© Blackwell Publishing Ltd

Cephalalgia,

2004,

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, 161–172

161

Blackwell Science, Ltd

Oxford, UKCHA

Cephalalgia

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243161172

Review Article

Neurobiology of chronic tension-type headacheM Ashina

REVIEW

Neurobiology of chronic tension-type headache

M Ashina

Department of Neurology and Danish Headache Center, Glostrup Hospital, University of Copenhagen, Glostrup, Copenhagen, Denmark

Ashina M. Neurobiology of chronic tension-type headache. Cephalalgia 2004;
24:161–172. London. ISSN 0333-1024

central sensitization, microdialysis, muscle tenderness, nitric oxide, tension-type

headache

Messoud Ashina MD, PhD, DrSci, Department of Neurology and Danish Headache
Centre, Glostrup Hospital, University of Copenhagen, DK-2600 Glostrup, Copenhagen,
Denmark. Tel.

+

45 4323 2300, ext. 3068, fax

+

45 4323 3926, e-mail [e-mail]

Received 30 January 2003, accepted 16 June 2003

Introduction

Chronic tension-type headache is one of the most
common and important types of primary headaches
(1) and represents a considerable health and socio-
economic problem (2). Increased tenderness of
pericranial myofascial tissues to manual palpation
is the most prominent abnormal finding in patients
with chronic tension-type headache (3–6). Painful
impulses from these tissues may be referred to the
head and perceived as headache, and myofascial
mechanisms may therefore play a major role in
the pathophysiology of tension-type headache (7).
Progress in molecular neurobiology of pain (8) and
an increasing number of studies on tension-type
headache (9) have increased our knowledge about
the mechanisms underlying chronic head pain.
Thus, substantial experimental evidence indicates
that central sensitization, i.e. increased excitability of
neurons in the central nervous system (CNS) gener-
ated by prolonged nociceptive input from the peric-
ranial myofascial tissues, plays an important role in
the pathophysiology of chronic pain (8) and chronic
tension-type headache (9). Furthermore, discovery
of neurotransmitters and neuromodulators such as
nitric oxide (NO), calcitonin gene-related peptide
(CGRP), substance P (SP), neuropeptide Y (NPY)
and vasoactive intestinal polypeptide (VIP) involved
in the pain processing provides new insights to our
understanding of the biology of chronic head pain.
To explore the neurobiology of human chronic pain
conditions, it is necessary to utilize advances made
in basic research. The purpose of the present thesis

was to study the neurobiology of chronic tension-
type headache. Specific aims were: (i) to investigate
NO mechanisms in chronic tension-type headache
sufferers; (ii) to study plasma levels of CGRP, SP,
NPY and VIP in patients with chronic tension-type
headache; and (iii) to study

in vivo

skeletal muscle

blood flow during static exercise in patients with
chronic tension-type headache.

Nitric oxide in chronic tension-type headache

Biosynthesis of nitric oxide

The free radical NO is a messenger molecule involved
in various biological functions (10–12). NO is synthe-
sized by a complex family of nitric oxide synthase
(NOS) enzymes (13). Because NO is highly reactive
and unstable, much of the research on its functions
is based on characterization of NOS. Three distinct
NOS enzymes [neuronal NOS (nNOS), endothelial
NOS, inducible NOS) have been purified, cloned and
biochemically characterized (13). The precise mech-
anism of NO formation is not fully understood. It is
known that NO is synthesized from L-arginine and
that the reaction also yields citrulline (Fig. 1).

NO and nociception in animal studies

NO or NOS immunoreactivity has been identified in
the peripheral and central nervous system. In the
peripheral nervous system, nNOS immunoreactivity
was demonstrated in dorsal root ganglia in both man
and rat (14) and in perivascular nerves of large cere-

162

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© Blackwell Publishing Ltd

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, 161–172

bral arteries of the rat (15) and human brain (16). In
the central nervous system, nNOS is present exclu-
sively in neurons and NOS neurons were demon-
strated in the spinal trigeminal nucleus (17) and in
the dorsal horn of the spinal cord (14, 18).

