DISCUSSION
Taking together, these results identified independent factors associated
with subsequent exacerbation frequency rates within the year following
fibreoptic bronchoscopy in severe preschool wheezers. These factors
include atopic dermatitis, age at first episode of wheezing, the
presence of Haemophilus in the BALF, the RBM-BSM distance and the
BSM area. The BSM area was the most important factor and was associated
with each exacerbation frequency group.
The risk of subsequent exacerbations has been previously associated with
a range of various parameters such as markers of uncontrolled disease
(i.e. , uncontrolled symptoms of wheezing, at least one severe
exacerbation in the previous year) in preschool wheezers (12,17). Thus,
it was not surprising to note that three quarters of the patients in our
study that included severe preschool children who experienced at least
one exacerbation or one hospitalization for wheeze within a year prior
to inclusion, continued to experience exacerbations during the follow-up
year. Moreover, using univariate multinominal regression analysis, we
identified a significant association between the number of
hospitalizations within a year prior to inclusion and the exacerbation
frequency in the follow-up year in agreement with previous reports
(12,17).
The risk of exacerbations in preschool wheezers has also been associated
with type-2 inflammation, highlighted by the presence of atopic
dermatitis or allergic sensitizations, increased levels of blood
eosinophils and/or exhaled NO levels (17–21). On the one hand, we do
confirm, in the present study, that atopic dermatitis was an independent
risk factor associated with the exacerbation frequency in severe
preschool wheezers and that its prevalence was increased in the high
exacerbation group in agreement with previous studies (17–19,21).
However, the presence of atopic dermatitis was also associated with the
no-exacerbation group. Therefore, this result suggest that isolated
atopic dermatitis was not a risk factor of subsequent exacerbations but
when combined with other exacerbation risk factors, its presence
increased this risk. On the other hand, previous studies demonstrated an
increased risk of exacerbations in atopic and/or smoking mothers’
infants with elevated exhaled NO levels after birth (20) and in
preschool children with single (17) or early multiple allergic
sensitizations (18,21) or increased blood eosinophils (19). However, we
did not find any difference in exacerbation rate according to allergic
sensitization or blood eosinophils levels. The association between
exacerbation risk and blood eosinophil levels or allergic sensitization
has previously been reported in populations including patients treated
or not with ICS (17–19,21). In our study, all patients were treated at
inclusion with high doses of ICS with a second controller which may have
decreased the impact of both blood eosinophils level and allergic
sensitization on the risk of exacerbations (22). Indeed, Fitzpatricket al . showed that the risk of exacerbations was significantly
decreased by ICS and abolished the statistical significance between the
sensitized or blood eosinophils based groups (18,19).
Non-type 2 inflammation related parameters (i.e. , neutrophilic
inflammation, isolation of bacteria on BALF culture) have also been
associated with an increased risk of subsequent exacerbations (9,11).
Despite no significant differences between exacerbation frequency-groups
according to BALF and bronchial inflammation, the univariate analysis
showed that a decreased percentage of macrophages and an increased
percentage of PMN were significantly associated with infrequent
exacerbations whereas a low density of lymphocytes in bronchial biopsies
was a significant risk factor for frequent exacerbations. Unfortunately,
these parameters were not included in the multivariate analysis due to
the great number of missing values. We also demonstrated that bacteria
in the BALF represent an independent risk factor for subsequent
exacerbations but was mostly associated with a low annual exacerbation
rate. This was consistent with studies reporting the efficacy of
antibiotic or bacterial lysate therapies to decrease wheezing attacks
rates and the use of oral corticosteroids compared to placebo
(11,23,24).
In the present study, the risk of subsequent exacerbations was
associated with several bronchial remodelling parameters (i.e .,
submucosal fibrosis area, BSM area, RBM thickness, RBM-BSM distance) as
previously reported (25–28). However, only the BSM area and the RBM-BSM
distance appeared as independent risk factors of subsequent
exacerbations in severe preschool wheezers using multivariate analysis.
Moreover, it is interesting to note that the group with the highest
exacerbation frequency is characterized by the greatest BSM area, the
lowest RBM-BSM distance and the earliest age at first episode of
wheezing. Thus, it is tempting to speculate that early-life respiratory
tract infections promote BSM remodelling which in turn increased the
risk of virus- and/or bacteria-induced exacerbation creating a vicious
circle. Indeed, the first year of life appeared as a period of high
susceptibility of BSM remodelling (29) characterized by an increased
mitochondria-dependent BSM cell proliferation (30), which has been
involved in BSM remodelling of both preschool wheezers (31) and adult
asthma (32). Moreover, in adult asthma, we recently demonstrated an
increased viral replication within the bronchial epithelium induced by
the BSM which can increased exacerbation susceptibility (33) and in turn
induced a specific BSM cell migration toward bronchial epithelium
decreasing RBM-BSM distance (34). However, further mechanistic studies
would be necessary to assess both the underlying mechanisms of increased
BSM mass and the impact of BSM remodelling on viral replication in the
specific population of preschool wheezers.
All these findings reinforce the usefulness of fibreoptic bronchoscopy
in the specific population of severe preschool wheezers. Indeed, while
it can be argued that fibreoptic bronchoscopy with bronchial biopsies is
an invasive procedure requiring anaesthesia and of limited interest
especially in such young children. Our study has, on the contrary,
demonstrated its usefulness to provide information on bronchial
inflammation, dysbiosis and remodelling and then on the risk of
subsequent exacerbations. This well-tolerated examination allows for
better characterization of the disease and helps the clinician to
optimize the therapeutic strategy with a favourable benefit-risk ratio
in severe preschool wheezing (14).
Several limits of the present study must be pointed out: (i ) The
relatively small number of patients limited the size of each group may
have increased the risk of type 2 error in the group comparisons and the
95% confidences interval in the logistic regression analysis. In
addition, the small size of our population has limited the maximal
number of parameters included in the multinomial logistic regression
model number. Indeed, some of the significant parameters in univariate
analyses (i.e. , Macrophages and PMN in the BALF as well as
density of lymphocytes) had too many missing values in bronchial
biopsies, and were assessed only in the P’tit Asthme” cohort (14).
However, this pooled cohort represent the largest cohort describing both
clinical and fibreoptic bronchoscopy parameters in severe preschool
wheezers. (ii ) As highlighted recently (35), our data did not
include long term follow-up data (i.e ., more than one year)
allowing for the assessment of the stability of exacerbation frequency
over time and the consequences of bronchial remodelling on lung function
and asthma persistence at school age. Additional, long term longitudinal
studies are thus necessary.