Idiopathic Chronic Cough: Real Disease or a Failure in Diagnosis?

Chronic cough is a common reason for seeking medical assistance. Approximately 10% of the global population suffers from a chronic cough, severely impairing quality of life. In addition, chronic cough poses many diagnostic challenges since there are complex etiologies that interplay with one another.

Most coughs often have underlying causes diagnosed following a thorough clinical investigation. However, sometimes this diagnosis is of an idiopathic chronic cough, otherwise known as an unexplained chronic cough.

Idiopathic chronic cough has been observed more frequently among women. Studies into cases of chronic cough suggest patients often note an incident they attribute as the inciting moment of their chronic cough. But the prolonged nature and lack of an underlying cause for cough at the time of presentation adds difficulty to understanding idiopathic chronic coughing.

Understanding Idiopathic Chronic Cough

A chronic cough is a cough that has lasted beyond eight weeks. This cough is often persistent and deteriorates the person’s quality of life over time. The most common causes of a chronic cough are found to be postnasal drip, asthma, and gastroesophageal reflux disease.

However, even though the majority of the chronic cough presentations can be categorized following investigation, approximately 20% of chronic coughs remain undiagnosed even after systemic investigation and basic treatment trials are carried out.

What is Classified as an Idiopathic Cough?

An idiopathic cough is diagnosed when a series of tests and treatment trials have failed to find a cause for someone’s chronic cough.

During the patient’s initial presentation, it is vital to consider potential underlying causes for coughs, including conditions that can aggravate cough reflexes. This includes recent viral infections, current medications, chronic respiratory conditions, cardiac health, and gastroesophageal function.

Additionally, it is also crucial to thoroughly document medical history, particularly for conditions having autoimmune components. Organ-specific autoimmune conditions, such as hyperthyroidism, hypothyroidism, pernicious anemia, celiac disease, vitiligo, and autoimmune hepatitis, are frequently observed among those who have an idiopathic chronic cough.

When documenting a chronic cough, it is also vital to note down any additional symptoms present during a clinical examination. Chronic cough can often be accompanied with -

• Hoarseness
• Wheezing
• Sputum or blood with cough
• Chest discomfort
• Runny nose
• Constant discomfort in the throat

In most cases, the only presentation is a chronic cough, often recorded as persistent, and can be either of a dry or productive variant.

Pathogenesis of Idiopathic Chronic Cough

Idiopathic conditions do not have a particular cause linking them to the outcome observed.

One postulated cause of idiopathic chronic coughs is the modification of the “plasticity” of the vagal afferent cough receptors. Neurons can undergo a lot of changes that lead to increased cough sensitization are still vastly unknown. Persistent inflammation and accumulation of inflammatory mediators following an upper viral respiratory infection have been linked to this altered cough sensitivity. However, as of yet, it cannot be conclusively associated as a cause for the presentation of chronic cough.

Women have been recorded to have higher cough sensitivities among both healthy individuals and cough subjects. This sex difference is more frequently observed among adults than children, highlighting the possibility of hormones playing a role in chronic coughing cases.

Autoimmune processes have also been linked with unexplained chronic cough. Among patients with documented autoimmunity, the constant exposure to autoantibodies can lead to the homing of certain lymphocytes from the target organ (e.g., thyroid) to the lung and cough receptors. Studies of bronchoalveolar lavage among patients with autoimmune diseases and chronic idiopathic cough have recorded excess lymphocytes, suggesting a possible link

Studies have still, however, not linked a specific cause to chronic idiopathic cough. It could in fact be a syndrome of many different disorders currently lumped together under one name, rather than a pathology with a single cause.

This naturally creates the question as to whether, when faced with labeling someone with idiopathic chronic cough, the diagnosis has been inefficient or treatment inadequate.

Diagnosing Idiopathic Chronic Cough

When presenting with any chronic cough, patients should be recommended to track their coughs, especially when inciting events, such as respiratory infection, which might be a cause of a chronic cough.

Preliminary Tests During Chronic Cough Presentation

The initial presentation of idiopathic chronic cough is of a patient who has a cough that has lasted for greater than eight weeks. During clinical examination, any pertinent symptoms and history of recent illness or medications should be recorded.

Ideally, during clinical examination, possible underlying disease processes are indicated. Symptoms such as a runny nose, fever, metallic taste in the mouth, or dyspnea can suggest respiratory illness.

For any presentation of a chronic cough, the following tests should be indicated along with routine blood investigations -

• Chest X-ray
• CT scan
• Lung function tests
• Methacholine provocation
• ECG monitoring
• 24h esophageal pH monitoring
• Sputum examination

These basic tests are capable of analyzing the most frequent causes of chronic coughing. Fiberoptic bronchoscopy might be indicated especially for those who have an autoimmune process, to investigate possible lung involvement. Autoantibodies are also a vital facet to consider when investigating the likely cause of idiopathic cough — many people presenting with idiopathic chronic cough have autoimmune disorders.

