Carpal tunnel syndrome and the use of computer mouse and keyboard: A systematic review

Abstract

This review examines evidence for an association between computer work and carpal tunnel syndrome (CTS).
A systematic review of studies of computer work and CTS was performed. Supplementary, longitudinal studies of low force, repetitive work and CTS, and studies of possible pathophysiological mechanisms were evaluated.
Eight epidemiological studies of the association between computer work and CTS were identified. All eight studies had one or more limitation including imprecise exposure and outcome assessment, low statistical power or potentially serious biases. In three of the studies an exposure-response association was observed but because of possible misclassification no firm conclusions could be drawn. Three of the studies found risks below 1. Also longitudinal studies of repetitive low-force non-computer work (n = 3) were reviewed but these studies did not add evidence to an association. Measurements of carpal tunnel pressure (CTP) under conditions typically observed among computer users showed pressure values below levels considered harmful. However, during actual mouse use one study showed an increase of CTP to potentially harmful levels. The long term effects of prolonged or repeatedly increased pressures at these levels are not known, however.
There is insufficient epidemiological evidence that computer work causes CTS.

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BMC Musculoskeletal Disorders
Open Access
Research article
Carpal tunnel syndrome and the use of computer mouse and
keyboard: A systematic review
Jane F Thomsen*1, Fred Gerr†2 and Isam Atroshi†3
Address: 1Department of Occupational Medicine, Copenhagen University Hospital in Glostrup, Nordre Ringvej, DK-2600 Glostrup. Denmark,
2Department of Occupational and Environmental Health, College of Public Health, University of Iowa, Iowa City, IA 52245. USA and
3Department of Orthopaedics, Hässleholm and Kristianstad Hospitals SE-28125 Hässleholm. Sweden
Email: Jane F Thomsen* - jfth@glo.regionh.dk; Fred Gerr - fred-gerr@uiowa.edu; Isam Atroshi - isam.atroshi@skane.se
* Corresponding author †Equal contributors
Abstract
Background: This review examines evidence for an association between computer work and
carpal tunnel syndrome (CTS).
Methods: A systematic review of studies of computer work and CTS was performed.
Supplementary, longitudinal studies of low force, repetitive work and CTS, and studies of possible
pathophysiological mechanisms were evaluated.
Results: Eight epidemiological studies of the association between computer work and CTS were
identified. All eight studies had one or more limitation including imprecise exposure and outcome
assessment, low statistical power or potentially serious biases. In three of the studies an exposure-
response association was observed but because of possible misclassification no firm conclusions
could be drawn. Three of the studies found risks below 1. Also longitudinal studies of repetitive
low-force non-computer work (n = 3) were reviewed but these studies did not add evidence to an
association. Measurements of carpal tunnel pressure (CTP) under conditions typically observed
among computer users showed pressure values below levels considered harmful. However, during
actual mouse use one study showed an increase of CTP to potentially harmful levels. The long term
effects of prolonged or repeatedly increased pressures at these levels are not known, however.
Conclusion: There is insufficient epidemiological evidence that computer work causes CTS.
Background
Carpal tunnel syndrome (CTS) is a compression neuropa-
thy of the median nerve as it passes through the carpal
tunnel. It is regarded as the most frequent compression
neuropathy. Based on both clinical symptoms and nerve
conduction tests (NCT), overall prevalences of 3.0–5.8%
among women and 0.6–2.1% among men have been
found in general population samples [1,2]. CTS is gener-
ally believed to be caused by increased pressure in the car-
pal tunnel. It has been a matter of discussion whether
biomechanical factors may cause the condition. It is now
widely accepted that exposure to hand-arm vibrations and
exposure to a combination of repetitive hand use and the
use of hand force may be causal agents [3]. In recent years,
with the expanding use of computers, it has been a matter
of concern if computer use could be a risk factor for the
development of CTS, and if so, should the condition be
recognised as an occupational disease. The issue was
partly addressed in a recent review where it was concluded
that the evidence did not point at any important associa-
Published: 6 October 2008
BMC Musculoskeletal Disorders 2008, 9:134 doi:10.1186/1471-2474-9-134
Received: 2 November 2007
Accepted: 6 October 2008
This article is available from: http://www.biomedcentral.com/1471-2474/9/134
© 2008 Thomsen et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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tion between keyboard and computer work and CTS [3].
