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ORIGINAL CONTRIBUTION
Computer Use and Carpal Tunnel Syndrome
A 1-Y ear Follow-up Stud y
Johan Hviid Andersen, MD, PhD
Jane Froelund Thomsen, PhD
Erik Overgaard, MD
Christina Funch Lassen, MD
Lars Peter Andreas Brandt, MD, PhD
Imogen Vilstrup, MSc
Ann Isabel Kryger, MD, PhD
Sigurd Mikkelsen, MD, DMedSc
U
SE OF THE KEYBOARD AND THE
mouse as key interface de-
vices to the computer has led
to much debate concerning
their role in development of injuries to
the nerves in the upper limbs, particu-
larly carpal tunnel syndrome (CTS).
The proposed mechanism for this re-
lationship is regional compression or
nerve stretching, which has been con-
firmed in animal models, but no mod-
els exist concerning the effects of re-
petitive hand-finger loading on nerve
structure and function.
1
There is a large epidemiological lit-
erature on CTS, but the interpretation
and conclusions regarding the impor-
tance of workplace factors for CTS have
been intensively discussed during the
last 30 years, especially in the United
States, where CTS is reported most fre-
quently
2
and has been central in the sci-
entific and political struggle concern-
ing work-related musculoskeletal
disorders.
3
There is evidence that in-
dustrial repetitive, forceful work is a
contributing factor in CTS,
2,4
but the
role of computer use, which mainly
consists of repetitive, nonforceful move-
ments, is less clear. A recent study found
that the frequency of CTS in com-
puter users was similar to that in the
general population.
5
The reported prevalence of CTS in
the general population ranges from
0.7% to 9.2% among women and from
0.4% to 2.1% among men,
5-10
and the
prevalence and 1-year incidence among
intensive keyboard users in a data en-
try service company were found to be
1.1% and 0.27%, respectively, which
were confirmed by diagnostic inter-
view and nerve conduction studies.
11
In
a prospective study of computer us-
ers, Gerr et al
12
found a baseline preva-
lence of 0.5% and an overall annual
incidence of 0.9 cases per 100 person-
years. In earlier studies of computer us-
ers, based on symptoms and clinical ex-
aminations, the prevalence was slightly
higher.
13,14
The aim of the current study was to
determine the contribution of weekly
Author Affiliations: Department of Occupational
Medicine, Herning Hospital, Herning, Denmark (Drs
Andersen and Overgaard and Ms Vilstrup); Depart-
ment of Occupational Medicine, Copenhagen Uni-
versity Hospital, Glostrup, Denmark (Drs Thomsen, Las-
sen, Kryger, and Mikkelsen); and Department of
Occupational and Environmental Medicine, Odense
University Hospital, Odense, Denmark (Dr Brandt).
Corresponding Author and Reprints: Johan Hviid
Andersen, MD, PhD, Department of Occupational
Medicine, Herning Hospital, 7400 Herning, Den-
mark (e-mail: hecjha@ringamt.dk).
Context Computer use is increasingly common among many working populations,
and concern exists about possible adverse effects of computer use, such as carpal tun-
nel syndrome (CTS).
Objectives To estimate the prevalence and incidence of possible CTS and to evalu-
ate the contribution of use of mouse devices and keyboards to the risk of possible CTS.
Design and Setting A 1-year follow-up study with questionnaires conducted in 2000
and 2001 at 3500 workplaces in Denmark, followed on each of the 2 occasions by a
clinical interview on symptom distribution and frequency.
Participants The questionnaire was sent to 9480 members of a trade union, with
an initial response rate of 73% (n=6943), and 82% (n=5658) at follow-up.
Main Outcome Measures At baseline, there were 3 outcome measures: tingling/
numbness in the right hand once a week or more as reported in the questionnaire; tin-
gling, numbness, and pain in the median nerve in the right hand confirmed by clinical
interview; and tingling, numbness, and pain in the median nerve in the right hand at night
confirmed by clinical interview. At 1 year of follow-up the main outcome of interest was
onset of symptoms among participants who had no or minor symptoms at baseline.
Results The overall self-reported prevalence of tingling/numbness in the right hand
at baseline was 10.9%. The interview confirmed that prevalence of tingling/
numbness in the median nerve was 4.8%, of which about one third, corresponding to
a prevalence of 1.4%, experienced symptoms at night. Onset of new symptoms in
the 1-year follow-up was 5.5%. In the cross-sectional comparisons and in the fol-
low-up analyses, there was an association between use of a mouse device for more
than 20 h/wk and risk of possible CTS but no statistically significant association with
keyboard use.
Conclusions The occurrence of possible CTS in the right hand was low. The study
emphasizes that computer use does not pose a severe occupational hazard for devel-
oping symptoms of CTS.
JAMA. 2003;289:2963-2969 www.jama.com
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use of mouse devices and keyboards,
work-related physical factors, work-
related psychosocial factors, and indi-
vidual characteristics in the occur-
rence and onset of possible CTS.
