Sensory re-training (also referred to as sensory re-education) is a cognitive behavioral therapy technique that helps the patient with a nerve injury to meaningfully interpret the altered profile or neural impulses reaching his conscious level after the altered sensation area has been stimulated (30
). Moreover, the repetitive neural input from sensory re-training exercises can produce plastic changes in the somatosensory cortex via the same mechanisms underlying those evoked by altered input from the nerve damage. This reorganization through re-training can compensate, in part, for some of the impairments associated with nerve injury (31
Animal studies have shown that behavioral sensory training alters the central neural representation of the involved skin sites, alters the response of individual somatosensory cortical cells to tactile stimulation, and increases synapse to neuron ratios and improves behavioral function after induced brain damage more than simple repetitive exercise (38
). Neuroimaging studies indicate that similar changes occur in human subjects following sensory denervation and sensory training (45
). Sensory experience or retraining results in somatosensory cortical maps that exhibit higher sensory resolution and greater topographical organization which facilitate better interpretation of sensory inputs. In contrast to the central neural changes, sensory re-training does not alter the course of nerve regeneration or the absolute thresholds to touch (39
) but does improve both the patient’s cognitive and adaptive response to stimulation of the affected skin region. (12
Although improvement has been reported when re-training isn’t initiated soon after the injury, reorganization of the cortex after changes in peripheral input happens quite quickly. Persistent chronic altered sensation may result in irreversible cortical changes. One of the goals of re-training is to avoid, minimize or modulate the central functional re-organization (52
The process of sensory re-training can be likened to the brain learning a new language in progressive phases of difficulty. Initially, use of the words is slow, challenging and error prone. With time and practice, verbal fluency may be acquired. Unfortunately, no research has been conducted to determine the optimal number of phases or the exercises required to obtain the maximum benefit to patients with orofacial nerve injuries.
Historically, in the early phase of sensory retraining (), the intent is to re-educate constant vs moving touch perceptions. That is, a patient must re-learn what constant touch feels like compared to moving touch and where on the skin the touch is actually occurring. In the early phase, a greater stimulus intensity may be necessary for the patient to differentiate constant from moving touch but the intensity should never be so great as to evoke pain. If hyperesthesia or dysesthesia occurs, desensitization with gentle stroking using different textures or gentle tapping is recommended(53
). In the late phase of retraining (), the intent is to re-educate the directionality of movement perceptions of the patient. For example, is the movement of an external object across the skin from left to right or right to left?
General Concepts of Sensory Re-training
For orofacial sensory retraining, an important component of the retraining exercises is the visual feedback provided by performing the exercises in front of a mirror. This elicits two different sensory events, the sensation of the brush on the facial skin and the sight of the brush on the face. Recent experimental studies have shown that viewing a body surface can directly enhance tactile perception and detection (57
) even when the “touch” is not physical but a mirrored reflection.(59
). The frequency with which the exercises are performed each day is much more important than the length of time spent at any given time. It may be that encouraging patients to perform orofacial sensory retraining exercises with a small handheld mirror for a short period of time, perhaps 1–2 minutes, 4 to 6 times per day would be as or more effective than a longer less frequent protocol.
Both the potential for acquiring the “second language” of sensory retraining and its effectiveness decreases with age (49
), varies with the verbal learning capacity and visuo-spatial cognitive skills of the patient, and depends on motivation and positive reinforcement (45
Sensory re-training as a rehabilitative approach has been used extensively over the past several decades for patients who had nerve injuries affecting the hand. The emphasis of the sensory re-training exercises for hand injury and stroke patients has been to teach the patient to interpret the percepts of objects manipulated by the fingers in a meaningful and functional way (30
). Hand injury patients learn to recognize and to discriminate the shapes of small objects (various buttons, coins and keys). Patients gain the ability to button their own shirt and to identify shapes without visual cues (ex. a key versus a coin). Although the touch percepts produced by the objects remain abnormal after re-training, patients become more comfortable with, and accepting of, the situation since the percepts are no longer functionally disabling.
The same therapeutic approach, incorporating meaningful and graded stimuli, active participation, and accurate feedback, has successfully been used to improve tactile and proprioceptive discrimination following a stroke (65
) and recovery of function in people with brain damage (67
). An adaptation of sensory re-education, mirror box therapy, has successfully been used with patients with phantom limb pain (68
), hemiparesis after stroke (69
), and complex regional pain syndrome type I (70
). Patients have regained functionality and mobility with reduced pain and evidence of cortical reorganization of the primary somatosensory cortex that paralleled their clinical improvement (71