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Occupational Medicine2014;64:483489Advance Access publication 7 July 2014 doi:10.1093/occmed/kqu094

The Author 2014. Published by Oxford University Press on behalf of the Society of Occupational Medicine.All rights reserved. For Permissions, please email: [email protected]

Risk factors for occupational acute mountain

sickness

D. Vinnikov1, N. Brimkulov2, V. Krasotski1, R. Redding-Jones1and P. D. Blanc3,4

1Medical Department, Kumtor Gold Company, Bishkek 720017, Kyrgyzstan, 2Chair of Internal Diseases, Kyrgyz State Medical

Academy, Bishkek 720020, Kyrgyzstan, 3Department, Division of Occupational and Environmental Medicine, University of

California San Francisco, San Francisco, CA 94143-0924, USA, 4Department of Occupational and Environmental Medicine,

University of Gothenburg, Gothenburg 40530, Sweden.

Correspondence to: D. Vinnikov, Medical Department, Kumtor Gold Company, Isanova Street, 25A, Bishkek 720017,

Kyrgyzstan. Tel: +996 772 864260; fax: +996 312 900740; e-mail: [email protected]

Background Studies of occupational acute mountain sickness (AMS) have not focused on the more severe end of

the spectrum to date.

Aims To examine risk factors associated with the development of occupational AMS severe enough to

receive treatment in a compression chamber.

Methods A nested case referent study in a cohort of high-altitude (4000 m) mine workers, comparing cases

of severe, chamber-treated AMS to matched referents. Using logistic regression, we tested potential

risk factors based on premorbid surveillance examinations, including cigarette smoking (current

smoking, smoking intensity and exhaled carbon monoxide [CO]).

Results There were 15 cases and 30 controls. In multivariate analysis including age, sex and place of resi-

dence, current smoking was associated with increased risk of severe AMS (odds ratio [OR] 10.0;

95% confidence interval [CI] 1.567.4), taking into account any prior, less severe AMS event, which

was also a potent risk factor (OR 33.3; 95% CI 2.8390). Smoking intensity (cigarettes per day) and

exhaled CO were also statistically significantly associated with severe AMS.

Conclusions Cigarette smoking is a strong, previously under-appreciated risk factor for severe AMS. Because

this is a modifiable factor, these findings suggest that workplace-based smoking cessation should be

tested as an intervention to prevent such morbidity.Key words Altitude; mining; mountain; occupation; smoking.

Introduction

Acute mountain sickness (AMS) is a common complica-

tion of hypobaric environmental stress [1,2]. It can be

associated with significant morbidity leading to compro-

mise in both physical and cognitive functioning. AMS

remains a persistent and even growing problem, despite

decades of research into the factors that may influence itsincidence and severity. Beyond the well-established AMS

risk factors such as degree of elevation, rate of ascent to

a given altitude [13] and level of exertion under hypo-

baric conditions [4], the identification of other poten-

tially modifiable variables remains elusive. Obesity and

migraine history have been shown to impart AMS risk

[5,6], but beyond that individual susceptibility is gen-

erally considered an idiosyncratic phenomenon [710].

Thus, risk profiling has focused on physiological charac-

teristics of an AMS-prone state, such as excessive desat-

uration under hypobaric conditions, a low ventilatory

response to exercise or variability in nitric oxide metabo-

lism [4,11].

It is unclear whether certain established or suspectedfactors are associated with an increased risk of severe

AMS. These factors include obesity, decrements in lung

function, hypertension and previous less severe AMS

episodes that have not led to chamber treatment. There

is also a need to assess whether current cigarette smok-

ing might be associated with AMS as smoking reduction

is an achievable workplace wellness intervention. We

have previously shown in a longitudinal analysis that a
mailto:[email protected]?subject=mailto:[email protected]?subject=

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484 OCCUPATIONAL MEDICINE

secondhand smoke exposure reduction policy that was

successfully instituted at the mine site was temporally

associated with improved lung function [12]. We, there-

fore, carried out this study to examine AMS risk through

a retrospective analysis of cases in a population employed

at high altitude.

