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					       Electroencephalography.
                (EEG)
   The electroencephalogram is recorded
    from scalp electrodes on 16 channels
    simultaneously for 10-30 minutes. The
    main value of the EEG is in diagnosing
    epilepsy and diffuse brain diseases.

   Videotelemetry combines continuous EEG
    and video recording ,and is valuble in the
    specialist assessment of difficult cases of
    episodes of disturbed consciousness.
   Epilepsy:
     Spikes ,or spike-and wave abnormalities ,are
    the hallmarks of epilepsy ,but it should be
    emphasized that patients with epilepsy often
    have a normal EEG between seizures.

   Diffuse brain disorders:
     Recognizable patters of slow wave EEG
    abnormalities appear in encephalitis
    ,,dementia [Creutzfeld-Jakob disease] and
    metabolic states [e.g. hypoglycaemia and
    hepatic coma]

   Brain death:
The EEG is isoelectric [i.e.flat].This EEG finding
 is no longer necessary to confirm brain death
 in the UK.
   Cerebral-evoked potentials:

 Visual –evoked potentials record the time
  for visual stimulus to reach the occipital
  cortex.
 Their value is chiefly in documenting
  previous retrobulbar neuritis, which leaves
  a permanent delay in latency despite
  recovery of vision.
 Similar techniques are used for auditory
  and somatosensory potentials [from the
  ear or a limb].
 These are also used during neurosurgery to
  monitor brain and spinal cord function.
   Cerebral-evoked potentials:

 Either visual ,auditory or Somatosensory
  from limbs ,are senstive measures of
  damage in these sensory pathways.
 Visual evoked potentials are recorded from
  the scalp over the occiptal cortex.

 A flash or regular pattern reversal stimulus
  produces a small visual evoked
  potential,which can be averaged.
 The latency is prolonged if there has been
  significant demyelination at any time in
  the past.
   Similar informtion can be obtained
    concerning brain stem auditory
    pathways and large fibre ,dorsal
    column somatosensory pathways
    from the limbs .

   The evoked potentials are often
    delayed in the absence of any
    present or past symptoms.
   Electromyography (EMG) is a technique for
    evaluating and recording the activation
    signal of muscles.

   EMG is performed using an instrument
    called an electromyograph, to produce a
    record called an electromyogram.

   An electromyograph detects the electrical
    potential generated by muscle cells when
    these cells contract, and also when the
    cells are at rest.
   EMG is used to diagnose two general
    categories of disease: I. neuropathies
    and II.myopathies:
   I. Neuropathic disease has the following
  defining EMG characteristics:
 An action potential amplitude that is twice
  normal due to the increased number of fibres
  per motor unit because of reinnervation of
  denervated fibres.
 An increase in duration of the action potential
 A decrease in the number of motor units in the
  muscle (as found using motor unit number
  estimation techniques)
   II. Myopathic disease has these
    defining EMG characteristics:

 A decrease in duration of the action
  potential
 A reduction in the area to amplitude
  ratio of the action potential
 A decrease in the number of motor
  units in the muscle (in extremely
  severe cases only)