Patients with Carpal Tunnel Syndrome show increased reliance on vision in reaching-to-grasp: a study of in-flight grasp kinematics in compressive nerve injury

The fluid efficiency of everyday hand actions such as reaching-to-grasp is underpinned by finely calibrated, anticipatory, in-flight control of the hand. Peripheral nerve dysfunction could affect this control. We used Carpal Tunnel Syndrome (CTS), a compressive neuropathy of the median nerve, as a model of nerve dysfunction. Whether CTS affects in-flight aspects of reaching-to-grasp is unknown. We compared kinematics of movements in CTS and healthy controls, using motion capture. We varied object properties to determine whether anticipatory signatures of reaching-to-grasp are preserved in CTS. We also examined the effect of removing visual feedback at movement onset. This manipulation forces greater reliance on non-visual control signals, which should highlight impairments due to CTS, while indexing how much movements rely on vision. Participants also completed standardised clinical tests of sensorimotor function. Reaching-to-grasp in CTS showed normal anticipatory signatures, including reliable scaling of movement speed and hand opening with object distance and size, respectively. Removing visual feedback caused both patients and controls to increase hand opening by similar amounts, to compensate for increased probability of errors. Distinct from controls, however, CTS patients also showed slower movement speeds and longer movement onset times without visual feedback. These CTS-specific responses suggest that CTS causes greater reliance on vision for reaching-to-grasp. They also demonstrate adaptive compensation for nerve dysfunction (consistent with normative, statistical-decision-theoretic accounts of movement control). The qualitative similarity of reaching-to-grasp in CTS and controls suggests that normal control processes are preserved in moderate peripheral nerve dysfunction, taking the effects of injury into account appropriately.