MethodsStudy data were extracted from the clinical illness case

files and baseline hire and annual surveillance exami-

nation records of the occupational health service for

an active gold mine operating at an altitude of 3800

4200 m. The mine is located in the Tyan Shan Mountains

of the Kyrgyz Republic, ~400 km from its capital, Bishkek.

This occupational cohort, provided a unique opportu-

nity to study AMS. First, this is a relatively large cohort

and AMS is an endemic problem in this setting. Second,

there is a medical service at the mine allowing easy iden-

tification of people with symptoms of AMS and on-site

use of a compression (hyperbaric) chamber for treat-ment of clinically severe cases. Finally, electronic medical

records including premorbid surveillance data facili-

tated the identification of cases as well as the selection

of matched referent mine workers without compression

chamber-treated AMS, permitting a nested casecontrol

study. Mineworkers generally work 2 week periods on-

site, with a 12 hour on, 12 hour off shift schedule while

at the mine. Mining personnel commute on buses from

their places of permanent residence either in the Issykul

Lake plateau area of Kyrgyzstan (1600 m elevation) or in

Bishkek and its environs (700 m).

All mine employees undergo pre-placement and

annual health surveillance screening in Bishkek, whichprovided the premorbid data for both cases and referents.

Although screening does select out individuals based on

the presence of selected health conditions, prior AMS

events are not considered an employment exclusion cri-

terion. The medical database includes all screening data

and test results for each employee as well as records

of any symptomatic mine clinic visits, with mandatory

entry of an associated diagnosis code. This includes sep-

arate coding options for mountain sickness (MS) and

AMS. Although distinct options, they are applied too

interchangeably in practice to allow their use as sepa-

rate distinct diagnostic entities for these purposes of this

investigation.We queried this database to identify all on-site mine

clinic intake visits involving a diagnosis of MS or AMS

during a period of 4 years, starting on 1 January 2009 and

ending on 31 December 2012. The calendar year 2009

marked the introduction of the medical electronic soft-

ware system. The MS or AMS codes are typically entered

when employees present with complaints that are con-

sistent with high-altitude syndromes and when another

aetiology for such symptoms is not readily apparent.

Management of AMS cases is based on conventional

assessment and management protocols, with cases des-

ignated as mild, moderate or severe [1,2]. Symptoms

that prompt the clinical diagnosis include persistent

headache (especially with a poor response to acetami-

nophen [paracetamol] or ibuprofen), dizziness, shortness

of breath, gastrointestinal complaints, sleep disturbance

and fatigue. Suspected MS/AMS is generally treated

empirically with acetazolamide 125 mg orally every 12

hours and with inhalation of supplementary oxygen for

at least 30 minutes, following which the patient is reas-

sessed clinically. In practice, systematic standard elicita-

tion of complaints that could yield Lake Louise Scoring

(LLS) is not recorded. This tool is a scoring system to

assess the presence and severity of AMS, which quanti-

fies headache with other symptoms of AMS [13].

The occupational medicine clinic on-site at the mine

is equipped with a fully functional multiplace hyper-

baric chamber (Southern Oceanic, Republic of South

Africa). Patients considered on clinical grounds to be suf-

fering from severe AMS are treated in the chamber. Werestricted the case definition of AMS to those persons who

were deemed ill enough to undergo compression chamber

treatment. We selected two referents for each case, one on

each side of a numeric sorting by employee identification

number. When annual surveillance data were missing (e.g.

when that employee was no longer in the workforce), we

selected the next listed employee. The mean time elapsed

from the screening examination until the incident AMS

event was 3.5 months among the cases and 4 months

among the incidence density-matched referent employees.

Annual screening data closest in date prior to the

event were used to derive demographics, place of per-

manent residence, physical examination and testing dataand duration of employment. Clinical data included

height and weight (yielding body mass index [BMI]),

blood pressure, findings of fundoscopic examination by

a trained ophthalmologist (a routine in the surveillance

protocol), pulmonary function test results (MicroLoop

portable spirometer with Spida5 software, FusionCare,

Hampshire, UK) and electrocardiogram (ECG) assess-

ment. The cigarette smoking status of each mine

employee is reassessed at every annual screening. Self-

reported daily smoking at the time of examination and

numbers of cigarettes smoked per day are elicited and

further assessed with an exhaled carbon monoxide (CO)

measurement by means of portable breath CO analyser(Smokerlyzer, Bedfont, UK).