NO derived from neurons was first recognized

when N-methyl-D-aspartate (NMDA) receptor acti-
vation in cerebellar cultures resulted in NO generation
(19). Involvement of NO in neuronal signalling was
confirmed by demonstrating that NOS inhibitors
blocked the stimulation of cyclic guanosine mono-
phosphate in brain that is associated with activation
of NMDA receptor (20, 21). Later, it was demonstrated
that sensitization of pain pathways in the spinal cord
may be caused by or associated with activation of
nNOS and the generation of NO (22–24). Moreover,
prolonged elevation of NO levels within the spinal
dorsal horn is important in maintaining the central
sensitization (25). Finally, it has been shown that inhi-
bition of NOS reduces central sensitization in pain
models (22, 26, 27) and that nociceptive responses in
these models are enhanced by NO donors (28, 29).
Taken together, these data suggest that NO is an
important transmitter in pain pathways of the spinal
cord and that NO contributes to development and
maintenance of central sensitization at the spinal level.

Central sensitization and chronic tension-type
headache

Nociception from pericranial myofascial tissues may
play a major role in the pathophysiology of tension-

type headache. Thus, several studies have consis-
tently reported increased myofascial tenderness as
the most prominent abnormal finding in patients
with chronic tension-type headache (3–6). Further
support for myofascial involvement is the finding of
increased muscle hardness (30) and a positive corre-
lation between muscle hardness and tenderness in
chronic tension-type headache (31). In addition to
findings in the periphery, chronic tension-type head-
ache sufferers also exhibit signs of increased sensi-
tivity in the CNS. Thus, pressure pain detection and
tolerance thresholds to mechanical stimuli have been
found decreased in these patients (32, 33). Further-
more, Bendtsen et al. (34) demonstrated that patients
with chronic tension-type headache had a qualita-
tively altered pain perception. On the basis of these
findings and data from basic pain research (35), it
has been suggested that the central sensitization and
thereby the chronic pain state in patients with
chronic tension-type headache may be due to sensi-
tization at the level of the spinal dorsal horn/trigem-
inal nucleus induced by prolonged nociceptive input
from pericranial myofascial tissues (9, 36).

Inhibition of NOS in chronic tension-type headache

In order to test the hypothesis that inhibition of NO
and thereby central sensitization would reduce
chronic headache, Ashina et al. (37) investigated the
analgesic effect of the NOS inhibitor L-N

G

methyl

arginine hydrochloride (L-NMMA) in patients with
chronic tension-type headache. In a double-blind,

Figure 1

Nitric oxide synthase-catalysed oxidation of L-arginine. Nitric oxide is synthesized from L-arginine and the reaction

also yields citrulline. (Reproduced from ‘Nitric oxide in the nervous system’. In: V. B. Mayer, editor. Biochemistry and molecular
pharmacology of nitric oxide synthases, Chapter 2, 1995:21–38, with permission from Elsevier.)

NH

H

2

N NH

2

H

+

H

2

N

1 NADPH

1 NADPH

L-arginine

NH

+

N OH

H

N

G

-hydroxy-L-arginine

NH

COO

–

H

3

N

+

COO

–

H

3

N

+

COO

–

H

3

N

+

NH

2

O

2

O

2

O

+

•

N=O

L-citrulline

Neurobiology of chronic tension-type headache

163

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Cephalalgia,

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placebo-controlled crossover study patients received
L-NMMA or placebo on 2 days. L-NMMA reduced
headache intensity significantly more than placebo
(Fig. 2). To explore the mechanisms of this analgesic
effect Ashina et al. (38) also studied the relationship
between myofascial factors and NOS inhibition. This
study showed that both muscle hardness and ten-
derness were significantly reduced following treat-
ment with L-NMMA, while there was no significant
reduction at any time after treatment with placebo
(Figs 3 and 4). The muscle hardness was significantly
reduced following treatment with L-NMMA com-
pared with placebo. The reduction in tenderness
following treatment with L-NMMA did not reach
statistical significance compared with placebo. The
pressure pain detection thresholds in the finger and

temporal region were largely unchanged following
treatment with L-NMMA. Thus, these studies (37,
38) demonstrated that the NOS inhibitor, L-NMMA,
reduces headache intensity and muscle hardness in
patients with chronic tension-type headache.

