Thunderstorm Asthma
Pasture grasses rely on the distribution of
numerous small pollen grains cast over a wide area to ensure reproduction.
A single hectare of ryegrass, for example, may release hundreds
of kilograms of pollen per season. High-speed winds promote the
distribution of pollen grains over many kilometres, although the
absolute concentration will be highest nearest its source.
Not all allergen, however, is contained within intact pollen grains.
Other allergen-carrying particles (as small as 0.1 um diameter)
have been described for grass and tree allergens as well as mold
spores. Unlike most intact pollen grains (generally 2-60 um diameter),
these small particles are capable of reaching the lower airways
and promoting bronchospasm. They may be detected prior to, and
following, the main hay fever season. Their origin has been postulated
to included sap-derived aerosols (such as that released from grass
blades when mowing the lawn), the decay of organic matter or the
leaching of allergen onto other particles from atmospheric pollution.
A significant proportion of patients with allergic rhinitis inappropriately attribute symptoms of tightness and wheeze to their hay fever, and are surprised when appropriate treatment of their upper airway symptoms fails to provide relief. Unless specifically questioned about symptoms, seasonal asthma may, in fact, be the main complaint.
Intuitively one would expect that rain would result in symptom relief by "washing"pollen out of the atmosphere. Paradoxically, some patients experience an exacerbation of symptoms. So-called "thunderstorm" induced asthma attacks have been described in epidemics in Melbourne, Wagga and London. An explanation has been found in recent studies of the kinetics of allergen release and its effects on patients.
One ryegrass allergen (Lol pIX) is located on
the surface of starch granules within pollen grains. A single
pollen grain contains up to 700 starch granules of 0.6 to 2.5
um (small enough to reach the lower airways), and these may be
released with the osmotic shock of exposure to moisture. These
particles rise in concentration up to 50 fold after rain, and
deliberate challenge of patients experiencing "thunderstorm
asthma" has reproduced symptoms. It is interesting to note
that not all patients experiencing acute asthma requiring hospitalization
have had preexistent symptoms. Some have experienced de novo
symptoms, yet almost all have had allergic rhinitis and have been
found to be allergic to ryegrass on testing. Since many patients
with allergic rhinitis will have demonstrable bronchial hyperreactivity
(even in the absence of clinical asthma), presumably the high
load of respireable allergen particles provokes the attacks.
Source:: American Academy of Allergy, Asthma, and Immunology
Appropriate management of chronic "pollen asthma" (which probably has a similar mechanism) includes commencing anti-inflammatory asthma medication either prophylactically or with the first "wheeze" of spring. Some patients desensitised for their allergic rhinitis will experience an improvement in their seasonal asthma as well.
http://www.bmj.com/cgi/content/full/312/7031/601
http://www.bmj.com/cgi/content/full/312/7031/604
http://www.bmj.com/cgi/content/full/312/7031/590
http://www.bmj.com/cgi/content/full/309/6947/131/c
