Dynamic changes in the perceived posture of the hand during ischaemic anaesthesia of the arm

• Published in: The Journal of physiology 2011-12, Vol.589 (23), p.5775-5784
• Main Authors: Inui, N., Walsh, L. D., Taylor, J. L., Gandevia, S. C.
• Format: Article
• Language: English
• Subjects: Adult; Anesthesia; Arm; Female; Fingers; Hand - anatomy & histology; Hand - physiology; Humans; Ischemia; Male; Middle Aged; Neuroscience: Behavioural/Systems/Cognitive; Perception - physiology; Phantom Limb - physiopathology; Phantom Limb - psychology; Wrist; Young Adult
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2011.219949

Non‐technical summary  Even when the hand is stationary we know its position. This information is needed by the brain to plan movements. If the sensory input from a limb is removed through an accident, or an experiment with local anaesthesia, then a ‘phantom’ limb commonly develops. We used ischaemic anaesthesia of one arm to study the mechanisms which define the phantom hand. Surprisingly, if the wrist and fingers are held straight during anaesthesia, the perceived phantom hand becomes bent at the wrist and fingers, but if they are bent during anaesthesia, the final phantom is extended at the wrist and fingers. There is no ‘default’ posture for the phantom hand. Further, the hand appears to increase gradually in size as anaesthesia develops. The start of these perceptual changes occurs when input from large‐diameter sensory nerve fibres is declining. These results provide new information about how the brain generates phantom limbs.   Contorted ‘phantom’ limbs often form when sensory inputs are removed, but the neural mechanisms underlying their formation are poorly understood. We tracked the evolution of an experimental phantom hand during ischaemic anaesthesia of the arm. In the first study subjects showed the perceived posture of their hand and fingers using a model hand. Surprisingly, if the wrist and fingers were held straight before and during anaesthesia, the final phantom hand was bent at the wrist and fingers, but if the wrist and fingers were flexed before and during anaesthesia, the final phantom was extended at wrist and fingers. Hence, no ‘default’ posture existed for the phantom hand. The final perceived posture may depend on the initial and evolving sensory input during the block rather than the final sensory input (which should not differ for the two postures). In the second study subjects selected templates to indicate the perceived size of their hand. Perceived hand size increased by 34 ± 4% (mean ± 95% CI) during the block. Sensory changes were monitored. In all subjects, impairment of large‐fibre cutaneous sensation began distally with von Frey thresholds increasing before cold detection thresholds (Aδ fibres) increased. Some C fibres subserving heat pain still conducted at the end of cuff inflation. These data suggest that changes in both perceived hand size and perceived position of the finger joints develop early when large‐fibre cutaneous sensation is beginning to degrade. Hence it is unlikely that block of small‐fibre afferents is