Hopefully, one or more of these tests will lead to the cough’s cause, so that it can be treated. However, sometimes there are no useful results from the test.

Diagnosing an Unexplained Cough

Among patients with idiopathic chronic cough, symptoms are minimal; most often, only the chronic cough is present. Clinical examination of systems, such as the ear, nose, throat, and other respiratory systems, is unlikely to show any evident clinical findings.

A study has highlighted the need for long-term documentation for those with unexplained coughs. Outcomes for these patients over time are uncertain. There was a decline in lung function (FEV1) of about 44ml/year among study subjects. A small percentage of patients also documented airflow obstruction over time.

While conducting diagnostic tests might seem futile in these cases, it is crucial to consider changes that could be documented over time. Documentation also contributes to big data for idiopathic chronic cough, for which the basic understanding for diagnosis and management is still in its infancy.

Treating Idiopathic Chronic Cough

Once likely causes such as infection, eosinophilic bronchitis, gastroesophageal reflux, postnasal drip, and asthma are ruled out, approaching treatment trials can be considered.

Considering A Therapeutic Approach

Since the diagnostic phase has not highlighted an underlying cause, it then becomes vital to limit the chance of over-medicating, as that could become another source further aggravating cough. Patients should have already documented their use of over-the-counter medications to assist with reducing the cough, which makes prescribing them again futile.

A trial of inhaled corticosteroids could be considered to reduce potential airway inflammation. Postulated causes for idiopathic chronic cough indicate that inflammation could play a role in its pathogenesis. Additionally, causes such as rhinitis, asthma, and eosinophilic bronchitis can also be managed with inhaled corticosteroids. While this cannot determine a deficiency in diagnosis, it can facilitate the improvement in patient quality of life.

Other drugs to be considered are neuromodulators since alterations in cough sensitivity have been indicated as a possible cause of unexplained chronic cough. Gabapentin has been proven to be effective among patients who have a refractory chronic cough. It modulates the cough sensitization at the central reflex centers, enabling improvement within eight weeks of treatment. Additionally, gabapentin is well-tolerated by most in small doses.

A viral infection can often be an inciting incident following the presence of a chronic cough, which is later diagnosed as idiopathic chronic cough. Vagal neuropathy has been reported among those with a post-viral chronic cough. Studies have indicated effectiveness in the trial of amitriptyline to manage these chronic coughs. Similarly, this could be indicated for idiopathic chronic cough, especially for those who have reported viral infection during the initial presentation.

Opiates have long been documented as antitussive agents for cough relief. Studies have indicated trials of morphine might prove to have some benefits in managing chronic cough. Morphine has been shown to improve cough frequency but does not alter cough sensitivity.

Randomized controlled trials have also investigated the non-pharmacological approach to managing idiopathic cough, such as speech therapy. Breathing, cough suppression techniques, and counseling were suggested for these study subjects. These studies recorded an improvement in cough severity and frequency for those with a chronic cough.

Advising Patients On Managing Their Chronic Cough

For those who present with an idiopathic chronic cough, the outlook to management is one of trial and error. There is no one-size-fits-all approach, which makes regular follow-up and patient counseling cornerstones of management.

The approach toward managing unexplained chronic cough should always include investigating possible causes which could have been missed during diagnosis. Patients should be encouraged to seek more than one option from respiratory specialists to be able to have additional perspectives on their treatment strategy.

Patients should be advised to diligently track their symptoms and remain compliant with medications prescribed for their cough and other health conditions. Changes in doses should be monitored, along with their effect on the chronic cough. The onset of new health conditions, such as heart failure or tumors, can contribute to variations in the cough presentation. This can then confound the overall investigative results each time a patient is monitored solely for their cough.

As a healthcare professional, it becomes vital to liaise with other specialists to have a more holistic outlook on a patient’s diagnosis and treatment.

Conclusion

An idiopathic chronic cough is a complex condition, with very little insight into its causes and effect. The key facet for treating such a chronic cough is to improve a patient’s quality of life, which visibly deteriorates.

As highlighted above, some studies show that having a chronic cough can decrease lung function over time, making studies on more effective treatment strategies crucial for improving patient outcomes.

With all idiopathic conditions, it is difficult to understand inciting incidents, which could assist with prevention. In such scenarios, it becomes important to encourage all individuals to track all symptoms and general health regularly, to facilitate a better understanding of conditions that have no known causes.