METHODS
Design and Study Population
The Neck and Upper Extremity Disor-
ders Among Technical Assistants (NU-
DATA) study is a 1-year follow-up
study examining the relation between
computer use and neck and upper-
limb musculoskeletal symptoms and
disorders. The cohort was established
in January 2000 and was recruited from
the Danish Association of Profes-
sional Technicians (a trade union), and
represents a population of 9480 par-
ticipants at 3500 workplaces with a
wide distribution of both mouse de-
vice and keyboard use.
At baseline and at 1-year follow-up,
participants completed a question-
naire, and those meeting specific crite-
ria for a symptom case were offered a
standardized clinical interview and
physical examination of the neck and
upper extremities. All of the partici-
pants were employed at the time of in-
clusion in the cohort. They represent 2
occupational groups from the Danish As-
sociation of Professional Technicians;
namely, technical assistants (drafts-
men) and machine technicians, jobs re-
quiring a vocational education of about
3 years and carrying out technical draw-
ing tasks, administrative and graphical
tasks, and other mainly office-based
tasks. The participation rate was 73%
(n=6943) at baseline and 82% (n =5658)
at follow-up. The National Scientific Eth-
ics Committee approved the study.
Outcome Measures
At baseline, 3 outcome measures were
of interest. The first was any tingling/
numbness in the fingers at least once a
week or daily within the last 3 months
(except after sitting or lying with arms
in an awkward position), as reported in
the questionnaire. Response options in
the questionnaire were no, seldom, at
least once a month, at least once a week,
or daily. The second outcome measure
was tingling, numbness, and pain in the
median nerve at least once a week within
the last 3 months, confirmed by clini-
cal interview among participants fulfill-
ing the first outcome measure. The in-
terview focused on verification of
tingling/numbness, hand distribution,
and frequency of symptoms. The exam-
iner did not obtain any information on
mouse and keyboard use. The third out-
come measure at baseline was a combi-
nation of the second outcome measure
and symptoms at night.
At 1-year follow-up, the outcomes of
interest were tingling/numbness in the
right hand at least once a week within
the last 3 months among participants
with no or minor tingling/numbness at
baseline, and median nerve symptoms
in the right hand.
Work-Related Factors
The baseline and follow-up question-
naires included the same questions about
occupational physical factors, which en-
abled extraction of information about
time spent per week using a computer
(in hours per week) and time spent us-
ing mouse devices and keyboards sepa-
rately. On the basis of the self-reported
proportion of computer time during
which a mouse was actively being used
(and, correspondingly, the keyboard), as
well as information about hand use, we
estimated mouse use in hours per week
for the right hand and keyboard use in
hours per week (formulas available from
the authors on request). Mean mouse use
was 14.7 h/wk for women and 12.5 h/wk
for men, and mean keyboard use was 9.3
h/wk for women and 8.0 h/wk for men.
Other noncomputer work tasks includ-
ing office work not using a computer,
meetings, and supervision accounted for
the remainder of work time.
Posture-related variables included (1)
abnormal mouse position, with the
mouse positioned more than 40 cm to
the right of the shoulder or more than
40 cm from the desk’s front edge; (2) ab-
normal keyboard position, with the key-
board placed to the right or left side of
the body; (3) forearm/wrist support (no
support, support less than half of the
time, or support more than half of the
time) while using mouse devices and
keyboards; (4) whether the work desk
chair had been adjusted suitably (no, yes,
or cannot be adjusted); and (5) overall
satisfaction with physical workplace en-
vironment (on a 5-point scale of very dis-
satisfied to very satisfied, dichoto-
mized between satisfied and unsatisfied).
Psychosocial work characteristics
were assessed by a Danish version of the
Karasek job content questionnaire,
15
in-
cluding questions about job demands,
job control, and social support from co-
workers and supervisors. On the basis
of 2 questions about the ability to meet
current deadlines and quality require-
ments of the job, a dichotomous vari-
able termed “time pressure” repre-
sented inability vs ability to meet
requirements.
Personal Characteristics
Information about age, sex, height, and
weight was obtained. Negative affect and
type A behavior were determined by 2
questions designed for the study: “Do
you tend to be worried, nervous, or
somewhat pessimistic?” and “Do you
tend to be competitive, jealous, ambi-
tious, and somewhat impatient?” Re-
sponses were reported on a 7-point
nominal scale from not at all to very
much, and both were dichotomized be-
tween quite a lot and much (5-7 vs 0-4).
Leisure-time activity was categorized
into low physical activity (almost none
or light physical activity for ⬍2 h/wk,
light activity for 2-4 h/wk), and high
physical activity (light physical activity
⬎4 h/wk or 2-4 h/wk with hard physi-
cal activity, or hard physical activity for
⬎4 h/wk). Poor private social network
was measured only at baseline by 1 ques-
tion: “If you have problems, is it pos-
sible to obtain the necessary support
from family or friends?” Responses on
a nominal scale (always, nearly always,
usually, often, sometimes, or seldom/
never) were dichotomized between of-
ten and sometimes. Whether pain, tin-
gling, or numbness was related to a
specific accident was reported. Concur-
rent medical disorders such as inflam-
matory rheumatic diseases, diabetes, hy-
perthyroid or hypothyroid disease, and
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disorders of the nervous system were re-
corded. Participants with pain after an
accident were not invited to the clini-
cal examination.
