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  Disease summary:

  
  
  
  
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    Asthma is a disease that is defined by reversible airflow obstruction, airway inflammation, and respiratory symptoms of wheezing and cough. The disorder is closely associated with allergy, or immediate type hypersensitivity to aeroallergens. Roughly 70% of all childhood asthmatics are allergic. Epidemiologically the disease is very common, affecting roughly 6% of the US population. Asthma commonly starts in early childhood. It has a variable early natural history; roughly 30% of children have wheezing with colds or occasionally apart from colds in the first year of life. This decreases to about 15% at age 3, with a similar percentage at age 6. Unfortunately, the phenotype does not breed true, and clinicians cannot predict which of those children wheezing at age 1 will be wheezing at age 3 or age 6. Adult asthma is often a recrudescence of childhood disease and has a less clear-cut relationship to allergy. There is a growing body of evidence that persistent asthma leads to reduced growth in lung function and an increased risk for the development of chronic obstructive lung disease (COLD) in adult life. Major morbidity is incurred from uncontrolled disease by reduced quality of life, days lost from work, hospitalizations, and emergency room visits. More details of the clinical features of asthma are given in references 1 and 2.,

  
  
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  Differential diagnosis:

  
  
  
  
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    In children asthma can be confused with chronic bronchitis, cystic fibrosis, bronchopulmonary dysplasia, and bronchiectasis. Asthma in adults is often confused with COLD. It can be differentiated from chronic obstructive pulmonary disease (COPD) via chest x-ray, and spirometry (lung function tests) before and after a short-acting bronchodilator. Occasionally a computed tomography (CT) scan is helpful in distinguishing these two conditions. In children, asthma must be differentiated from bronchiolitis or bronchitis. This is usually done by the same sort of clinical testing, for example, x-ray and spirometry, and clinical response to anti-inflammatory medication.

  
  
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  Monogenic forms:

  
  
  
  
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    None described

  
  
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  Family history:

  
  
  
  
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    Roughly two-thirds of all asthma patients will have a family history of allergies or asthma in a first-degree relative. This does not prove a genetic origin to the disease as this also occurs with any prevalent complex condition of reasonably high prevalence.

  
  
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  Twin studies:

  
  
  
  
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    Concordance for dizygotic twins ranges from 15% to 40% and for monozygotic twins ranges from 50% to 80%.

  
  
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  Environmental factors:

  
  
  
  
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    The most important environmental risk factors are aeroallergens (house dust mite, alternaria, cockroach) and smoking. In utero and postnatal passive smoking are important risk factors in children increasing the disease occurrence from 40%-200%. Active smoking is important in adolescents and adults. Additional environmental exposures of importance are viral infections and pets, particularly cats and dogs. In adults, postmenopausal estrogen is a risk factor.

  
  
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  Genome-wide associations:

  
  
  
  
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    This analytic approach that uses single-nucleotide polymorphisms (SNPs) across the whole genome has identified a three-gene locus on chromosome 17q21 containing ORMDL3, GSDMB, and ZPBP2 as the strongest locus for asthma. Other genes identified by this approach include IL1RL1/IL18, TSLP, HLADQ, IL2RB, SLC22A5, RORA, IL33, and SMAD3.

  
  
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  Pharmacogenomics:

  
  
  
  
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    Genome-wide association studies (GWASs) have implicated the GLCCI1 and the T gene as determinants of change in lung function with inhaled corticosteroid. Short-acting bronchodilator GWAS has implicated SPATS2L as a gene for response to short-acting beta-2 agonists.

Diagnostic Criteria for Asthma

Diagnostic evaluation should include at least two of the following

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   History of intermittent or continuous wheezing or shortness of breath (SOB)

   
   
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   Exertional symptoms of wheeze and/or SOB

   
   
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   Normal chest-x-ray

   
   
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   Normal CT scan of the chest

   
   
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   Obstructive pattern on spirometry (reduced FEV₁, normal forced vital capacity [FVC]) reversal with short-acting bronchodilator. Asthmatics have variable levels of lung function as defined by the FEV₁. This is the amount of air that is breathed out in 1 second after a maximal inhalation. This variability occurs spontaneously, but is best elicited in response to drugs. Asthmatics have an exaggerated bronchodilator response to short-acting beta-2 agonist drugs that act through the beta-2 adrenergic receptors on airway smooth muscle to bronchodilate the airway. They also have an exaggerated response to bronchoconstrictor drugs such as histamine and methacholine and bronchoconstrict dramatically to low doses of these drugs.

And the absence of

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   Any evidence of emphysema or bronchiectasis on chest-x-ray or CT scan

   
   
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   Lack of reversibility of lung function with bronchodilator

Clinical Criteria

Asthma is a syndrome without a sensitive or specific diagnostic test. In general, the symptoms of wheezing and reversible airflow obstruction are sufficient to make the diagnosis.