The review, however, did not include a systematic search
for studies on computer work and the evaluation was
based on a limited number of epidemiological studies
only. Furthermore, there were no considerations on pos-
sible mechanisms.
Thus, the core of this review was a detailed evaluation of
the existing epidemiological evidence of an association
between computer work and CTS. Because of some simi-
larities between computer work and repetitive, low force
work, studies examining associations between repetitive
work and CTS were also evaluated (longitudinal studies
only). Though not epidemiological in design, studies of
median nerve function among computer users, exposure
characteristics in computer work and the influence on car-
pal tunnel pressure (CTP) and median nerve function was
also evaluated in order to determine possible pathophys-
iological mechanisms.
The review was originally conducted on behalf of the Sci-
entific Committee of the Danish Society for Occupational
and Environmental Medicine for use by The Danish
National Board of Industrial Injuries in its evaluation of
whether specific musculoskeletal disorders among com-
puter workers should be included on its list of occupa-
tional injuries and diseases that may be compensated
through the Danish Workers' Compensation Act [4].
Methods
Literature search
The identification of epidemiological studies examining
associations between computer work and CTS began with
a search in the following databases: Pubmed, Embase,
Web of Science, and Arbline. The language had to be Eng-
lish and the article published in a journal with a peer-
review process. Only papers with original data were con-
sidered. The original search was performed in June 2005
and covered all years included in the databases. For the
purpose of the present review this part of the search was
further updated in August 2008 (human studies only).
The text search terms were: 'carpal tunnel syndrome or
CTS or median nerve and computer or visual display unit
or keyboard or mouse'. All titles and relevant abstracts
were read. Reference lists in relevant articles and personal
files were also searched for articles not identified in the
original database search.
The criteria for inclusion in the review of epidemiological
evidence were: Cross-sectional or longitudinal studies that
included participants exposed to computer work (mouse
or keyboard) or typing and participants without such
exposure and case-referent studies where computer work
(mouse or keyboard) or typing was specified as an expo-
sure. In all studies, the health outcome had to be CTS
ascertained with 1) symptoms (questionnaire or interview
or both) in combination with NCT or 2) symptoms alone
but confirmed by qualitative interview.
Studies in which CTS was ascertained with questionnaire
symptoms only, NCT only or Tinel's or Phalen's test only
were not included. Studies using workers' compensation
data were not included (Table 1).
Supplementary to the epidemiological evidence of the risk
of computer work, longitudinal studies of the association
between CTS and high repetition and low force work were
identified with the search terms 'carpal tunnel syndrome
and repet*' (Pubmed only). All titles and relevant
abstracts were read. Only studies with a prospective design
and a focus on the association between repetitive work
and CTS as defined above were included.
With the use of the above search terms studies of median
nerve function in computer users were retrieved and
included. The search term 'computer use and ergonomic
risk factors' was also used (Pubmed only). Studies describ-
ing the arm and hand position and force level in computer
work were retrieved and included.
Table 1: Inclusion and exclusion criteria for the epidemiological
studies of the association between computer work and carpal
tunnel syndrome.
CTS: Carpal tunnel syndrome
NCT: Nerve conduction test
Inclusion criteria
English language, peer-reviewed articles with original data
Study design
Longitudinal studies
Cross sectional studies
Case referent studies
Population
Should include both an exposed group and a control group
Exposure
Computer work (keyboard or mouse) or typing should be defined
as the exposure
Outcome definition
1. Symptoms (questionnaire or interview or both) of CTS in
combination with NCT or
2) Symptoms alone but confirmed by qualitative interview.