Statistical Analyses
Baseline and follow-up analyses used lo-
gistic regression analyses, and all risk fac-
tors were retained in the models irre-
spective of level of significance. Mouse
device use and keyboard use were ana-
lyzed by assignment of dummy vari-
ables for weekly use to the categories 0
to less than 2.5, 2.5 to less than 5, 5 to
less than 10, 10 to less than 15, 15 to less
than 20, 20 to less than 25, 25 to less than
30, and 30 or more h/wk. In some of the
analyses, high levels of keyboard use
were collapsed further because of small
numbers of cases. In all multivariate
analyses, participants who used both
hands interchangeably were excluded
(n=623).
In analysis of follow-up data, the risk
of developing new or worsened tingling/
numbness was examined by logistic re-
gression among participants with no or
minor tingling/numbness at baseline. In
these follow-up analyses, 4 dummy
variables for increased or decreased (dif-
ference of ⬎5 h/wk in either direc-
tion) use of a mouse or keyboard since
baseline were introduced to adjust for
the difference between mouse use and
keyboard use in the 1-year follow-up
period. Because of a small number of
incident cases with extensive median
nerve symptoms (n =35), we intro-
duced only mouse use and keyboard use
into the model. Mouse and keyboard
use were negatively correlated in all
analyses, and introduction of an inter-
action term did not contribute signifi-
cantly to any of the models (P =.45). To
check for colinearity, we calculated the
correlation coefficients between all pro-
posed risk factors, and all were less than
0.25. Stata version 7.0 software (Stata
Corp, College Station, Tex) was used
for the analyses.
RESULTS
Survey
The distribution of characteristics for
mouse use, keyboard use, sex, age, job
tasks, and baseline outcome status
among respondents and nonrespon-
dents at follow-up is shown in T
ABLE 1.
There was an overrepresentation
among nonrespondents at follow-up of
young men with executive jobs, but
there was no remarkable difference in
exposure time or the level of possible
CTS symptoms at baseline.
Symptoms
The overall self-reported prevalence of
tingling/numbness in the right hand at
baseline was 10.9%. The interview con-
firmed that prevalence of tingling/
numbness in the median nerve was
4.8%, of which about one third, corre-
sponding to a prevalence of 1.4%, ex-
perienced symptoms at night.
The incidence of new or worsened
more frequent CTS symptoms reported
in the questionnaire at 1-year fol-
low-up was 5.5% (n=198), but only 41
participants (1.2%) had symptoms in the
median nerve. Changes in symptom level
between baseline and follow-up are
shown in T
ABLE 2. The majority of par-
ticipants remained at the same symp-
tom level (42.8%). A slightly higher pro-
portion improved than worsened in
symptoms (9.1% and 7.7%, respectively).
The proportion missing at follow-up was
greatest among participants with no
symptoms at baseline (1761/4488
Table 1. Baseline Characteristics According to Status at 1-Year Follow-up
*
Baseline Status
Follow-up Status
Respondents
(n = 5658)
Nonrespondents
(n = 1285)
Right-handed mouse use, h/wk
0to⬍2.5 1279 (23.4) 273 (22.3)
2.5 to ⬍5 380 (7.0) 94 (7.7)
5to⬍10 676 (12.4) 169 (13.8)
10 to ⬍15 980 (18.0) 220 (18.0)
15 to ⬍20 877 (16.1) 209 (17.1)
20 to ⬍25 706 (12.9) 155 (12.7)
25 to ⬍30 313 (5.7) 59 (4.8)
ⱖ30 248 (4.5) 44 (3.6)
Keyboard use, h/wk
0to⬍2.5 532 (9.5) 134 (10.7)
2.5 to ⬍5 864 (15.4) 195 (15.6)
5to⬍10 1633 (29.2) 367 (29.3)
10 to ⬍15 1043 (18.6) 207 (16.5)
15 to ⬍20 686 (12.3) 168 (13.4)
20 to ⬍25 400 (7.1) 75 (6.0)
25 to ⬍30 236 (4.2) 48 (2.6)
ⱖ30 201 (3.6) 58 (4.6)
Sex, female 3616 (63.9) 731 (56.9)
Age, mean (SD), y 41.7 (8.9) 39.1 (8.9)
Job title
Technical assistant 3586 (63.4) 697 (54.2)
Machine technician 746 (13.2) 192 (14.9)
Executive, sales manager, engineer 391 (6.9) 138 (10.7)
Other 739 (13.1) 191 (14.9)
Baseline outcome status
Questionnaire case† 585 (10.9) 129 (10.9)
Median nerve symptoms‡ 255 (4.9) 53 (4.7)
Median nerve symptoms at night§ 70 (1.3) 20 (1.8)
*
Data are expressed as No. (%) unless otherwise noted. For right-handed mouse use, n = 5459 for respondents and
n = 1223 for nonrespondents. For keyboard use, n = 5595 for respondents and n = 1252 for nonrespondents.
