Whilst sat nursing several strained intercostal muscles following a particularly severe bought of coughing on return from a recent trip to Nepal I thought it prudent to consider the evidence behind the notorious ‘Kumbu cough’.
A cough at high altitude has long been recognised as a hindrance and was originally thought to occur as a result of the low temperatures and low humidity associated with high altitudes. (Although, interestingly, such a couch is not well described in polar regions where similar ambient conditions exist). It has been known to cause muscle strains and even rib fractures.
The most comprehensive review article I could find on ‘Altitude-related cough’ was published in 2013 in the Journal ‘Cough’ and is authored by Nicholas Mason. It is freely available on PubMed (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4176487/).
This paper highlights that high altitude cough is common, in a survey of 283 trekkers in the Everest region in Nepal, 42% suffered from a cough.
Various mechanism of this mysterious cough were considered and are summarised below:
1. Acute Mountain Sickness (AMS)
There has been no demonstrable relationship between acute mountain sickness and cough in any of the papers considered and is though unlikely to play a significant role.
2. Sub clinical high altitude pulmonary oedema (HAPE)
HAPE is a non-cardiogenic, hydrostatic pulmonary oedema which occurs predominantly in unacclamatised individuals up to 72 hours after ascent to a new altitude. Thought to be related to increased pulmonary artery pressures, treatment focuses on the acute reduction of this pulmonary hypertension in order to facilitate descent.
What remains unclear however is if pulmonary oedema can cause cough and this lack of understanding stems from an incomplete understanding of the complex pathways involved in the cough reflex. What is clear is that many anecdotal cases of high altitude cough (Including mine currently!) have persisted on return to sea level which would seem to rule out HAPE as a causative factor as any small rise in pulmonary artery pressures which may have been a causative factor, would have returned to normal.
3. Central control of cough
Part of the acclimatisation process involved the hypoxic ventilator response and therefore an increase in ventilation. It is hypothesised that these changes in central respiratory control could be responsible for high altitude cough, there has been no evidence to support this though.
4. Respiratory water loss
Water loss from the respiratory tract is thought to play a large contributory role in altitude related cough and there is much evidence to support this. The exact mechanisms of how this water loss stimulates afferent cough pathways remains up for debate.
5. Respiratory tract infections
Infections are the commonest cause of acute cough at sea level and likely contribute to many coughs at altitude. Evidence has shown an increased predisposition to developing an upper respiratory tract infection at altitude but the microbiological aetiology remains unclear and more research is needed.
Cough may be the only presenting symptom of asthma and bronchoconstriction can be exacerbated by exposure to cold air. It is unlikely to play a significant role in non-asthmatic patients.
7. Vasomotor rhinitis and post nasal drip
Post nasal drip can cause a chronic cough but its relationship with a cough at high altitude is unknown.
8. Gastro oesophageal reflux disease
A cough is a common sign of GORD but its prevalence at high altitude is unknown.
This paper concludes that, as a sea level, altitude related cough is a combination of a number of possible perturbations in the cough reflex arc that may exist independently or together. The most likely causative factors are infection or water loss from the respiratory tract leading to trauma of the respiratory mucosa and trachea-bronchitis. More work is clearly needed to investigate these relationships further.
Mason, N. Altitude-related cough. Cough. 2013;9:23
Ericsson, C. et al. Infections at High Altitude. Clinical Infectious Diseases. 2001. 33(11):1887-1891
Litch, J. Tuggy, M. Cough induced stress fracture and arthropathy of the ribs at extreme altitude. Int J Sports Med. 1998. 19(3):220-2