Exclusion criteria
Population
Studies using workers' compensation data
Outcome definition
Studies where CTS was diagnosed using questionnaire symptoms
only, NCT only, or clinical tests as Tinel's and Phalen's tests only

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Finally, the search term 'carpal tunnel pressure' was used
(Pubmed only). All titles and relevant abstracts were read
and human studies with a focus on carpal tunnel pressure
(CTP) and the effect of force and position of arm, wrist
and fingers were included. No cadaver studies were con-
sidered.
In the quality assessment of each of the epidemiological
studies, no scoring system was used. Instead, descriptive
data for each study were provided and assessed, as fol-
lows: Study design: Longitudinal better than cross sec-
tional and case referent but dependent on other of the
mentioned factors as well. Exposure assessment: Objec-
tive measurements better than self-reported exposure bet-
ter than job title. Outcome definition: Symptoms and
NCT better than symptoms only. Length of follow-up
period: If baseline exposure was used then a short follow-
up period was better than a long period. Controlling for
potential confounders: Age and gender as a minimum.
Reporting or selection bias was considered. Sample size
was assessed in relation to statistical power. Blinding of
the participant and/or the examiner was considered.
Level of evidence
The overall evaluation was based on a classification sys-
tem established by The Scientific Committee of the Dan-
ish Society of Occupational and Environmental Medicine,
2005 and used in other recent reviews, e.g. [3,5]. The fol-
lowing categories were used:
Strong evidence of a causal association: A causal relation-
ship is very likely (chance, bias, and confounding could
be ruled out with reasonable confidence) between an
exposure to a specific risk factor and a specific outcome. A
positive relationship has been observed between exposure
to the risk factor and the outcome in several studies.
Moderate evidence of a causal association: Some convinc-
ing epidemiological evidence exists (chance, bias, and
confounding are not the likely explanation) for a causal
relationship between an exposure and a specific outcome.
A positive relationship has been observed between expo-
sure to the risk factor and the outcome in several studies.
Limited evidence of a causal association: Some convincing
epidemiological evidence exists in some studies for a
causal relationship between an exposure to a specific risk
factor and a specific outcome. A positive relationship has
been observed between exposure to the risk factor and the
outcome, but it is not unlikely that this relationship could
be explained by chance, bias, or confounding.
Insufficient evidence of a causal association: The available
studies are of insufficient quality, consistency, or statisti-
cal power to permit a conclusion regarding the presence or
absence of a causal association.
The likelihood that chance, bias and confounding may
explain observed associations was based on the quality
assessment criteria (study design, exposure assessment,
outcome definition, confounder control, length of follow-
up period, selection and information bias, study size and
statistical power).
Biological plausibility and contributory information may
add to the evidence of a causal association.
Results
Epidemiological studies
Computer work and carpal tunnel syndrome
In the literature search on the association between com-
puter work and CTS 4661 references were identified (Fig-
ure 1). Eight epidemiological studies met the criteria for
inclusion [6-14] (two of the papers were from the same
population [11,12]). Four of these studies were prospec-
tive in design [6-8,12], one was a case-referent study [10],
one was cross-sectional but with a case-referent approach
[9], and two were cross-sectional[13,14]. The studies are
listed in Table 2 with information on design, population,
response rate, control group, exposure, CTS case defini-
tion, confounders controlled for, results and strengths
and weaknesses [see Additional file 1].
The large population-based study by Atroshi et al.
included both physical examination and NCT [14]. It
showed a significant protective effect of keyboard work, i.e.
the prevalence of CTS increased with decreasing hours/
day with self-reported computer work. The study was care-
fully conducted with adequate blinding of the interviewer
regarding exposure and of the technician regarding the
symptom status.