†Defined as tingling/numbness at least once a week within the last 3 months in the right hand.
‡Defined as tingling/numbness at least once a week within the last 3 months in the area of the right hand innervated by
the median nerve.
§Defined as tingling/numbness at night at least once a week within the last 3 months in the area of the right hand
innervated by the median nerve.
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[39.2%]) and lowest among those who
reported more frequent symptoms at
baseline (185/714 [25.9%]); this points
against a selection out of the study of
those who had the most symptoms at
baseline.
Risk Factors at Baseline
Tingling/numbness in the right hand
was associated with time spent using a
mouse device but not time spent using
a keyboard (T
ABLE 3). Although the 2
broadest definitions of the outcome
showed an exposure response pattern
for hours per week using a mouse, the
inclusion of symptoms at night revealed
a significant association only when time
spent using a mouse exceeded 30 h/wk.
None of the included posture vari-
ables were associated with any of the
outcomes, but participants who were
dissatisfied with their physical work-
place design had a slightly elevated risk
(odds ratios [ORs] for the 3 outcomes
were 1.6 [95% confidence interval {CI},
1.2-2.1], 1.7 [95% CI, 1.1-2.6], and 1.9
[95% CI, 0.9-4.2], respectively). Psy-
chosocial risk factors were not signifi-
cantly associated with possible CTS.
Women had an elevated risk ranging
from 2.1 to 7.4 for the 3 outcomes, and
older age, other medical disorders, and
smoking were also associated with pos-
sible CTS, although nonsignificantly, for
all 3 outcomes.
Risk Factors at Follow-up
Onset of symptoms of possible CTS af-
ter 1 year was associated with mouse use
with a somewhat irregular exposure re-
sponse pattern at less than 20 h/wk, but
mouse use of 20 h/wk or more was ob-
served to be a risk factor for becoming
an incident case of self-reported tingling/
numbness with elevated ORs (for 20 to
⬍25 h/wk, OR, 2.6; 95% CI, 1.2-5.5; for
25 to ⬍30 h/wk, OR, 3.2; 95% CI, 1.3-
7.9; and for ⱖ30 h/wk, OR, 2.7; 95% CI,
1.0-7.6) (T
ABLE 4). At follow-up, only
35 participants had symptoms in the me-
dian nerve, which was too few to be in-
cluded in a thorough multivariate model.
A model including mouse use and key-
board use showed an elevated risk when
weekly use of the mouse exceeded 20
h/wk (OR, 3.6; 95% CI, 1.4-9.4). There
was only a slight indication of an asso-
ciation between keyboard use and on-
set or worsening of symptoms.
A total of 60.8% reported that their
weekly computer use was at the same
level as in the previous year, and 83.1%
of the participants reported mouse use
at follow-up at nearly the same level (±5
h/wk). Introducing variables into the
logistic regression model for those who
used a mouse more and who used a
mouse less at follow-up contributed to
the final model with ORs of 0.98 (95%
CI, 0.50-1.90) and 0.77 (95% CI,
0.38-1.56), respectively. The mean (SD)
difference in time spent using a mouse
in the right hand between baseline and
1-year follow-up was 0.3 (6.9) h/wk for
participants who experienced new or
more symptoms (Table 2). The mean
(SD) difference for those whose symp-
toms resolved or improved was 1.5
(7.9) h/wk (Table 2). The correspond-
ing mean (SD) differences for time spent
using a keyboard were 0.5 (5.7) h/wk
and 0.7 (5.9) h/wk, respectively.
Other predictors for onset of tingling/
numbness were other medical disor-
ders, female sex (with a lower risk than
in the baseline comparisons; OR, 1.6;
95% CI, 1.1-2.4), previous accident, and
smoking. As in the baseline compari-
sons, posture variables and psychoso-
cial risk factors were not associated with
an elevated risk of possible CTS.
COMMENT
This study found a prevalence of pos-
sible CTS between 1.4% and 4.8% based
on a screening questionnaire and a clini-
cal interview, and an incidence of new
or aggravated symptoms of possible CTS
of 5.5%. When the median nerve was in-
cluded, the incidence dropped to 1.2%.
We did not include nerve conduction
studies, but based on other results us-
ing the same diagnostic screening pro-
cess, the prevalence and annual inci-
dence of CTS confirmed by nerve
conduction studies would have dropped
to approximately one third of the val-
ues for interview-based possible CTS.
11
Another study also found a decrease from
10.5% among those who met clinical cri-
teria to 3.5% when nerve conduction was
included.
5
With that reasoning, our in-
cidence would drop to less than 1%.