An important question is how L-NMMA modu-

lates myofascial factors in patients with chronic
tension-type headache and whether the effects of L-
NMMA are due to an action in muscle, peripheral
nerves or the CNS. It is well established that persis-
tent activity in peripheral nociceptors may lead to
sensitization of neurons in the spinal dorsal horn,
partly via activation of NMDA receptors (8). Many
of the effects of NMDA receptor activation are medi-
ated via production of NO (23) and, as described
earlier, animal models of persistent pain have shown
that inhibitors of NOS decrease sensitization of the
spinal dorsal horn induced by continuous painful
input from the periphery (22, 24, 26, 27). On the basis
of these findings Ashina et al. (37, 38) suggested that
the anti-nociceptive effect of NOS inhibition in
patients with chronic tension-type headache is prob-
ably due to reduction of central sensitization at the
level of the spinal dorsal horn/trigeminal nucleus.
One should, however, also consider other possible
mechanisms of action. Thus, it is possible that L-
NMMA has direct anti-nociceptive effects in myofas-
cial tissues. It has been demonstrated that NOS
inhibitors have anti-nociceptive effects after periph-
eral administration, probably due to inhibition of
endothelial NOS (eNOS) (22, 39, 40). However, the
exact role of NO in the periphery is still far from
understood, and additional research is needed to

Figure 2

Percent changes from baseline pain intensity on a

100-mm visual analogue scale (VAS) in 16 patients with
chronic tension-type headache. The pain intensity was
significantly more reduced following treatment with L-
NMMA (



) compared with placebo (



) (

P

=

0.01). *

P

<

0.05

compared with baseline (time

=

0). The plots represent mean

scores. (Modified from Ashina

et al

. 1999, by permission of the

Lancet

.)

0

VAS (%)

*

*

*

*

*

Min

0

15

30

60

90

120

–10

–20

–30

Figure 3

Percent changes in muscle hardness in 16 patients

with chronic tension-type headache. Muscle hardness was
significantly more reduced following treatment with L-
NMMA (



) than with placebo (



) in patients with chronic

myofascial pain (

P

=

0.04). *

P

<

0.05 compared with baseline

(time

=

0). The plots represent mean scores. (Reproduced from

Ashina

et al

. 1999, by permission of Oxford University Press.)

0

120

Change in hardness (%)

Min

*

*

60

0

–1

–2

–3

–4

–5

–6

Figure 4

Percent changes in total tenderness score (TTS) in 16

patients with chronic tension-type headache. The TTS tended
to be reduced following treatment with L-NMMA (



)

compared with placebo (



) (

P

=

0.11). Within each treatment,

the TTS was significantly reduced at 60 and 120 min after start
of the infusion of L-NMMA, while there was no significant
change at any time after treatment with placebo. **

P

<

0.01

compared with baseline (time

=

0). The plots represent mean

scores. (Reproduced from Ashina

et al

. 1999, by permission of

Oxford University Press.)

0

60

120

Change in tenderness (%)

**

**

Min

0

–5

–10

–15

–20

–25

–30

164

M Ashina

© Blackwell Publishing Ltd

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clarify whether NO may activate or sensitize nocice-
ptors in myofascial tissues.