An Indian cross-sectional study, on the contrary, showed
a significant effect of both years and hours/day with com-
puter work [13]. The participation rate was 100% which
raises a question of the selection process. There was not
the expected effect of gender (men had twice the fre-
quency of CTS compared to women), age or BMI. Blind-
ing was not described and NCT was not used.
In the follow-up study of Andersen et al. (The NUDATA
study), two case definitions relevant for this review were
used [see additional file 1] [8]. When applying case defi-
nition 1, analysis of baseline data showed odds ratios of
2.3 (95% CI 1.2–4.5) among participants reporting 5–9
h/w of mouse use increasing to 3.6 (95% CI 1.8–7.1)
among participants reporting 20–24 h/w of mouse use.
There was no further increase of risk among participants
reporting more than 24 h/w of mouse use. Case definition

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2 showed only borderline significance at 30+ hours and a
very irregular pattern of association. In the follow-up
analyses, a significantly increased risk was found among
participants reporting working more than 20 h/w with the
mouse.
In the extensive prospective study of computer users by
Gerr et al. the prevalence of CTS was too low to allow any
analyses of association [6]. It is worth noting, however,
that with the use of an outcome definition including both
symptoms and NCT the prevalence and incidence of the
disease was low in this working population.
As a substudy of the Danish PRIM study (Project on Inter-
vention and Research in Monotonous work), 731 partici-
pants from three companies, one bank and two postal
centres, were included. [7]. Data entry was the main repet-
itive task. Electrogoniometer measurements of wrist
movements showed that this work task was highly repeti-
tive. Blinding of physician and technician was observed.
The overall prevalence of CTS was 1.1% (8 cases) on the
working hand and 0.3% (2 cases) on the contralateral
hand. The risk of CTS was statistically significantly
increased for every 10 hours of repetitive work (OR = 1.86
(95% CI 1.06–3.19)) after adjusting for forceful work and
personal characteristics. The annual incidence of CTS was
0.62% (4 cases) and thus no further analyses of incidence
data could be made.
Nathan et al. had followed a cohort established in 1984
for 11 years [11,12]. Originally, the cohort included 471
participants from 4 industries representing a wide variety
of hand activities. It was not stated if the at-risk popula-
tion had normal NCT at baseline nor if the examiners
were blinded to exposure or health status. In multivariate
analyses adjusting for various potential baseline con-
founders (but not age and gender), neither an effect of
keyboard work nor of repetitive work (as ascertained in
1984) was found.
In the study by Stevens et al., all participants were identi-
fied as "frequent computer users" working in a medical
facility [9]. Exposure and demographic characteristics
were compared between the two groups but without for-
mal statistical testing. Hours of daily keyboard use and
years with keyboard did not differ between the groups.
Frequent mouse use was more prevalent in the CTS group
than the non-CTS group (48.1% vs. 27.9%). This differ-
ence was not tested statistically. However, testing the
reported figures with a Mantel-Haenszel chi-square test (1
df) showed a statistically significant difference (p = 0.04).
In another study with a case-referent approach de Krom et
al. identified 156 CTS cases from a population based sur-
vey and from an outpatient department of neurology [10].
Referents were persons without CTS symptoms from the
population based survey (n = 473). Care was taken to
keep the purpose of the study blinded to the participants
in order to avoid information bias. One question docu-
mented hours per week of typing during the last 5 years.
The odds ratios were all below 1 and not statistically sig-
nificant.
A large population based study with main focus on hand-
arm vibration and CTS did not meet the inclusion criteria
because of insufficient outcome definition [15]. There was
one questionnaire item concerning keyboard use more
than 4 hours per day. No excess risk for self reported tin-
gling/numbness was found (PR = 1.1, 95% CI 0.8–1.3)
after adjusting for age, smoking, headaches and tiredness/
stress.