The NUDATA study benefits from a
large cohort with a wide range of expo-
sure and simultaneous analysis of physi-
cal, psychosocial, and nonoccupational
personal characteristics measured at 2
separate points. The prevalence of symp-
toms was at the level of the general
Table 2. Frequency of CTS Symptoms at Baseline and 1-Year Follow-up
*
Baseline CTS
Symptom Level
CTS Symptom Level at 1-Year Follow-up, No. (%)
No Symptoms Mild CTS Symptoms More Frequent CTS Symptoms Missing Data Total
No symptoms Stable Incident Incident
2322 (33.4) 331 (4.8) 74 (1.1) 1761 (25.4) 4488 (64.6)
Mild symptoms Resolved Ongoing Worsened
353 (5.1) 400 (5.8) 124 (1.8) 348 (5.0) 1225 (17.6)
More frequent symptoms Resolved Improved Chronic
107 (1.5) 172 (2.5) 250 (3.6) 185 (2.7) 714 (10.3)
Missing data 190 (2.7) 36 (0.5) 20 (0.3) 270 (3.9) 516 (7.4)
Total 2972 (42.8) 939 (13.5) 468 (6.7) 2564 (36.9) 6943 (100)
Abbreviation: CTS, carpal tunnel syndrome.
*
Symptom levels were defined as follows: no symptoms, no tingling/numbness within the last 3 months; mild symptoms, tingling/numbness seldom or at least once a month within
the last 3 months; and more frequent symptoms, tingling/numbness at least once a week or daily within the last 3 months. Percentages do not exactly sum to totals because of
rounding.
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Table 3. Prevalence of Tingling/Numbness in the Right Hand at Baseline and Results From Logistic Regression Analyses by Case Definition
*
Characteristics
Questionnaire Case
(n = 602-612)†
Median Nerve Symptoms
(n = 260-265)‡
Median Nerve Symptoms
at Night (n = 76-77)§
No. (%) OR (95% CI) No. (%) OR (95% CI) No. (%) OR (95% CI)
Physical Work Characteristics
Right-handed mouse use, h/wk
0to⬍2.5 51 (5.9) 1.0 19 (2.2) 1.0 5 (0.6) 1.0
2.5 to ⬍5 25 (5.7) 1.0 (0.6-1.7) 7 (1.6) 0.7 (0.3-1.9) 2 (0.5) 0.8 (0.1-4.7)
5to⬍10 62 (7.9) 1.5 (0.9-2.4) 33 (4.3) 2.3 (1.2-4.5) 12 (1.6) 2.3 (0.7-7.9)
10 to ⬍15 111 (10.0) 1.8 (1.2-2.8) 50 (4.6) 2.2 (1.1-4.2) 11 (1.0) 1.1 (0.3-3.9)
15 to ⬍20 137 (13.2) 2.8 (1.9-4.3) 55 (5.5) 3.1 (1.6-6.0) 15 (1.5) 1.9 (0.6-6.7)
20 to ⬍25 115 (14.2) 3.3 (2.2-5.0) 54 (6.8) 3.6 (1.8-7.1) 15 (1.9) 1.9 (0.6-6.8)
25 to ⬍30 60 (17.2) 4.3 (2.6-7.0) 25 (7.4) 3.5 (1.6-7.4) 4 (1.2) 0.9 (0.2-4.4)
ⱖ30 42 (15.4) 3.8 (2.2-6.5) 19 (7.2) 3.5 (1.6-8.0) 11 (4.2) 4.1 (1.0-16)
Arm/wrist support during mouse use
Never 85 (8.9) 1.0 33 (5.0) 1.0 7 (0.8) 1.0
⬎0% to 50% of time 71 (12.5) 1.2 (0.8-1.7) 26 (4.7) 1.2 (0.7-2.2) 6 (1.1) 2.0 (0.6-7.1)
⬎50% to 100% of time 446 (10.9) 0.9 (0.7-1.2) 201 (5.1) 1.1 (0.7-1.8) 63 (1.6) 1.7 (0.6-5.0)
Keyboard use, h/wk
0to⬍2.5 59 (12.6) 1.0 22 (4.8) 1.0 8 (1.8) 1.0
2.5 to ⬍5 115 (11.1) 0.7 (0.5-1.0) 60 (5.9) 1.0 (0.6-1.7) 20 (2.0) 0.9 (0.4-2.3)
5to⬍10 190 (9.9) 0.7 (0.5-1.0) 82 (4.3) 0.8 (0.5-1.2) 26 (1.4) 0.7 (0.3-1.6)
10 to ⬍15 140 (10.4) 0.9 (0.6-1.2) 56 (4.3) 0.7 (0.4-1.3) 14 (1.1) 0.7 (0.3-1.8)
15 to ⬍20 72 (11.4) 0.8 (0.5-1.2) 30 (4.9) 0.8 (0.4-1.5) 7 (0.8) 0.3 (0.1-1.0)
