The key to success when utilizing topical therapy involves an accurate assessment of the lesion or problem as well as an understanding
of the basic principles of topical preparations. In addition, knowledge of the various formulations available as well as
of the drugs and their limitations when applied topically, along with choosing an appropriate physical mode of therapy are
all critical factors to consider when utilizing topical therapy.
Pharmacokinetics: the skin
The skin serves as a protective organ and maintains an effective barrier, thus preventing escape of water and essential electrolytes
as well as keeping harmful substances out. This is accomplished via a two-compartment system comprised of hydrophobic lipids
and hydrophilic proteins. Vehicles utilized in topical therapy serve not only to increase penetration, especially through
the hydrophobic lipid portion, but also to decrease permeation and target the skin appendages, in some situations. Concentration
and solubility of drug in the vehicle agent also play a role in the effectiveness of a topical product. Most topically applied
drugs are absorbed systemically to some degree, influenced by dermal blood flow, increased body temperature and vasoconstriction.
Types of topicals
Emulsions are a dispersion of two or more immiscible liquid phases. Alone they are unstable and will separate, however when
combined with an emulsifying agent, such as an ionic (sodium lauryl sulfate) or nonionic (polysorbate, sorbitan ester) surfactant
they can be used effectively in topical form.
These come in the form of wet dressings, baths or soaks, lotions and aerosols or sprays. Wet dressings are used to hydrate
the skin and may be useful when the goal is to debride necrotic or devitalized tissue. Bathing therapy serves to cleanse
the skin and hair coat, deliver an active ingredient to the affected skin and is very helpful in decreasing exposure to environmental
allergens. This is especially of importance when managing patients with atopic dermatitis. Hydrotherapy can either rehydrate
or even dehydrate the skin when an occlusive type rinse is not part of the regimen. Novasomes and spherulites are types of
sustained-release microvesicle technology that improves the ability to maintain skin hydration, in particular. Rinses or
dips are medications in a water-soluble form, typically associated with antiparasitic agents and antifungal agents. Lotions
are suspensions of powders in water that dry leaving a fine powder of medication behind. The residual, or leave-on products
are available in this form.
Powders are uncommonly used in veterinary medicine since they are often messy and difficult to use in abundantly haired patients.
They do promote drying and commonly contain talk or some type of starch. Some formulations contain potent topical corticosteroids
that can cause significant suppression of the hypothalamic-pituitary-adrenal axis if used frequently.
Most of the localized topical agents utilized in veterinary medicine come in the form of creams, gels, ointments, pastes and
fixed dressings. Creams are emulsions of oil-in-water that are miscible with the skin and absorb water, thus they are non-occlusive
and often desirable treatment delivery forms for our dermatologic patients. Gels are colloidal dispersions that liquefy upon
contact with the skin and are also non-occlusive in nature. Ointments can be water soluble, containing polyethylene glycol,
or emulsifiable, containing water-in-oil mixtures. Water repellant ointments contain white petroleum or mineral oil. Most
are occlusive, however these provide better penetration than creams or lotions. Incorporating powders into an ointment
creates a paste. They are well tolerated with residual activity, however because they are messy they are rarely used in veterinary