The anti-nociceptive effect of L-NMMA might

also be due to blocking of vascular input. Excessive
vascular nociception may contribute to a primary
myofascial nociception in patients with tension-
type headache (41). L-NMMA inhibits all three
types of NOS, including eNOS. Thus, it is possible
that L-NMMA exerts its action by blocking eNOS
and thereby moderate vasodilatation of cephalic/
extracephalic arteries in patients with chronic ten-
sion-type headache. Because of convergence of
nociceptive input from facial tissues at the spinal/
trigeminal level (42), blocking of vascular input
might also lead to reduction of myofascial nocicep-
tion from pericranial muscles (38).

Taken together, these data indicate that the NOS

inhibitor L-NMMA elicits its anti-nociceptive effect
in chronic tension-type headache by modulation of
nociceptive information from myofascial tissues.
This anti-nociceptive effect may mainly be due to
reduction of central sensitization at the level of the
spinal dorsal horn or trigeminal nucleus, or both.

NO induction in chronic tension-type headache

In 1989, Iversen and colleagues introduced a NO
donor, glyceryl trinitrate (GTN), model of experi-
mental headache (43). Using this model, it has been
demonstrated that patients with migraine are hyper-
sensitive to NO, i.e. migraineurs develop signifi-
cantly stronger headache after GTN infusion than
healthy subjects (44, 45). Furthermore, it has been
shown that GTN may induce strong immediate
headache in patients with episodic tension-type
headache compared with healthy subjects (44). To
explore further the role of NO in development of
headache and modulation of myofascial pain input,
Ashina et al. (46) studied the effect of GTN in
patients with chronic tension-type headache. GTN
infusion in patients resulted in a biphasic response
with an immediate and a delayed headache (46).
Patients developed significantly stronger immediate
and delayed headache on a GTN day than on a pla-
cebo day (Fig. 5). Furthermore, patients developed
significantly stronger headache after GTN than con-
trols (Fig. 5).

The mechanisms responsible for the GTN-induced

headache in patients with primary headaches are
unknown. CGRP is an important neuromodulator of
the sensory system (47). An experimental study has
shown that GTN dilates cerebral arteries in cats via
liberation of CGRP, and it has been suggested that
this mechanism may explain the occurrence of

vasodilatation and headache in humans (48, 49).
However, this effect of GTN was not confirmed in
isolated guinea pig basilar arteries (50). Further-
more, Iversen and colleagues (51) reported that
plasma levels of CGRP are unchanged in the cranial
circulation of healthy subjects after GTN infusion. To
study the role of CGRP in NO-induced immediate
headache, Ashina et al. (52) measured plasma levels
of CGRP during and after infusion of GTN in
patients with chronic tension-type headache (52). No
significant changes in plasma CGRP after GTN infu-
sion were found in either patients or controls. Inter-
estingly, the dosage of GTN used in that study is
known to liberate CGRP in cats (48) and vasodilata-
tion in humans (53).

The unchanged sensitivity of pericranial myofas-

cial pain pathways seems also to rule out sensitiza-
tion of myofascial peripheral and central pathways
as a mechanism of the immediate headache (52).
Unchanged pressure-pain detection thresholds in
the finger, i.e. outside of the pain area, may also
indicate no alteration in sensitivity of third-order
neurons (52).

NO evokes pain in humans when injected

paravascularly or perfused through a vascularly iso-
lated hand vein segment (54). These findings suggest
that NO may directly activate or sensitize nocicep-
tors around blood vessels. Intravenous infusion of
GTN induces dilatation of the middle cerebral artery
in healthy subjects (53), and in migraineurs and
patients with episodic tension-type headache (49). In
these studies the dilatation lasted at least until 1 h

Figure 5

Median headache intensity over time during

(20 min) and after infusion of glyceryl trinitrate (GTN) (



,

patients;



, controls) and placebo (



, patients;

D

, controls) in

16 patients with chronic tension-type headache and in 16
healthy subjects. Headache was scored on a 10-point verbal
rating scale (VRS). The patients developed significantly
stronger headache than healthy controls both during the first
hour (immediate headache) (

P

=

0.02) and during the

subsequent 11 h (delayed headache) (

P

=

0.008). (Modified

from Ashina et al. 2000, by permission of Oxford University
Press.)