ⱖ20 27 (10.8) 1.5 (0.9-2.6) 12 (4.9) 1.6 (0.7-3.7)
Arm/wrist support during keyboard use
Never 240 (11.2) 1.0 105 (5.0) 1.0 29 (1.4) 1.0
⬎0% to 50% of time 122 (10.5) 1.0 (0.8-1.4) 48 (4.2) 1.0 (0.7-1.5) 11 (1.0) 1.1 (0.5-2.3)
⬎50% to 100% of time 249 (10.3) 1.0 (0.8-1.3) 112 (4.8) 1.1 (0.8-1.6) 37 (1.6) 1.3 (0.7-2.4)
Abnormal mouse position 40 (9.1) 0.9 (0.6-1.3) 16 (3.7) 0.8 (0.4-1.4) 4 (0.9) 0.9 (0.3-2.5)
Abnormal keyboard position 85 (10.9) 1.0 (0.8-1.3) 38 (5.0) 1.1 (0.8-1.6) 8 (1.1) 0.9 (0.4-2.0)
Work chair not adjusted 26 (15.2) 1.4 (0.8-2.6) 8 (4.9) 0.8 (0.3-2.0) 1 (0.6) 0.8 (0.1-6.5)
Work desk not adjusted 131 (9.1) 0.8 (0.6-1.0) 55 (3.9) 0.8 (0.6-1.2) 9 (0.6) 0.4 (0.2-1.0)
Not satisfied with workplace design 115 (14.7) 1.6 (1.2-2.1) 46 (6.1) 1.7 (1.1-2.6) 10 (1.3) 1.9 (0.9-4.2)
Psychosocial Work Characteristics
High demands 276 (12.0) 1.3 (1.1-1.6) 117 (5.2) 1.2 (0.9-1.6) 36 (1.6) 1.7 (0.98-2.8)
Low control 256 (13.9) 1.3 (1.1-1.6) 106 (5.9) 1.2 (0.9-1.6) 30 (1.7) 1.2 (0.7-2.0)
Low social support 297 (13.0) 1.2 (1.0-1.5) 120 (5.4) 1.1 (0.8-1.4) 33 (1.5) 1.1 (0.7-1.9)
Time pressure 184 (13.3) 1.2 (0.9-1.5) 84 (6.2) 1.4 (1.0-1.9) 22 (1.6) 1.1 (0.6-1.9)
Personal Characteristics
Negative affect 100 (12.9) 1.0 (0.8-1.3) 45 (5.9) 1.0 (0.7-1.5) 9 (1.2) 0.5 (0.2-1.3)
Type A behavior 78 (10.6) 1.0 (0.8-1.3) 32 (4.5) 1.0 (0.6-1.5) 9 (1.3) 0.9 (0.4-2.1)
Age㛳 NA 1.1 (1.1-1.3) NA 1.3 (1.1-1.6) NA 1.4 (1.1-1.9)
Female sex 480 (13.2) 2.1 (1.7-2.7) 219 (6.2) 3.0 (2.1-4.4) 70 (2.0) 7.4 (2.9-19)
High body mass index 24 (14.0) 1.3 (0.8-2.0) 10 (6.0) 1.3 (0.7-2.5) 3 (1.8) 1.5 (0.4-4.9)
Low body mass index 39 (12.5) 1.1 (0.7-1.6) 17 (5.6) 1.0 (0.5-1.8) 8 (2.6) 2.0 (0.9-4.6)
Poor social network 69 (14.3) 1.3 (0.9-1.7) 31 (6.6) 1.2 (0.8-1.9) 7 (1.5) 1.4 (0.6-3.2)
High physical activity 236 (10.0) 1.0 (0.9-1.3) 110 (4.7) 1.1 (0.9-1.5) 32 (1.4) 1.3 (0.8-2.1)
Medical disorder 63 (17.3) 2.1 (1.5-2.9) 21 (6.2) 1.5 (0.9-2.4) 7 (2.1) 1.7 (0.7-4.0)
Smoking 353 (11.6) 1.2 (1.0-1.5) 153 (5.2) 1.2 (0.9-1.6) 40 (1.4) 1.0 (0.6-1.7)
Symptoms started after accident 17 (15.9) 1.8 (1.0-3.2) 0 NA 0 NA
Abbreviations: CI, confidence interval; NA, not applicable; OR, odds ratio.
*
Mouse related variables refer to right-handed use. Participants who operated the mouse with both hands (n = 623) were excluded. The ORs shown are adjusted for effects of
psychosocial and personal factors listed in this table. Because of missing values for the outcomes and explanatory variables, sample sizes for each model were different.
†Defined as tingling/numbness at least once a week within the last 3 months in the right hand.
‡Defined as tingling/numbness at least once a week within the last 3 months in the area of the right hand innervated by the median nerve.
§Defined as tingling/numbness at night at least once a week within the last 3 months in the area of the right hand innervated by the median nerve.
㛳Continuous variable; data show effect of 10-year increments.
