Transection of the major hand nerves leads to significant and long-lasting functional impairments. Animal models show functional changes in the contralesional hand area of primary sensory cortex (S1-deprived) in response to both stimulation of the repaired nerves of the injured hand as well to the (ipsilateral) uninjured nerves of the uninjured hand. Evidence for brain changes in human peripheral nerve repair patients is limited and their impact on functional recovery is unknown.
 
Objectives
We present preliminary results from an ongoing project using functional MRI to measure cortical responses to cutaneous stimulation of the hand in peripheral nerve repair patients and healthy controls. The results are preliminary since our target participant sample size has not been reached; current numbers include ten patients—three median, four ulnar, and three ulnar and median cases (time since repair, mean: 41 months; range: 10 – 82 months)—and 16 healthy controls.
 
Method
Vibrotactile stimulation was applied to the distal digit pads of each hand while inside the MRI scanner. S1 was defined functionally in each individual based on responses to stimulation of the digits of the contralateral hand combined with the estimated location of area 3b based on their anatomical data. We then extract estimates of fMRI response levels within S1 to stimulation of the digits of the ipsilateral hand.
 
Patients and controls also completed novel tests of touch localisation on the volar surfaces of each hand; mean absolute and directional error were computed. Patients also completed a number of standardised clinical assessments, including the sensory Rosen test.
 
Results
We find significant hand-hemispheric differences in S1 ipsilateral responses levels in patients but not controls. A two-way mixed ANOVA identifies a significant interaction between group (patient, control) and hand (injured, uninjured) (F(1, 24) = 4.767, p = .0.039). Patient ipsilateral responses in S1-deprived (i.e., to the uninjured hand) are elevated compared to their ipsilateral response levels in the opposite hemisphere (i.e., to the injured hand), while no such hand-hemispheric differences are identified in healthy controls.
 
Inconsistent with our expectations, however, elevated ipsilateral response levels in patients do not exceed estimates of the range of normal variation as defined by our healthy controls. Greater ipsilateral responses in S1-deprived are nonetheless found to reliably correlate with higher sensory Rosen scores (r = 0.82, p = .0035) and better touch localisation (r = -0.74, p = .0151).
 
Conclusions
Our current results are difficult to interpret. There is a suggestion that ipsilateral response levels are elevated in the S1-deprived of patients, yet these levels do not exceed our estimates of normal variation in healthy controls. This makes it difficult to interpret these responses as possible markers of functional reorganisation following peripheral nerve repair. At the same time, the results identify a significant positive relationship between the strength of ipsilateral responses in S1-deprived and patient recovery. More data are needed to clarify these findings. Cautious interpretation is necessary, at present.
 
Compliance with ethical standards
The study has been approved by the ethic committees of Bangor University and Health and Care research Wales.