Because the medical electronic record system did not

cover the earlier period between hire and the years of

our study, prior AMS events were identified through

review of hard copy medical files. From this data source,

the number and date of such episodes along with their

severity and treatment were extracted. We excluded any

potential referent with a record of prior chamber treat-

ment: this had occurred in only one potential referent

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D. VINNIKOV ET AL.: RISK FACTORS FOR OCCUPATIONAL AMS 485

and an employee with the closest adjacent identifica-

tion number was substituted. No previous chamber

treatments were identified among the cases prior to the

case-defining event.

We used logistic regression analysis to estimate the

relative odds of severe AMS (defined by compression

chamber treatment) using a hierarchical approach to

model building. In the first step, we compared cases and

referents in terms of two groups of variables: potential

study confounders and potential risk factors of pri-

mary study interest (t-test, Fishers exact test or 2*2

tables were used). For confounders, we assessed dif-

ferences between cases and referents in age, sex and

altitude of residence (because those dwelling at higher

altitude might be protected from AMS). Among risk

factors for disease, we were interested in obesity, lung

function, hypertension or its sequelae, previous MS/

AMS and cigarette smoking. The latter was of partic-

ular interest because it is potentially modifiable at the

workplace [12]. Moreover, in addition to self-reported

smoking status, we also had exhaled CO measurements.The characteristics of cases and referents are shown in

Table 1. We limited logistic regression modelling only

to those variables of primary study interest that dif-

fered between cases and referents (P< 0.20). We tested

each of these in bivariate models and then in multi-

variate models adjusted for age, sex and place of resi-

dence (Issykul plateau versus Bishkek and its environs).

We further explored the relationship between cigarette

smoking and AMS, taking into account previous AMS

events. To assess potential mediation and effect modera-

tion, we tested these models with and without a prior

AMS*smoking interaction term. These final models did

not include sex, age and residence. The study protocol

was reviewed and approved by the committee on bioeth-

ics of the Kyrgyz State Medical Academy.

Results

MS/AMS cases amounted to 216 in 2012, 388 in 2011,

350 in 2010 and 350 in 2009, a total of 1304. Of these,

15 (1%) underwent chamber pressurization treatment

with 7 treated in 2009, 1 in 2010, 3 in 2011 and 4 in

2012. As shown in Table 1, the study population was

predominantly male, but females were over-represented

among cases and cases were slightly younger than refer-

ents (P< 0.20, meeting criteria for model inclusion; see

Methods). Most saliently, a substantially greater propor-

tion of cases had experienced at least one prior episode

of less severe AMS compared with referents (53% ver-sus 7%; P < 0.001). Lung function parameters, blood

pressure and BMI were similar among cases and refer-

ents. Resting heart rates were marginally faster among

the cases compared with referents, but not at a level of

statistical significance. Cigarette smoking status differed

between cases and referents for each of the three meas-

ures studied: there were almost twice as many smokers

in the case group, they smoked twice as many cigarettes

Table 1. Initial risk factors of interest in cases and referents

Indicators Cases, n(%) Referents, n(%) P-value

N 15 30

Male/female 11/4 (73/27) 27/3 (90/10) NS

Issykul residents/lowland residents 9/6 (60/40) 17/13 (57/43) NS

Age in years, mean SD 33 7 36 9 NS

Employment duration in years, mean SD 5.9 3.1 8.6 4.3

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per day and their exhaled CO level was higher compared

with referents (Table 1).

The clinical symptoms and treatment of the 15 cases

of severe AMS are summarized in Table 2. The most

prevalent symptom group was neurological, with at least

one of the following symptoms present: headache, diz-

ziness or numbness of the extremities. Gastrointestinal

and cardiopulmonary symptoms were less common. In

accordance with standard recommendations [13], all

cases were treated with acetazolamide and systemic cor-

ticosteroids were administered in all but two. By study

definition, all 15 cases underwent compression cham-

ber treatment. None evolved into more serious disease

requiring emergency transport to lower altitude (the

protocol for high-altitude pulmonary oedema or high-

altitude cerebral oedema).

