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CHAPTER 25
CUTANEOUS VASCULAR RESPONSES TO MECHANICAL AND
CHEMICAL STIMULATION (TRIPLE RESPONSE, REFLEX
ERYTHEMA, DERMOGRAPHISM)
The term triple response (Lewis), which is now generally used for reflex erythema and
dermographism, is characterized
a. by an initial, sharply defined vasodilatation of all skin vessels restricted to the line of
stroke evoked by the end of the reflex hammer or by a dragging needle;
b. a flare with irregular margins spreading for variable distances outside the region of
the stroke and caused by dilatation of the arterioles of the surrounding area of the skin;
c. local oedema (weal) of variable intensity over the line of stroke.
The neurogenic mechanism of the various components of the triple response has
been investigated by various authors (Breslauer, 1919, L.R.Miiller, 1931, and in parti
cular by Lewis & Grant, 1925), and this reflex phenomenon is considered as an axon reflex
resulting in vasodilatation of the skin arterioles due to direct mechanical stimulation of
peripheral vaso-motor nerves or as a result of chemical stimulation by a histamine-like
substance (H-substance of Lewis), liberated by cells of the skin along the path traced
by the stimulation.
My own observations on this cutaneous reflex response in complete transverse lesions
of the spinal cord can be summarized as follows:
In the initial stages following cord transection, mechanical stimuli by stroking with
blunt or sharp instruments elicit no significant difference in the extent of the spreading
flare between the normal and paralysed area of the skin but the flush may last longer in
the dermatomes below the transection. This is understandable if one remembers that, in
the initial stages following transection, the paralysis of the vasoconstrictors may be very
conspicuous. However, once the reflex function of the isolated cord is developed and
with the recovery and, indeed, increase of the vasoconstrictor tone, the effect in the
paralysed area to cutaneous stimulation is vasoconstriction—i.e. the line made by the
stroke is of white colour (dermatographia alba). This is in distinct contrast to the spread
ing flush above the level of the lesion, which is particularly obvious in the segment or
segments just above the level of the lesion (dermatographia rubra). Thus, in complete
transection of the thoracic cord, the level of the lesion is clearly defined by the increased
cutaneous vasodilatation and sometimes weal formation in the border zone above the
transection. Chemical stimulation by intracutaneous histamine injection confirmed these
findings obtained by mechanical stimulation—i.e. decrease of spreading flush in the
paralysed area and increase and longer lasting spreading flush in the border zone above
the level of the lesion. This is in accordance with Cooper's (1950) findings. These
investigations have clarified the neurogenic mechanism of the triple response. While it is
accepted that the triple response is primarily an axon reflex, this reflex is modified by
the reflex action of the spinal cord.
DISTURBANCES OF THERMOREGULATION
Changes of blood flow and temperature regulation, as a result of the large scale redistri
bution of blood which follows the vasoconstriction in the paralysed part of the body
caused by bladder distensions, have been described in this book (page