In summary, of the eight studies identified, four studies
were designed as follow-up studies. One of these studies
was well performed with a short follow-up period, a large
study population and thus sufficient statistical power. The
study found some positive associations. However, the
possibility of information bias in combination with an
outcome definition not involving NCT made inferences
difficult [8]. Two of the follow-up studies had too few CTS
cases to perform analyses [6,7]. The last follow-up study
identified had a very long follow-up period, only baseline
exposure information and no adjustment for age and gen-
der. They found no association but because of the impor-
Flow-chart showing the identification of epidemiological studies of the association between computer work (key-board and mouse) or typing and carpal tunnel syndromeFigure 1
Flow-chart showing the identification of epidemiolog-
ical studies of the association between computer
work (keyboard and mouse) or typing and carpal tun-
nel syndrome.
Duplicates
n=1039
Did not fulfil the inclusion
criteria
n=3617
Retrieved
references Fulfilled the inclusion
criteria
n=5 [6,8,9,13,14]
n=4661
Identified from own files
n=2 [10,12]
Studies
included
n=8
Identified from
supplementary search
(‘carpal tunnel
syndrome and repet*’)
n=1 [7]

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tant limitations the result was difficult to interpret [12].
One of the follow-up studies reported positive associa-
tions from baseline data but here the computer exposure
was mixed with other kinds of repetitive work [7]. One of
the two cross-sectional studies was very well performed
and had enough statistical power and found the opposite
association of what was expected [14]. The other cross-sec-
tional study showed a positive association but had some
confusing results, e.g. a much higher prevalence among
men compared to women. Two of the eight studies were
case-control studies. One study had limited statistical
power and showed no association [10]. The other showed
a weak association but only in a crude analysis with no
adjustment for age [9].
Repetitive work and carpal tunnel syndrome
In the literature search on repetitive work and CTS, 229
studies were retrieved and three studies fulfilled the inclu-
sion criteria [16-18]. Two of the three studies were based
on one population [16,17]. Werner et al. identified 49
asymptomatic participants with an abnormal NCT and 59
with a normal NCT and assessed them after 17 months
and after 70 months. Work tasks were video-filmed and
categorized according to repetition rate on a scale from
1–10. The incidence of developing CTS symptoms was
equal in the two groups after 17 months but significantly
higher after 70 months in the group with initially positive
NCTs. Repetitive work was a risk factor for developing
symptoms after 17 months (OR 1.35, 95% CI 1.03–1.77)
and after 70 months (risk estimate not provided).
In a well designed follow-up study by Gell et al. no asso-
ciation between level of repetitive hand tasks and devel-
opment of CTS was found [18]. The case definition
included both symptoms as well as NCT. Each job was
assessed and rated for ergonomic exposures.
In all, results from studies on other kinds of repetitive, low
force work and CTS did not add to evidence of to an asso-
ciation between computer use and CTS.
Studies of nerve involvement among computer workers
Seven studies comparing median nerve function in com-
puter users with groups without computer use were found
[19-25]. Two of the studies assessed median nerve func-
tion with NCT [23,25] whereas the other five studies used
vibration perception threshold testing. Vibration sense
perception, however, is not a good indicator of CTS [26].
The selection of participants was not described in the five
studies using vibration perception threshold testing and
selection bias may have affected the results.
The study of Murata et al[23] used nerve conduction tests
among 27 female life insurance company employees
entering data for six hours or more per day and 24 female
students. Significant differences in median nerve sensory
conduction velocities were found for measurements
across the carpal tunnel whereas values proximal and dis-
tal to the wrist did not differ. The two groups differed in
symptom profile. The findings of Murata et al[23] were in
contrast to a recent study by Sandén et al[25] in which 82
secretaries with a median of 6 hours of daily computer
work were compared to 35 nurses with very limited com-
puter work. No statistically significant differences were
found in the median nerve conduction velocity or in the
vibration threshold between the two groups in t-test anal-
yses. Doezie et al. compared vibration thresholds among
transcriptionists with symptoms to a control group [21].