Based on a prioricriteria, we tested six independent

variables as risk factors for AMS (based on their bivari-

ate associations inTable 1; see Methods). These analy-

ses are shown inTable 3, which presents the unadjusted

odds ratio (OR; consistent with the previous bivariatedata) and then the OR adjusted for age, sex and place

of residence. We chose to include age, sex and residence

in the multivariate model because they were potential

confounding variables. Work duration was negatively

associated with severe AMS, while increased heart rate

was positively associated, although neither factor was

statistically significant in these models. In contrast, both

previous AMS and current cigarette smoking (assessed

dichotomously or by intensity) were associated with

increased odds of severe AMS. Smoking adjusted for

age, sex and place of residence increased the odds for

severe AMS by 10 times, while any previous but less

severe AMS increased odds by an even greater margin

(Table 3). Smoking, ascertained both by the number

of cigarettes smoked per day and the concentration of

exhaled CO, was tested in separate multivariate mod-

els included both AMS and an AMS*smoking interac-

tion term for each of the smoking measures (Table 4).

The interaction terms were not statistically significant

in either model. Moreover, the effect estimates for ciga-

rettes per day and exhaled CO were similar with (Model

2) or without the interaction term (Model 1) in the

model, although the prior AMS effect estimate became

less stable with inclusion of the interaction term.

Discussion

We found that active cigarette smoking was a potent

risk factor for severe AMS. The cigarette smoking

exposure-response was noted both for self-reported

smoking quantities and exhaled CO and was robusteven after taking into account any prior AMS event.

Moreover, prior AMS risk, as anticipated and sup-

porting our study methodology, was a powerful inde-

pendent risk factor for recurrent AMS, consistent with

the findings of other epidemiological investigations

[3,8,10,1416].

The chief focus in investigations of AMS risk has been

on physiological variables that predispose to adverse out-

comes, such as rate of ascent [1417], obesity [5], oxygen

desaturation [9] and hypoxic tolerance [4]. One prospec-

tive cohort study of six different physiological measures

(oxygen desaturation, low heart rate and low ventilator

rate, each at rest and at exercise) found all predisposedto severe high-altitude-related illness [4].

The potential importance of cigarette smoking

in AMS varies with the population studied. Among

mountain climbing enthusiasts, the prevalence of

active cigarette smoking is generally low (e.g. 9% in

Table 2. Symptoms reported and treatment administered among

15 cases of AMS

n(%)

Symptoms

Neurological

Headache 11 (73)

Dizziness 7 (47)

Numbness of extremities 4 (27)

Any neurological symptom 15 (100)

Cardiopulmonary

Palpitations 3 (20)

Shortness of breath 1 (7)

Any cardiopulmonary symptom 4 (27)

Gastrointestinal Nausea 6 (40)

Vomiting 3 (20)

Any gastrointestinal symptom 7 (47)

Other

Disturbed sleep 3 (20)

Fatigue 2 (13)

Treatments

Acetazolamide 15 (100)

Dexamethasone 13 (87)

Compression chamber treatment (case definition) 15 (100)

Table 3. Logistic regression of selected variables for severe AMS

Variable OR unadjusted OR adjusted for

age, sex and place

of residence

OR 95% CI OR 95% CI

Duration of employment 0.8 0.71.0 0.9 0.71.1

Heart rate/10 beats

per minute

1.7 0.83.7 2.2 0.95.3

Previous AMS 25.4 2.7237 33.3 2.8390

Smoking 2.7 0.79.6 10.0 1.567.4

Cigarettes per day/

10 cigarettes

3.0 1.09.2 5.5 1.421.9

Exhaled CO/10 p.p.m. 2.2 0.95.7 4.0 1.213.1

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one such study [8] and 11% in another study [15]).

