Case Report: Plasticity in Central Sensory Finger Representation and Touch Perception After Microsurgical Reconstruction of Infraclavicular Brachial Plexus Injury

Abstract

After brachial plexus injury (BPI), early microsurgery aims at facilitating reconnection of the severed peripheral nerves with their orphan muscles and sensory receptors and thereby reestablishing communication with the brain. In order to investigate this sensory recovery, here we combined functional magnetic resonance imaging (fMRI) and tactile psychophysics in a patient who suffered a sharp, incomplete amputation of the dominant hand at the axilla level. To determine somatosensory detection and discomfort thresholds as well as sensory accuracy for fingers of both the intact and affected hand, we used electrotactile stimulation in the framework of a mislocalization test. Additionally, tactile stimulation was performed in the MRI scanner in order to determine the cortical organization of the possibly affected primary somatosensory cortex. The patient was able to detect electrotactile stimulation in 4 of the 5 fingertips (D1, D2, D4, D5), and in the middle phalanx in D3 indicating some innervation. The detection and discomfort threshold were considerably higher at the affected side than at the intact side, with higher detection and discomfort thresholds for the affected side. The discrimination accuracy was rather low at the affected side, with stimulation of D1/D2/D3/D4/D5 eliciting most commonly a sensation at D4/D1/D3/D2/D5, respectively. The neuroimaging data showed a mediolateral succession from D2 to D5 to D1 to D4 (no activation was observed for D3). These results indicate a successful regrowth of the peripheral nerve fibers from the axilla to four fingertips. The data suggest that some of the fibers have switched location in the process and there is a beginning of cortical reorganization in the primary somatosensory cortex, possibly resulting from a re-education of the brain due to conflicting information (touch vs. vision).

Keywords: S1; brachial plexus injury; brain plasticity; sensory cortex; somatosensory.

Figure 1

Visualization of nerve injury and reconstruction. (A) The interrupted nerves indicate the level of disruption of the infraclavicular brachial plexus distal to the conjunction of the ventral roots of C5-7 (lateral cord) and C8 and T1 (medial cord) to the final contributions of the median nerve. The proximal and distal nerve ends were retracted toward the forearm by up to 7 cm. From the union of the lateral and medial root to the median nerve the proximal ends of the radial, ulnar, median, and musculocutaneous nerves could be identified. (B) Sural nerve and median cutaneous antebrachii nerve grafts (indicated in violet) were doubled, reversed, and used to bridge the gaps between the proximal and distal nerve stumps by epineural suture under loupe magnification.

Figure 2

Stimulation types. (A) Electrotactile stimulation. Five electrodes were fixed on the according fingertips for psychophysical examination. (B) Tactile stimulation. Five piezo-electric vibrators were used for somatosensory stimulation during fMRI, one at each finger, each with an 8-dot Braille display.

Figure 3

Results of the electrotactile stimulation and tactile mapping in S1 via fMRI as opposed to the expected cortical pattern in intact-bodied subjects. Confusion matrices for tactile psychophysics using electrostimulation for the (A) intact and (B) impaired hand. Shown are averaged results across the three blocks, percentage of how often the subject perceived one finger upon stimulation of the same (in white squares) or a different finger. The respective percentages are stated in numbers and further emphasized by color coding. The two lower panels show the hand representation in BA 3b expected in intact-bodied subjects and observed at the subject's affected side. In both panels, the same detail, covering the motor hand knob (Ω) and expected somatosensory hand area (white border) of the central sulcus of the subject's hemisphere contralateral to the affected side is shown. A, anterior' P, posterior; M, medial; L, lateral. (C) Schematic representation of the finger representation from lateral (D1) to medial (D5) as expected for an intact-bodied limb. (D) The subject's somatosensory representation of the affected fingers in assumed SI (cortical activations masked by that area). The most significant vertices are below the finger labels. Color code: Magenta = D1, yellow = D2, green = D3, blue = D4, red = D5.