Thresholds of the second and fifth fingers were signifi-
cantly elevated in the transcriptionists compared to the
control group but only for the high frequencies (125–500
Hz). Very little information about the control group was
shown.
Greening et al. conducted two studies examining associa-
tions between vibrotactile thresholds and computer use
[19,20]. In both studies, they found that patients with
musculoskeletal symptoms in the upper limbs had higher
thresholds than healthy individuals. The studies did not
compare computer users without symptoms to non-com-
puter users. Thus, the design of the studies did not allow
conclusions considering an effect of computer work per
se.
Similar methods were used in a Danish study and similar
results were observed [24]. Finally, another Danish study
studied vibration thresholds in computer users but with a
focus on symptoms and not levels of computer use [22].
Pathophysiological mechanisms
It is widely believed that biomechanical factors (e.g. force-
ful exertions, repetition, and awkward postures) increase
the risk of CTS by increasing carpal canal pressure with
subsequent nerve ischemia [27]. Therefore, in addition to
epidemiological evidence of associations between com-
puter work and CTS, insight into the role of computers on
the development of CTS may be found in studies examin-
ing wrist biomechanics or carpal canal pressures during
computer use.
Wrist position and exertion of force in computer work
Wrist positions and forces exerted by computer users have
been measured in several studies. Keir et al[28], reported
that wrist extension ranged from 23° to 30° and that
ulnar deviation from -3.2° to 5.2° during mouse work. In
a study of wrist position in keyboard work (entering of
data), electrogoniometer measurements showed wrist
extension of 14° and 20° at the 50th and 90th percentile,
respectively [7]. In another study of keyboard work, mean

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ulnar deviation using a conventional keyboard was 18.9°
(SD 6.8°) [29]. Gerr et al. reported wrist postures
observed among 379 computer users. Mean wrist exten-
sion was 24.3° (SD 9.6°) during keyboard use and 23°
(SD 8.8) during mouse use. Mean ulnar deviation was
5.0° (SD 7.3) during keyboard use and 1.0° (SD 7.7°)
during mouse use [30].
Several investigators have measured finger tip forces
among computer users. The finger tip force exerted while
keying varied from less than 1 N to 7 N but in most studies
was between 1 and 4 N [31-34].
Carpal tunnel pressure
In the literature search, 253 studies were retrieved and
nine studies were found with measures of carpal tunnel
pressure in relation to finger, wrist or arm use. Several
studies have measured the carpal tunnel pressure (CTP)
among persons free of CTS and among those with CTS
[28,35-42]. In aggregate, these studies suggest that CTS
development is associated with elevated CTP. The resting
CTP with the wrist in neutral position among persons free
of CTS ranges from 3 to 13 mmHg (results from 7 studies
are summarized in [37] and [39]). CTP in CTS patients
varies between 10 and 43 mmHg [39] though higher val-
ues have been found [38]. In an often cited study by Lun-
dborg et al[35], CTP was increased experimentally among
16 human volunteers. In four participants the CTP was
increased to 60 mmHg and in four other participants the
CTP was increased to 90 mmHg. In these two groups, the
sensory and subsequently the motor response were
blocked within an hour. In a third group of 4 participants
CTP was increased to 30 mmHg. This produced minor
and varying effects but "pins and needles" was reported in
2 of 4 subjects. This was further studied by Gelberman et
al. who found some functional loss at 40 mmHg and
complete motor and sensory block at 50 mmHg among
healthy subjects [36].
Several studies have measured the CTP profile associated
with different wrist angles, finger flexion and forearm
position [39,40,42,43]. The studies show that CTP is
dependent on the position of the forearm, wrist and met-
acarpophalangeal joint (MCP). In particular, supination
showed higher CTPs than pronation and MCP flexion
increased CTP [28,39,42]. With wrist positions between
40° flexion and 40° extension the CTP did not exceed 20
mmHg regardless of MCP angle. Ulnar and radial devia-
tion had only small effects [40].