Cigarette smoking is, however, much more common

in occupational cohorts, tourists and military person-

nel assigned to high altitude. It is all the more notable,

therefore, that two relatively large studies of cohortsin which smoking is common have found that this

factor appears to be protective against AMS. One of

these analysed nearly 400 railway construction work-

ers in China, the highest worksite being at 4905 m

[18]. Current smokers had lower LLS values for AMS

compared with non-smokers, driven largely by head-

ache symptoms, yet conversely, smoking was associ-

ated with lung function and oxygenation deficits at

follow-up [18]. A second study from China found that

for military recruits taken to a high altitude, cigarette

smoking was also protective for LLS-defined AMS

[19]. Several other studies of AMS have also reported

an association suggestive of smoking being a protec-tive factor for AMS, albeit not a statistically significant

one [3,4,8,15]. Other data on carboxyhaemoglobin

or exhaled CO as a potential AMS risk derive from

an interest in potential exposure to CO from cooking

stoves in mountaineering. This has not been found to

be associated with risk, positively or negatively [20].

Cigarette smoking is associated with lower exhaled

NO [21] and may confound the association with AMS.

The analysis of Chinese military recruits noted above

paradoxically found higher pre-ascent exhaled NO

among the cases, even though smokers had lower lev-

els; no multivariate analysis was reported [19].

A key aspect of our study is that we defined AMS asepisodes severe enough on clinical grounds to undergo

compression chamber treatment. It may be that smok-

ing protects against certain symptoms that contribute to

LLS-defined illness but nonetheless conveys increased

risk of more severe illness. In that light, we believe that

our studys limitation of a lack of systematic LLS data

is counterbalanced by its ability to identify cases at the

severe end of the AMS disease spectrum. This also would

explain the lack of a reduction in the risk associated with

current smoking by prior, less severe AMS events when

tested in the same model. Were smoking to be strongly

associated with milder AMS, including such events on

the multivariate modelling should have affected smok-

ing-associated risk estimates.Ours was a relatively small sample, comprising only

15 cases of severe AMS. Thus, we lack statistical power

to detect modest associations (e.g. in the observed

trends of lower forced expiratory volume in 1 second/

forced vital capacity and in heart rate). Even though

there was no marked difference in occupation status

of cases and referents when grouped broadly (produc-

tion versus service work), we acknowledge that a far

larger study would be needed to identify or exclude

specific job tasks as risk factors for severe AMS. Our

initial choice of variables beyond smoking was based

on the findings of other studies. Most of the selected

physiological variables were extracted from an annualscreening prior to the event, a strength of the study,

but the analysis was therefore limited in its range of

selected variables. Inclusion of other variables might

have identified additional associations that could not

be studied in this study. Certain other factors simply

cannot be studied in our working population (e.g. rate

of ascent or number of nights at a given altitude before

final ascent) because our subjects are all transported

to the mine site by bus, a 7 hour trip. Symptom-driven

bias in subjects reports of their smoking status is not

relevant as this was assessed prior to the AMS event.

Treatment bias (selection for chamber treatment on the

grounds not of illness severity but of clinical concernover smoking status) is unlikely and notably our analy-

sis took place after the study period. Moreover, if smok-

ing were simply a risk factor for occult lung disease that

led to greater symptom severity we should have seen

a stronger association with impaired lung function.

Finally, this study involves intermittent high-altitude

exposure in well-acclimatized workers and thus should

not be generalized to others such as non-acclimatized

non-occupational populations.

Table 4. Prior AMS and smoking intensity tested in multivariate regression models of severe AMS

Model without interaction term Model with interaction term

OR 95% CI P OR 95% CI P

Model 1. Smoking intensity/AMS

Cigarettes per day/10 cigarettes 1.1 1.01.3

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Our analysis raises the question of whether an oth-

erwise fit for duty employee who has had one or more

episode of AMS should be targeted for additional meas-

ures to prevent recurrent and potentially more severe

events. Interestingly, national legislation in Kyrgyzstan,

which is otherwise fairly prescriptive in terms of man-

dated surveillance and even work restrictions, does not

identify a prior AMS episode as a contraindication for

employment. The potential advantages of workplace-

based smoking cessation efforts are more obvious.

Smoking cessation programmes should be a priority,

particularly targeting those with prior AMS for addi-

tional efforts, given that effective smoking cessation

strategies do exist [22]. This is no panacea however and

middle-aged working populations without severe smok-

ing complications may be one of the hardest in which to

achieve successful cessation [23].

