NBDE Examination Practice Question # 111

NBDE Examination.

Practice Question # 111.

Local anaesthetics provide a differential block in a concentration-dependent manner. Aγ spindle efferents and the Aδ nociceptive fibres are most susceptible, whereas non-myelinated C fibres are relatively resistant. Differential sensitivity to local anaesthetics can be demonstrated during epidural block. Sympathetic fibres are most easily blocked, requiring the lowest concentration of local anaesthetic to block neuronal transmission.

Sympathetic blockade usually reaches a higher dermatome than other modalities. Temperature (cold) and pain (pinprick), followed by proprioception and finally motor fibres are next most easily blocked, demonstrated by a descending dermatomal level. During epidural anaesthesia for Caesarean section, sensation of touch and proprioception (Aβ fibres) may therefore still occur despite adequate sensory block, which can be distressing for patients.