COMPUTER USE AND CARPAL TUNNEL SYNDROME
©2003 American Medical Association. All rights reserved. (Reprinted) JAMA, June 11, 2003—Vol 289, No. 22 2967
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population and, in cross-sectional com-
parisons, we found an association with
time spent using a mouse device, al-
though the most rigorous definition in-
cluding symptoms at night showed an el-
evated risk only among those who used
a mouse for 30 h/wk or more. A differ-
ential loss of participants from baseline
to 1-year follow-up could explain the very
low incidence, but the prevalence at base-
line of tingling/numbness among non-
respondents was the same as for those
who remained in the study (Table 1), and
participants with more frequent symp-
toms at baseline remained in the study
to a higher extent than participants with-
out symptoms at baseline (Table 2).
Associations between mouse use
and onset or worsening of tingling/
numbness were confirmed by fol-
low-up analyses. Mouse use for 20 h/wk
or more seems to imply a slightly el-
evated risk of possible CTS. This risk
could be underestimated if partici-
pants with symptoms had moved to
lower exposure groups in the fol-
low-up period, but accounting for de-
crease or increase in mouse/keyboard
use did not change the risk estimates.
Other Studies
In a national survey assessing occupa-
tional exposures to vibration, 1 item
concerned regular use of keyboards.
16
This study found no association be-
tween keyboard use for more than 4
hours in an average workday and tin-
gling/numbness during the previous
week. The ORs for men and women
were 1.1 (95% CI, 0.8-1.3) and 1.1
(95% CI, 0.9-1.3), respectively. In a re-
cent follow-up study of newly hired
computer users with jobs requiring
more than 15 h/wk of computer use,
Gerr et al
12
found a baseline preva-
lence of CTS of 0.5%, verified by nerve
conduction studies. During the fol-
low-up phase of that study, 3 new cases
with CTS were found, corresponding
to an incidence of 0.9%. Other studies
that have investigated CTS among com-
puter users have reported prevalences
of 1.0% and 1.3%.
13,14
Taken together,
previous studies and the present study
are in contrast with a common belief
Table 4. Incidence of Possible CTS in the Right Hand at Follow-up Among Participants With
No or Minor Symptoms at Baseline and Results From Logistic Regression Analyses
*
Characteristics
Incidence of Possible CTS
in the Right Hand
(n = 166-173)
No. (%) OR (95% CI)
Physical Work Characteristics
Right-handed mouse use, h/wk
0to⬍2.5 16 (3.3) 1.0
2.5 to ⬍5 7 (2.8) 0.7 (0.3-1.9)
5to⬍10 24 (5.6) 1.9 (0.9-4.0)
10 to ⬍15 36 (5.8) 1.6 (0.8-3.3)
15 to ⬍20 34 (5.9) 2.0 (0.9-4.2)
20 to ⬍25 29 (6.4) 2.6 (1.2-5.5)
25 to ⬍30 16 (8.4) 3.2 (1.3-7.9)
ⱖ30 11 (7.0) 2.7 (1.0-7.6)
Arm/wrist support during mouse use
Never 16 (3.3) 1.0
⬎0% to 50% of time 16 (5.2) 1.5 (0.7-3.3)
⬎50% to 100% of time 137 (5.9) 1.9 (0.99-3.5)
Keyboard use, h/wk
0to⬍2.5 15 (6.0) 1.0
2.5 to ⬍5 32 (5.5) 0.9 (0.4-1.8)
5to⬍10 54 (5.1) 0.8 (0.4-1.5)
10 to ⬍15 51 (6.5) 1.2 (0.6-2.5)
15 to ⬍20 14 (4.0) 0.8 (0.4-1.5)
ⱖ20 7 (5.2) 1.4 (0.5-4.3)
Arm/wrist support during keyboard use
Never 67 (5.8) 1.0
⬎0% to 50% of time 45 (7.0) 1.2 (0.8-1.8)
⬎50% to 100% of time 61 (4.5) 0.7 (0.5-1.1)
Abnormal mouse position 6 (2.4) 0.4 (0.1-0.9)
Abnormal keyboard position 25 (6.0) 1.1 (0.7-1.7)
Work chair not adjusted 6 (6.5) 1.3 (0.5-3.3)
Work desk not adjusted 41 (5.3) 1.0 (0.7-1.6)
Not satisfied with workplace design 25 (5.9) 0.9 (0.5-1.6)
Psychosocial Work Characteristics
High demands 78 (6.4) 1.3 (0.9-1.8)
Low control 62 (6.0) 0.9 (0.7-1.4)
Low social support 78 (6.3) 1.2 (0.9-1.8)
Time pressure 44 (5.9) 1.0 (0.7-1.6)
Personal Characteristics
Negative affect 25 (6.0) 0.9 (0.6-1.5)
Type A behavior 25 (6.5) 1.1 (0.7-1.8)
Age† NA 1.1 (0.9-1.3)
Female sex 127 (6.2) 1.6 (1.1-2.4)
High body mass index 10 (5.9) 1.1 (0.6-2.3)
Low body mass index 3 (3.1) 0.6 (0.2-1.9)
Poor social network 19 (7.5) 1.2 (0.7-2.2)
High physical activity 72 (5.3) 1.1 (0.8-1.5)
Medical disorder 18 (8.8) 1.7 (1.0-3.1)
Smoking 111 (6.8) 1.7 (1.2-2.4)
Symptoms started after accident 11 (12.9) 3.1 (1.5-6.1)
Abbreviations: CI, confidence interval; CTS, carpal tunnel syndrome; NA, not applicable; OR, odds ratio.
