As a further means of assessing the physiological integrity of the peripheral C fibers in patients suffering from unusual pain syndromes or reduced pain sensitivity the investigators will elicit an axon reflex. This reflex involves the transcutaneous application of agents such as histamine via iontophoresis. The histamine will stimulate the peripheral nerve endings of the small diameter C fibres. Their stimulation induces a vasodilatation, which is visible as a flare response of the skin. The evaluation of the extent of the flare response is used as an indication of the integrity of the small diameter C fibre population. If the subject has any allergies they will be excluded from this test.