In summary, this study demonstrates more severe

AMS may be associated with some risk factors that are

similar to and some that may differ from those for less

severe AMS. In conditions of intermittent exposure inwell-acclimatized workers, smoking appears to play a

potent role in severe AMS, suggesting that in such popu-

lations effective smoking cessation measures may reduce

AMS-associated morbidity.

Key points

Severe acute mountain sickness requiring com-pression treatment may be associated with risk fac-

tors that differ from those for less severe sickness,

in particular in terms of smoking-related risk.

Pre-employment evaluation of prospective employ-ees for high-altitude work should take into account

previous acute mountain sickness and smoking, as

both may identify individuals at higher risk.

Work-based smoking cessation interventionsfor high-altitude employees should consider the

potential added benefit of severe acute mountain

sickness prevention.

Acknowledgements

The study group would like to thank the management of

Kumtor Gold Company (Kumtor Operating Company) for

their support. Kumtor mine site and Bishkek doctors and

nurses have together made this study possible.

Conflicts of interest

D.V., V.K. and R.R. are employees of the Kumtor Gold

Company. P.D.B. is involved in other research, partially sup-

ported by Kumtor Gold Company. No authors received pay-

ment for this research.

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Fifty years ago: General practice and industrial

medicine in the United States

B. H. Pentney, Appointed Factory Doctor, Camberwelland Lambeth

Given at a meeting of the Association on 3 April 1964

I am an Appointed Factory Doctor, but it was as a gen-

eral practitioner that I received a Nuffield Travelling

Fellowship for 1963. I had chosen to study the type

and range of general practice and the place of the

general practitioner in industry. I was also to con-

sider what part the general practitioner could play in

a future occupational health service in this country.

The tour was started in July 1963 and most of the time

was spent in two places5 weeks in California and

11 weeks in Cincinnati. In North America there is alarge and growing body of whole-time industrial physi-

cians, and these are of a high order of knowledge and

efficiency. They are the experts. For the greater part

they are whole time employees of big industrial organ-

izations. They are responsible for the running of their

medical services and research, and set the standard of

occupational health throughout the country.

A very comprehensive Workmens Compensation

system is really the basis of the greater part of the work

of the general practitioner in industrial medicine, and

it is at first sight of immense value in the creation of

occupational health systems generally. Unfortunately,

during its development the emphasis seems to havealtered. The ideals, including fair reimbursement for

suffering and loss of earnings, and early rehabilita-

tion, have sometimes been submerged in the processes

of preparing a case for presentation to the Accident

Commission. The disability will be assessed most care-

fullypossibly using the standard Packard Thurber

systemto achieve some uniformity. Compensation

will be awarded strictly in accordance with a standard

scale, taking into consideration the patients age, the

exact anatomical site of the lesion, the percentage of

loss of movement and expectancy in that mans spe-cific trade. With these tremendously detailed assess-

ments, time is lost and it seemed to me that early and

adequate rehabilitation was impeded. I was surprised

at the wide range of disabilities accepted as industrial

in origin and shocked at the amount of compensation

awardedfor example, for bronchitis and coronary

disease.

Undoubtedly the general practitioner has an estab-

lished place in occupational health in the United

States. It is, however, fairly limited in its application.

Individual and group practitioners provide good casu-

alty and routine examination services for large and

small industries.Although to us it is clearly desirable that all fact-

ories should have medical services, occupational health

has not yet been completely sold to industry. There

industrial medical practitioners dealing mainly with

Workmens Compensation cases have not established

free access into plants, and they appear to have little

enthusiasm for, or encouragement to be interested in,

sick factories. Though splendid and growing centres

for environmental studies are available there is much

left to be achieved in terms of getting the proprietor or

management to want medical services. Post-graduate

training in the speciality is available, but on the whole

poorly attended, and it would seem that though thismust be made more attractive, the subject of occupa-

tional health ought to be a mandatory undergraduate

subject.

From: General practice and industrial medicine in the

United States. Trans Ass Industr Med Offrs(1964) 14,

89. Available at: Occup Med (Lond) 1964;14:8990.

doi:10.1093/occmed/14.1.89.

The full Occupational Medicinearchive can be accessed

online at http://occmed.oxfordjournals.org/.
http://occmed.oxfordjournals.org/http://occmed.oxfordjournals.org/

483.full

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