Neuropathic Pain

  • Neuropathic pain is described by the patient as pain that is severe and “out of proportion” to the stimulus or pain that is often spontaneous, continuous, or lingering in nature. Stimulus induced neuropathic pains are: 
  • 1. Allodynia: painful response to normally non-painful stimulus. For example, moving a brush across the zone of injury causes a very painful stimulation of the area. Normally a brush moving across the adjacent uninjured area is non-painful. Axons of cutaneous afferent neurons become sensitive to mechanical stimulus. Local inflammatory mediators at or around the injured nerve terminals or axons decrease the threshold for activation and the response magnitude to constant stimuli may increase. This sensitization, which is caused by endogenous bradykinin, prostaglandin and others may enhance mechanosensitivity of afferent neurons. 2. Anesthesia Dolorosa: painful response in a zone of complete anesthesia. For example, pain is still reported in a zone that has no response to tactile, thermal, painful stimuli and not resolved with peripheral nerve blocks of the same zone. These findings indicate a central origin of the pain response.
  • 3. Hyperalgesia: enhanced pain response. For example, thermal pain tolerance over the mental fold in a healthy adult is between 48o and 50o C. If the recorded pain tolerance in the same area is now 45o C, then the patient has hyperalgesia. They have pain response and sensitivity, but it is abnormally low in threshold and therefore previously non-painful levels of a painful stimulus (cold, hot, pressure, etc.) is now painful and normal just detectable painful stimulus is enhanced.
  • 4. Hyperpathia: a painful syndrome characterized by an abnormally painful reaction to a stimulus, especially a repetitive stimulus, as well as an increased threshold. For example, in applying a neurosensory test protocol; a series of about ten tactile pulses with Semmes Weinstein monofilaments slowly administered (1/sec) followed by a 1 minute rest period. This protocol is used to identify delayed onset in pain. The pain may alternatively begin during the stimulation but continue for appreciable durations exceeding 1minute after the stimulation is terminated and is an example of aftersensation. If the pain begins during the stimulation, the approximate number of pulses required for its elicitation should be recorded to describe the summation. If the intensity of the pain is reported to increase with continued stimulation, overshoot is present. One or more observations of delayed onset, aftersensation, overshoot, or summation are sufficient for a diagnosis of a hyperpathic pain condition.
  • 5. Sympathetic Maintained Pain (SMP): Sympathetic maintained pain results from tonic activity in myelinated mechanoreceptor afferents, whose activity is induced by sympathetic efferent actions on sensory receptors, and this afferent input causes tonic firing in previously sensitized wide-dynamic (WDR) or mulitreceptive neurons that are part of a central nociceptive pathway. A burning painful sensation results from this chain of actions. A regional nerve block with local anesthesia would result in the reduction of the mechanical sensitivity but not the tonic firing of WDR neuron generated impulses. Brush-stoke directional testing of the involved area would demonstrated mechanical allodynia but would not be able to distinguish peripheral mechanical allodynia from SMP pain.