0

1

2

3

4

5

Infusion

Minutes

Hours

Headache intensity on VRS

0

20

40

60

90 120

4

8

12

Neurobiology of chronic tension-type headache

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after cessation of GTN infusion and 3 h in another
study (55). Collectively, these studies suggest that
immediate headache after GTN infusion in patients
with chronic tension-type headache may originate
from NO-induced activation or sensitization of sen-
sory nerves around cephalic arteries, or from NO-
induced arterial dilatation, or both (46).

The most important finding in the study by Ash-

ina et al. (46) was that systemic administration of NO
donor in patients with chronic tension-type head-
ache resulted in biphasic response with an immedi-
ate and a delayed headache (8 h after start of
infusion) (Fig. 5). Interestingly, the time profile of the
GTN-induced headache in patients with chronic ten-
sion-type headache was strikingly similar to the time
profile of GTN-induced headache in patients with
migraine (45). Thus, patients with migraine without
aura developed an immediate headache during
GTN infusion and a delayed headache fulfilling
International Headache Society (IHS) (56) criteria for
migraine several hours after cessation of the infu-
sion. The characteristics of the delayed headache in
chronic tension-type headache were, however, dif-
ferent from those in patients with migraine. Eighty
percent of migraine patients developed migraine
without aura after infusion of GTN (45), while 87%
of patients with chronic tension-type headache
developed a tension-type headache (46). These data
suggest that patients with chronic tension-type
headache are supersensitive to NO, similar to
patients with migraine, and that the majority of
patients in both groups develop their usual head-
ache several hours after the infusion of GTN.

What are the mechanisms of the delayed headache

and why do patients with chronic tension-type head-
ache and patients with migraine develop delayed
headache resembling their usual type of headache,
and why do most healthy subjects develop no
delayed headache or only a minor one? Studies in
rats and in humans have shown that after intrave-
nous administration of GTN very little drug remains
in the blood and that the majority of the GTN is
distributed to tissues (57). In the anaesthetized cat,
intravenous infusion of GTN induces a prolonged
(60 min) increase of NO in brain parenchyma (58),
and a prolonged increase of NO levels in the spinal
dorsal horn was demonstrated during central sensi-
tization (59). Furthermore, Wu and colleagues (60)
demonstrated that during central sensitization both
endogenous and exogenous nitric oxide induce

c-fos

(an immediate–early gene) which can further acti-
vate the production of other substances in the CNS.
In addition, Pardutz et al. (61) reported that subcu-
taneous GTN produced a significant increase of

NOS- and

c-fos

-immunoreactive neurons in the cer-

vical part of trigeminal nucleus caudalis in rats after
4 h. These data indicate that GTN infusion may
result in storage and subsequent liberation of NO or
it may trigger endogenous NO production in the
CNS, thereby enhancing sensitization of nociceptive
pathways in the CNS of patients with chronic ten-
sion-type headache.

Alternatively, sustained NO-induced vascular

nociception may lead to central sensitization and
subsequent convergence of nociceptive input from
blood vessels and myofascial tissue. Thus, NO may
activate or sensitize nociceptors around blood ves-
sels directly (54) or by dilatation (53). Dilatation of
meningeal blood vessels in rats causes sensitization
of central trigeminal neurons and facilitation of con-
vergent sensory responses (62). It is therefore pos-
sible that excessive vascular nociception caused by
GTN may gradually augment the sensitizing effect
of preexisting myofascial input in chronic tension-
type headache sufferers (41).

