Conduction velocity of nerve is reduced in all of the following conditions, except:
Answer is A (Acute Motor Axonal Neuropathy Preserved conduction velocity (No/minimal reduction in conduction velocity) is a feature of Axonal neuropathies. Acute Motor Axonal Neuropathy (AMAN) is pure axonal neuropathy characterized by normal motor conduction velocity. Demyelinating neuropathies are typically associated with reduced conduction velocities. AIDP, HSMN and MMN are all demyelinating neuropathies characterized by reduced conduction velocity on nerve conduction studies. Demyelinating Versus Axonal Neuropathies The primary function of myelin is to increase axonal conduction velocity without a significant increase in axonal diameter. This function is achieved by a process of saltatory conduction, in which nerve impulses jump between electrically excitable regions of the axon, called nodes of Ranvier located between elctrically insulated areas ensheathed by myelinating Schwann cells. A demyelinating process disrupts the normal saltatory conduction along some axons within the nerve slowing the conduction velocity of these axons. As demyelination becomes more profound, conduction through some areas is blocked completely (conduction block), functionally disconnecting the motor neuron from the innervated muscle resulting in weakness. In axonal neuropathies the conduction velocity is typically either preserved or only midly reduced. Electrophvsiologic Features: Axonal Degenaration vs. Segmental Demylination Features Axonal Degeneration Segmental Demylination Motor Nerve Conduction Studies CMAP amplitude Decreased Normal (except with C'13) Distal latency Normal Prolonged Conduction velocity Normal Slow Conduction block Absent Present Temporal dispersion Absent Present F wave Normal or Absent Prolonged or Absent H reflex Normal or Absent Prolonged or Absent Sensory Nerve Conduction Studies SNAP amplitude Decreased Normal Distal latency Normal Prolonged Conduction velocity Normal Slow Needle EMG Spontaneous activity Fibrillations Present Absent Fasciculation Present Absent Motor unit potentials Recruitment Decreased Decreased Morphology Long duration / polyphasic Normal
Acute Motor Axonal Neuropathy (AMAN)
Acute Inflammatory Demyelinating Neuropathy (AIDP)
Hereditary Sensory Motor Neuropathy (HSMN)
Multifocal Motor Neuropathy