From the Guidelines

Asthma is associated with Gastroesophageal Reflux Disease (GERD), but the strength of evidence to support a causal relation varies.

The association between asthma and GERD is complex and often controversial, leading to diagnostic and therapeutic challenges 1.
Asthma is considered a possible extraesophageal manifestation of GERD, along with other symptoms such as cough, laryngeal hoarseness, and dysphonia 1.
However, the causal relationship between GERD and asthma remains unproven, and the existing evidence supports the association between these syndromes, but data substantiating benefit from the treatment of reflux for the extraesophageal syndromes are very weak 1.

Diagnostic algorithms for extraesophageal reflux (EER) are difficult because the manifestations of EER are heterogeneous and often overlap with other conditions 1.
Patients with EER may not complain of heartburn or regurgitation, making it challenging for clinicians to determine whether acid reflux is a contributing factor to the symptoms 1.

Empirical therapy with proton pump inhibitors (PPIs) is a common practice, but the optimal dosage and duration of PPI therapy in patients with extraesophageal GERD syndromes are unclear 1.
Antireflux surgery may be considered in highly selected patients with EER, such as those with concomitant heartburn/regurgitation, a prior response to PPI, and a high burden of acid reflux demonstrated by pH monitoring 1.
Treatment of GERD in patients with asthma may benefit those who have frequent nighttime asthma symptoms or complain of frequent heartburn or pyrosis 1.

From the Research

The relationship between asthma and GERD has been investigated in several studies, with findings suggesting a weak association between the two conditions 2.
A systematic review and meta-analysis found that GERD is associated with asthma exacerbation, particularly in pediatric patients, with an odds ratio (OR) of 1.27 (95% CI 1.18-1.35) 2.
Another study found a bidirectional association between asthma and GERD in children, with a higher risk of developing GERD in children with asthma (OR 2.16,95% CI 1.6-2.91) than the risk of developing asthma in children with GERD (OR 1.55,95% CI 1.32-1.82) 3.

A longitudinal study in UK general practice found that patients with asthma are at a significantly increased risk of developing GERD, mainly during the first year following diagnosis, with a relative risk (RR) of 1.5 (95% CI 1.2 to 1.8) 4.
The same study found a non-significant increase in the risk of developing asthma among GERD patients, with an RR of 1.2 (95% CI 0.9 to 1.6) 4.

The underlying physiologic mechanisms of the relationship between asthma and GERD are not fully understood, but it is thought that asthma medications may promote GERD development in asthmatics 5.
Treatment of GERD may improve asthma outcomes in selected asthmatics, but a large multicenter trial found that treatment of asymptomatic GERD with proton-pump inhibitors did not improve asthma control 6.
Current guidelines recommend a trial of GERD treatment for symptomatic asthmatics, even without symptoms of GERD, but empiric treatment of asymptomatic GERD in asthmatics is not a useful practice 6.