As mentioned earlier, patients with chronic ten-

sion-type headache and patients with migraine
develop increased delayed headache with different
characteristics. The most likely explanation is that
preexisting facilitation of distinct nociceptive central
pathways in chronic tension-type headache sufferers
(myofascial pathways) and migraineurs (vascular
pathways) may be enhanced by NO-induced central
sensitization. This may explain why the delayed
headache fulfilled tension-type headache criteria in
patients with chronic tension-type headache and
migraine criteria in migraineurs. This could also
explain why NO does not induce strong delayed
headache in healthy subjects when no preexisting
sensitization is present.

Summary

Studies of NO mechanisms in chronic tension-type
headache suggest that NO plays an important role
in the pathophysiology of this disorder (37, 38, 46).
The anti-nociceptive effect of NOS inhibitor suggests
that inhibition of NOS may become a novel principle
in treatment of chronic tension-type headache. It is
probable that the anti-nociceptive effect is due to
reduction of central sensitization at the level of the
spinal dorsal horn or trigeminal nucleus, or both.
Studies with selective NOS inhibitors are needed to
determine which type of NOS is involved and its
exact site of action in chronic tension-type headache.
Data from the GTN model of experimental headache
indicate that NO-induced delayed headache in
patients with chronic tension-type headache is due

166

M Ashina

© Blackwell Publishing Ltd

Cephalalgia,

2004,

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, 161–172

to augmentation of preexisting central sensitization.
Moreover, these data indicate that NO contributes to
mechanisms of several types of primary headaches
and that NO-related central sensitization may be an
important common denominator in the pain mecha-
nisms of primary headaches, although their basic
pathophysiological mechanisms are different.

Neuropeptides in chronic tension-type
headache

CGRP, SP, NPY, VIP and nociception in animal
studies

Immunohistochemical studies have shown that
CGRP, SP, NPY, VIP are present in the peripheral and
central nervous system (47, 63–69). Experimental
studies demonstrated that acute and chronic nocice-
ption leads to altered release of CGRP, SP, NPY and
VIP from sensory nerve endings and from central
terminals of sensory afferents into dorsal horn of the
spinal cord (70–73). These data suggest that CGRP,
SP, NPY and VIP are important neurotransmitters or
neuromodulators in the nervous system and that
these neuropeptides may play a role in the process
of central sensitization.

CGRP, SP, NPY, VIP in chronic tension-type
headache

In the last 10 years there has been increasing interest
in the role of neuropeptides in primary headaches.
Particularly, a role for CGRP has been implicated in
the pathophysiology of migraine (74, 75) and cluster
headache (76, 77). However, the role of CGRP in
generating headache pain is still unclear (75). Studies
of SP, NPY and VIP in patients with primary head-
aches or other chronic pain conditions have not led
to consistent results (78). This is probably due to
variations in methodology [different methods of
analysis and/or field of sampling (plasma, platelets,
CSF, saliva)] or patient populations.

As mentioned above, nociception from the peric-

ranial myofascial tissues may be of importance in the
pathophysiology of chronic tension-type headache.
In rat muscle, CGRP sensory fibres are preferentially
located in the wall of arteries (79) and nerve fibres
containing CGRP, SP, NPY and VIP accompany
small blood vessels in human cranial muscles (80).
These findings indicate that ongoing activity in sen-
sory neurons in the cranial muscles may be reflected
in changes of plasma levels of neuropeptides in
patients with chronic tension-type headache. The
role of neuropeptides in chronic tension-type head-

