Healthcare and Medicine Reference
In-Depth Information
Clinical Features
SAC and PAC share the same signs, symptoms and pathogenesis, but differ
in onset and course (Bielory 2008). SAC is an acute or sub-acute condition,
whose manifestations are usually related to specifi c airborne seasonal
allergens, mostly grass and tree pollens. Instead, PAC persists throughout
the year with varying severity and periodical peaks. The allergens involved
are mainly dust mites and animal dander.
The peculiar symptom is itching, accompanied with redness, tearing,
mucous discharge, and burning. Usually both eyes are affected, although
with different degrees of infl ammation.
The conjunctiva looks hyperaemic, oedematous with papillary
hypertrophy on the tarsal plate ( Fig. 1a ). In severe manifestations, also lid
and periorbital oedema may develop. Cornea is never directly involved.
Differential diagnosis may include dry eye syndrome, infectious
conjunctivitis and contact allergic conjunctivitis.
SAC/PAC are mediated by a typical Type 1 hypersensitivity mechanism;
it is an immediate response that lasts clinically for 20-30 minutes.
After the sensitization, allergens bind to specifi c IgE on conjunctival
mast cells, resulting in the release of histamine and other pharmacological
mediators, as proteases, prostaglandins, leukotrienes and cytokines. All
these factors initiate the recruitment of other infl ammatory cells, such
as eosinophils, neutrophils, basophils and T lymphocytes, which are
responsible of the late-phase reaction and maintain local infl ammation
(Chigbu 2009). Specifi c IgE, histamine, eotaxine and ECP levels are increased
in tears of SAC/PAC affected subjects.
The most potent mediator in this immunological mechanism is
histamine, that acts through at least 4 different types of mediators (H1, H2,
H3, H4). It is involved in the regulation of infl ammatory cell migration,
cellular activation, modulation of T-cell function, vascular permeability,
smooth muscle contraction, and mucus secretion. It is responsible for itch
and was found elevated in tears after conjunctival antigen challenge.
Vernal keratoconjunctivitis is a potentially severe infl ammatory ocular
condition that usually affects young children for many years, mostly in
warm and hot regions of the world.
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