CTP has been studied among persons engaged in actual
work tasks. Rempel et al. measured CTP among 19 healthy
subjects engaged in a number of hand intensive tasks [37].
CTP increased from 8 (SD 6) mmHg at rest to 18 (SD 13)
mmHg after lifting 0.5 kilogram cans for 5 minutes at a
rate of 20 cans per minute. Keir et al. conducted a study on
the effect of computer tasks on CTP. Among 14 healthy
subjects the mean CTP rose from 5.3 mmHg during rest to
16.8–18.7 mmHg (varying between different kinds of
computer mice) with the hand static on the computer
mouse and to 28.8–33.1 mmHg while dragging or point-
ing and clicking with the mouse [28]. This was the only
study of computer work and CTP that was found.
To summarize, measurements of CTP under conditions
commonly observed among computer users showed
modest increases generally believed to be below potential
harmful levels. However, one study showed an increase of
CTP during actual mouse use to levels where possible neu-
rological effects were seen experimentally. These studies
have not been repeated in other studies and nothing is
known about the effects of prolonged or repeatedly
increased pressures to this level.
Discussion
The epidemiological evidence
The epidemiological evidence of an association between
computer use and CTS is inconsistent. All 8 studies iden-
tified in this review that examined the association
between computer work and CTS had important limita-
tions. Thus, a definitive study that clarifies the relation-
ship between computer use and CTS has not been
conducted yet. Such a study should involve a large popu-
lation with varying degrees of computer work, at least one
year of follow-up, a careful exposure description and a
precise CTS diagnostic procedure. Such a study would
require considerable resources to complete.
Based on evaluation of study design, sample sizes and
response rates, case definitions and the exposure informa-
tion, the studies by Andersen et al., Thomsen et al. and
Atroshi et al. were the most likely to yield valid inferences.
In two of the studies very intense computer work was rep-
resented (e.g. data entry, graphical work) [7,8]. Andersen
et al. observed an association between mouse use and
symptoms of CTS in the median nerve distribution area in
both the cross sectional and in the follow-up analyses. The
association was statistically significant for participants
reporting more than 20 h/w of mouse use with the risk
almost tripled compared to the control group. A similar
risk level was found in the study by Thomsen et al
Both studies had limitations. The study by Andersen et al.
was performed during a time of intense debate on the
potential hazards of mouse use in Denmark [8]. This may
have influenced the results and thus explain why only
associations with mouse use and not keyboard use was
found. Information bias caused by beliefs about certain
associations may have very strong effects. This was shown
in a study of indoor climate symptoms where reporting

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turned out to be dependent on the information given to
the participants about the purpose of the study [44]
Another draw back of the study by Andersen et al. was the
lack of NCT in the CTS case definition. Also of concern
was the observation that associations with the most spe-
cific CTS case definition were not as strong as associations
with the less specific CTS case definition.
Thomsen et al. used a sensitive and specific CTS case defi-
nition (including NCT) and precise estimates of exposure
with the use of questionnaires and direct measurements.
The odds ratio of 1.86 was based on only 8 cases among
the exposed and no cases among the control group. Fur-
thermore, the interpretation was complicated by the fact
that participants with data entry exposure were pooled
with participants performing manual letter sorting [7].
The study by Atroshi et al. showed quite convincingly the
opposite of the expected, i.e. a negative association [14].
A limitation in this study could be the rather limited
amount of keyboard work reported which would make it
more difficult to show an effect. The exposure was self-
reported and thus misclassification may have occurred.
The possibility of reporting bias was limited because the
participants were not aware of this special focus. The other
cross-sectional study was difficult to interpret because of
possible methodological bias, e.g. a much higher preva-
lence was found among men compared to women.
When statistical tests were applied to the results presented
by Stevens et al., a statistically significant positive associa-
tion was observed for the association between mouse use
and CTS (although the authors concluded otherwise) [9].
