Irritants stimulate sensory nerve endings and induce inflammation at the site of application. Depending on their nature, concentration and sensitiveness of the site, they produce cooling sensation or warmth, pricking and tingling, hyperaesthesia or numbness and local vasodilatation.
IRRITANTS AND COUNTER-IRRITANTS
Irritants stimulate
sensory nerve endings and induce inflammation at the site of application.
Depending on their nature, concentration and sensitiveness of the site, they
produce cooling sensation or warmth, pricking and tingling, hyperaesthesia or
numbness and local vasodilatation. Irritants which cause local hyperemia with
little sensory component are called Rubefacients.
Stronger irritants which in addition increase
capillary permeability and cause collection of fluid under the epidermis
(forming raised vesicles) are termed Vesicants.
Certain irritants also produce a remote effect which tends to relieve pain and
inflammation in deeper organs—called Counterirritants.
Cutaneous sensations
are precisely localized. Deeper sensations from muscles, joints and viscera are
perceived more diffusely. A spinal segment, receiving afferent impulses from
the surface as well as from deeper organs, modulates them—preferentially
conducting the former to the higher centers. When a counterirritant is applied
to the area of skin supplied by nerves form the same segment as the deeper
organ form which pain impulses are coming, the cutaneous impulses obscure the
deeper sensation. Irritation of afferent
nerve endings produces arteriolar dilatation in the adjoining areas of skin by
axon reflex (which mediates flare in triple response). Through segmental association
of afferents, vasodilatation also occurs in the corresponding deeper organ.
Increased blood supply helps to fight the cause of pain and inflammation in the
deeper organ.
Counterirritants are
generally massaged to relieve headache, muscular pain (torticollis, backache,
sprain), joint pain, pleural/peritoneal pain, colics, etc. Drugs are:
Volatile oils (essential oils) are terpene hydrocarbons
of plant origin having a characteristic odour. They have variable properties,
but all are irritants. Stearoptenes are solid volatile oils.
Turpentine
oil Obtained by distilling Pinus oleoresin; used as counterirritant in the form
of liniment or ‘stupes’.
Cloveoil Applied by cotton swab
for toothache.
Eucalyptus
oil
Used
in pain balms.
Camphor
It is obtained from the bark of Cinnamomum camphora or produced
synthetically. Produces cooling sensation on skin and is mildly
anaesthetic—relieves itching. It is added in liniments and pain balms. Taken
internally— small doses produce a warm and comforting sensation in epigastrium;
large doses are emetic.
Systemically it produces excitement and convulsions (especially
in children).
Thymol Obtained from Thymus vulgaris, has a pungent taste. It is included in pain balms.
Menthol From mint or prepared synthetically, has cooling and soothing action. It is added
to pain balms, throat paints, throat lozenges and inhalers for relief of nasal
congestion. It is also a carminative.
Mustard Seeds It contains a
glycoside sinigrin and an enzyme myrosin. When ground seeds are soaked in water, myrosin hydrolyses
sinigrin to release allyl isothiocyanate
which is a strong irritant. Mustard plaster has been used as rubefacient and
counterirritant. As a suspension in water 4–8 g, of ground seeds are emetic.
Capsicum (Chillies) It is a powerful irritant, hot in taste. The active principle is capsaicin. It is a popular condiment in
Indian cooking, and is included in some counterirritant preparations. After
initial stimulation, capsaicin depletes afferent nerve endings of the transmitter
substance P; may relieve postherpetic neuralgia on local application.
Canthridin A crystalline solid
obtained from Spanish fly. It is a
strong irritant, higher concentrations damage the epithelium and cause
vesication—has been used to remove warts, etc. It is added to hair tonics—claimed
to increase vascularity of scalp and promote hair growth.
Methyl salicylate (oil of wintergreen) In contrast to other
salicylates, it is not used internally (induces vomiting, gastritis and systemic
toxicity). It is combined with other irritants in liniments and ointments for
muscle and joint pain.
Alcohol Produces rubefaction when rubbed on skin and is a vehicle for liniments.
Some Counterirritant Combinations
ALGIPAN: Capsicum oleoresin 0.1%, histamine 0.1%, methyl
nicotinate 1%, glycol salicylate 5% cream.
ARJET SPRAY: Methyl salicylate 875 mg, menthol 1.6 g, camphor
1.5 g, benzyl nicotinate 20 mg, squalance 250 mg, glycol salicylate 875 mg per
50 ml spray.
EUTHERIA: Eucalyptol
7.2%, menthol 4.7%, methylsalicylate 11.25% balm.
MEDICREME:
Methylsalicylate 8%, menthol 2%, adrenaline 0.03%, mephenesin 2.5%,
chlorpheniramine 0.2%, cream.
RELAXYL: Capsicum oleoresin
0.05% mephenesin 10% methyl nicotinate 1% ointment.
VICKS VAPORUB: Menthol
2.8%, camphor 5.25%, thymol 0.1% turpentine oil 5.5% ointment.
IODEX: Methylsalicylate
5%, iodine 4% nonstaining ointment.
AMRUTANJAN: Eucalyptus
oil 17%, camphor 10%, thymol 1%, menthol 4.5%, methylsalicylate 7% ointment.
CAPSIGYLD: Capsaicin
0.075%, methyl salicylate 20%, menthol 10%, camphor 5%, eucalyptus oil 5%,
diclofenac 1% gel.
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