*
The ORs for incidence of possible CTS shown are adjusted for effects of psychosocial and personal factors. Because
of missing values for the outcomes and explanatory variables, sample sizes differ. The Hosmer-Lemeshow goodness-
of-fit test for 10 groups yielded P = .31.
†Continuous variable; data show effect of 10-year increments.
COMPUTER USE AND CARPAL TUNNEL SYNDROME
2968 JAMA, June 11, 2003—Vol 289, No. 22 (Reprinted) ©2003 American Medical Association. All rights reserved.
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that CTS is a frequent disorder among
computer users.
Keyboard Use
The NUDATA study is, to our knowl-
edge, the first study to attempt to dis-
sect computer use into mouse use and
keyboard use. Keyboard use for 20 h/wk
or more was slightly associated with tin-
gling/numbness at baseline and fol-
low-up for 2 of the outcome defini-
tions. This finding could be explained
by too little variation and contrast in
our cohort concerning use of a key-
board. Mean (SD) self-reported key-
board use was about 8 to 9 (5-6) h/wk.
Repetitive keying of 8000 to 12000 key-
strokes/h has been found to be a risk
factor for arm, hand, and elbow pain.
17
In a study of data entry workers in
which the keying speed was around
12000 keystrokes/h, a 10-h/wk in-
crease in data entry work was associ-
ated with an increased risk of CTS (OR,
1.8; 95% CI, 1.1-3.2), but this finding
was based on only 8 cases. In a sub-
group of our cohort (n=2146), the key-
ing speed in the 75th percentile was
measured to be 8000 to 22000 key-
strokes/h. There was variation and some
heavy keyboard use, but there were too
few cases in the highest exposure
groups to make proper use of the wide
exposure range.
Can keyboard use then be consid-
ered an occupational risk for develop-
ing CTS? From our data it seems un-
likely, but based on other studies, we
cannot exclude the possibility that very
intensive and repetitive keyboard use
could be a risk factor for CTS. How-
ever, our opinion is that it is not an im-
portant one.
Mouse Device Use
Use of a mouse device was associated
with symptoms in the cross-sectional
comparisons as well as in the follow-up
analyses. Despite efforts to introduce the
study as a general study of work envi-
ronment and health among computer us-
ers, the study funding was initiated be-
cause of public concern and discussions
in the media of “mouse-related disor-
ders,” “mouse arm,” and other such
terms, implying a focus on the mouse at
this time. This could explain the cross-
sectional associations, which could be
skewed by information bias. The find-
ing of an exposure response pattern be-
tween use and symptoms and the find-
ings from the follow-up make the
associations plausible. But the irregular
exposure response pattern for the
amount of use of a mouse or a key-
board makes it difficult to establish any
threshold time values for use of the de-
vices. One would expect low risks at the
lower end of exposure and a threshold
level at which the risk increases. We did
not find such a pattern, and, by intro-
duction of the time variables as continu-
ous instead of categorical, we could not
obtain a better fit of powers other than
linear in predicting any of the out-
comes (results not shown).
There were strong contributions to
the onset of new symptoms by an ac-
cident prior to symptom onset, other
medical disorders, and smoking—risk
factors that have been found in other
studies.
18
Tingling/numbness is re-
lated to nerve entrapment, but most
people who experience tingling/
numbness do so because of reasons
other than nerve entrapment. It is prob-
able that tingling and numbness are
common symptoms of either specific
medical conditions other than CTS or
are part of a large burden of medically
unexplained symptoms that reflect the
stresses and strains of everyday life.
Author Contributions: Study concept and design:
Andersen, Thomsen, Lassen, Brandt, Vilstrup,
Mikkelsen.
Acquisition of data: Andersen, Overgaard, Lassen,
Brandt, Vilstrup, Kryger, Mikkelsen.
Analysis and interpretation of data: Andersen,
Thomsen, Brandt, Kryger, Mikkelsen.
Drafting of the manuscript: Andersen, Overgaard,
Vilstrup.
Critical revision of the manuscript for important in-
tellectual content: Thomsen, Lassen, Brandt, Kryger,
Mikkelsen.
Statistical expertise: Andersen, Overgaard, Brandt,
Kryger.
Obtained funding: Andersen, Brandt, Mikkelsen.
Administrative, technical, or material support: Lassen,
Brandt, Vilstrup.
Study supervision: Andersen.
Funding/Support: This study was supported by grant
9801292 from the Danish Medical Research Council
and grant 20000010486 from the Danish Ministry of
Employment, National Work Environment Authority.
Acknowledgment: We thank the Danish Associa-
tion of Professional Technicians for useful assistance,
and we especially thank the study participants for their
time and effort.
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