However, the significant association was unadjusted for
potentially confounding risk factors, e.g. age.
The case referent study by de Krom et al. was inconclusive.
The number of exposed CTS cases in the study was very
low and thus statistically unstable [10]. However, no pat-
tern in the risk estimates was seen and all estimates were
below unity. One of the strengths of the study was that the
participants were blinded to the purpose of the study.
A number of methodological weaknesses substantially
limited inferences that could be made form the study by
Nathan et al. [11,12].
Findings in the longitudinal studies of repetitive low-force
work pointed in different directions. Thus, these studies
did not add further evidence.
The recent review by Palmer et al. reached the same over-
all conclusion concerning computer work based on only
two epidemiological studies of the association [3]. Even
though the present review managed to identify more stud-
ies of the association no further evidence was established
mainly because of limitations in the studies.
This review also had limitations as well as strengths. The
search strategy in the databases only identified six of the
eight epidemiological studies. Therefore, we may have
missed other studies with a focus on the use of keyboard,
mouse or typing. Because of the relatively few studies in
this field we preferred to describe the strengths and weak-
nesses of the studies in text in stead of using a scoring sys-
tem as scoring systems are not always valid [45]. The risk
of publication bias exists but is not obvious. There were
both positive and negative studies among both the large
and small studies.
Pathophysiological mechanisms
Computer work involves very little force. Experiments on
the effect of positions of fingers, wrist and forearm com-
parable to the positions common in computer use have
shown that CTP increases but not to levels generally
believed to be harmful [39,40,42]. Surprisingly, mean
CTP levels between 28–33 mmHg where observed when
study participants were dragging or clicking with the
mouse. Lower values were found with the hand static on
the mouse [28]. Although the experiment has never been
repeated the findings indicate a possible pathophysiolog-
ical mechanism for CTS among heavy mouse users.
Conclusion
In summary, because of insufficient quality, bias, lack of
consistency and statistical power evidence is insufficient
to conclude that computer work (mouse and keyboard)
causes CTS. As a consequence, this condition cannot be
recognised as an occupational injury because of computer
work. A large and unbiased prospective study is needed to
establish further evidence. Such a study is recommended
but the costs should be carefully considered.
Abbreviations
CTS: carpal tunnel syndrome; NCT: nerve conduction test;
CTP: carpal tunnel pressure; PPV: positive predictive
value; NUDATA: Neck and Upper extremity Disorders
Among Technical Assistants; PRIM: Project on Interven-
tion and Research in Monotonous work; MCP: metacar-
pophalangeal.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
J.F. Thomsen was the lead author of the original docu-
ment on which this manuscript was based and prepared
this manuscript from the original document. F. Gerr and
I. Atroshi made conceptual contributions to the original
document and reviewed and edited this manuscript.

BMC Musculoskeletal Disorders 2008, 9:134 http://www.biomedcentral.com/1471-2474/9/134
Page 8 of 9
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Additional material
Acknowledgements
The authors want to express their gratitude towards the valuable com-
ments during the process and on a one-day meeting from the members of
The Scientific Committee of the Danish Society for Occupational and Envi-
ronmental Medicine consisting of Sigurd Mikkelsen (chair), Staffan Skerfving,
Jørgen Olsen, Susanne Wulff Svendsen, Johan Hviid Andersen and Henrik
Kolstad.
The review was funded by the Danish National Board of Industrial Injuries.
The board was not involved in the collection, analyses and interpretation of
data, in the writing or in the decision to submit the paper.
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Additional file 1
Table 2. Epidemiological studies on carpal tunnel syndrome and use of
computer mouse and keyboard. The Table provides information on study
design, population, response rate, control group, exposure, CTS case defi-
nition, confounders controlled for, results and strengths and weaknesses.
Click here for file
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2474-9-134-S1.doc]

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