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In Depth Tutorials and Information
compressed upward as a result of short stature.
2
These
chest wall changes can result in poor ventilation, eventu-
ally accompanied by distortion of the trachea and major
airways. Reduction in alveolar ventilation can be general
or localized to compressed areas of the lung. Poorly ven-
tilated lung often has impairment of mucociliary clear-
ance and cough leading to increased risk of colonization
with microorganisms and infection.
But it is in individuals with mild disease that another
aspect of OI-related lung disease has surfaced. When
individuals with OI and no significant chest wall abnor-
malities were studied,
5
it was apparent that there were
intrinsic pulmonary connective tissue abnormalities that
restricted air movement. Since type I collagen is abun-
dant in lung connective tissue, especially in the connec-
tive tissue surrounding alveolar structures, a genetic
abnormality of type I collagen such as is seen in many
forms of OI is likely to cause lung parenchymal abnor-
malities. These produce restrictive defects in pulmonary
physiology. In intrinsic restrictive lung disease the act
of inhalation requires more work and pulmonary gas
exchange can be impaired.
on the capabilities of a particular pulmonary function
laboratory.
The diagnosis of pulmonary problems in a patient
with OI often requires complete pulmonary function
testing including spirometry, lung volumes and mea-
surement of gas diffusion, usually referred to as diffus-
ing capacity in the US. In general, spirometry alone is
reserved for follow-up monitoring, although in some pul-
monary conditions, spirometry alone can be insufficient
even for follow-up visits.
While pulmonary function testing can be important in
OI, in infants and young patients and in those with more
severe forms of OI testing can present unique difficulties.
Pulmonary function measurement is standardized to an
individual's height, age, sex and race. In individuals with
short stature, the usual surrogate for height is arm span,
but fractures can preclude even this workaround. Thus,
a computer generated printout of pulmonary function
testing can interpret a pulmonary function test as abnor-
mal in a healthy but small OI patient, or as normal in
someone with relatively severe disease. The most direct
method of correcting this problem is to get baseline pul-
monary function testing done as soon as an individual
with OI is old enough to perform testing accurately and
reproducibly, usually as a teenager. While there is test-
ing that can evaluate pulmonary function in the pediat-
ric population, these tend to be quite specialized and not
generally available.
Once one has a stable and reproducible baseline,
changes in lung function can be assessed over time. It is
well accepted that after about 20 years of age, everyone's
lung function tends to decline. The major factor affect-
ing prediction equations for normal lung function is age.
Thus, as an example, let's look at forced vital capacity
(FVC), the amount of air an individual is able to exhale
starting from the top of a maximal inhalation and exhal-
ing as forcefully as possible until no more air can be
exhaled. If an individual with no evidence of lung disease
and small stature due to OI has his or her FVC measured,
the spirometer would report this result as the number
of liters of air exhaled and would present a number that
equates to what percentage this number represents com-
pared with the predicted normal for this patient's height,
age, sex and race. If the individual's height is used in this
calculation, the percent predicted will likely be greater
than 100%; that is, they would have been expected to
have relatively small lungs because of their small height
and having normal or near normal sized lungs would
“confuse” this calculation. If arm span were used in
place of height and the individual had normal arm
length, the percent predicted value might be lower than
100%, because even though the lungs were relatively
normal, the chest architecture might be compromised as a
component of their short stature.
DIAGNOSTIC ISSUES IN OI LUNG
DISEASE
The diagnosis of pulmonary disease in OI relies on
measurement of gas exchange as well as evaluation of
pulmonary vessels and airways, patterns of respiration,
airway clearance capacity and respiratory anatomy. As
with any evaluation of the lungs this involves a compre-
hensive history and physical examination, routine and
specialized laboratory testing, radiologic evaluation and
pulmonary function testing.
Pulmonary Function Testing
Pulmonary function testing can be of particular
importance when evaluating individuals with restric-
tive pulmonary diseases and it is this restrictive physi-
ology that tends to predominate the intrinsic lung
problems associated with OI. There tend to be two
“levels” of pulmonary function testing available to
the clinician. The first is spirometry, the measurement
of maximal exhaled air low and volume that can be
done with simple equipment, often in an office setting.
The second is the type of testing that is available in a
hospital-based or freestanding pulmonary function lab-
oratory. In this setting, more complete measurement of
pulmonary physiology can be done as well as special-
ized testing such as simple and extended exercise test-
ing, pressure-volume measurement, sleep studies and
right heart vascular pressure measurement, depending