ache has been investigated in three studies (78, 81,
82). Bach and colleagues (81) reported normal CGRP
levels in the cerebrospinal fluid (CSF) in patients
with chronic tension-type headache. It was not
reported whether patients were examined during
headache or in headache-free period or whether
there was any relationship between CGRP levels and
headache quality. To explore a possible role of neu-
ropeptides in chronic tension-type headache, Ashina
and colleagues measured plasma levels of CGRP (82)
and SP, NPY and VIP (78) in the cranial and periph-
eral circulation of patients and controls. These stud-
ies showed that plasma levels of CGRP, SP, NPY and
VIP are normal in both cranial and peripheral circu-
lation of patients. Moreover, plasma levels of neu-
ropeptides were largely unrelated to headache state
(78, 82). Findings of normal plasma CGRP are par-
ticularly important because they are clearly different
from our previous study in migraine patients with
elevated levels of CGRP (75). However, exploratory
testing in relation to headache characteristics
showed that eight patients with a pulsating pain
quality, although fulfilling the IHS criteria for ten-
sion-type headache and not for migraine, had higher
plasma CGRP in the headache-free period than con-
trols (82). Plasma levels of CGRP in patients with
predominantly pressing headache in the past did not
differ from plasma CGRP in controls. In addition,
there was no relationship between CGRP levels and
muscular factors (82).

How can we explain increased plasma CGRP in

chronic tension-type headache patients with pulsat-
ing headache quality? Since CGRP levels are
increased in migraineurs (75) and in patients with
pulsating headache in the headache-free period (82),
ongoing nociception from cephalic or extracephalic
vasculature, or both, seems to be ruled out. Circulat-
ing CGRP may be involved in the regulation of
blood flow and in the maintenance of vascular tone
(83). It is possible that increased interictal CGRP
levels in patients with pulsating headache like
migraineurs may reflect altered vascular control due
to abnormal release of CGRP from sensory neurons
(75). Therefore, the most likely explanation is that
these patients, although fulfilling the IHS criteria for
chronic tension-type headache and not having any
migraine history, are in fact pathophysiologically
related to migraine. Although the finding of
increased plasma CGRP in patients with pulsating
headache quality is very interesting, one should be
very cautious with interpretation of

post hoc

analysis,

particularly with small numbers of patients. There-
fore, a prospective study with a large number of
patients is required to confirm this observation.

Neurobiology of chronic tension-type headache

167

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Cephalalgia,

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Summary

Studies on the role of neuropeptides in chronic ten-
sion-type headache indicate that plasma levels of
CGRP, SP, NPY and VIP are normal in patients and
largely unrelated to headache state (78, 82). Further-
more, findings of normal plasma CGRP add to the
growing list of differences between migraine and
tension-type headache. It is possible that changes of
neuropeptide levels at the spinal/trigeminal level or
in the myofascial tissue of these patients do not reach
the cranial or peripheral circulation. It would have
been optimal to measure SP, NPY and VIP levels in
the pericranial muscles and CSF or even at the neu-
ronal level. However, this is difficult for both practi-
cal and ethical reasons. Thus, the findings of normal
levels of CGRP, SP, NPY and VIP in the cranial and
peripheral circulation do not exclude that abnormal-
ities of these neuropeptides at the neuronal or
peripheral (pericranial muscle) levels play a role for
the pathophysiology of chronic tension-type head-
ache. Investigation of neuropeptides in other com-
partments with new sensitive methods of analysis is
necessary to clarify their role in chronic tension-type
headache. Finally, future CGRP studies in chronic
tension-type headache sufferers with pulsating pain
quality and in migraine patients with pressing pain
quality, but otherwise fulfilling the IHS migraine cri-
teria, are required to clarify whether increased CGRP
levels are associated with pulsating pain quality.

Microdialysis and chronic tension-type
headache

Muscle pathology in tension-type headache?

It is a common experience for individuals who have
been exposed to static or repetitive work for a long
period to develop tender areas in the pericranial
muscles and tension-type headache. It has been
hypothesized that local muscle ischaemia, distur-
bances in metabolism, microcirculation and mito-
chondria function in the tender areas may explain
myofascial pain in tension-type headache and in
other myofascial pain disorders such as trapezius
myalgia (84). Various

in vitro and in vivo

methods,

such as muscle biopsy, single-fibre laser-Doppler
and magnetic resonance spectroscopy, have been
used to explore the mechanisms responsible for
myofascial pain. The results of these studies have
been conflicting. While open studies suggested
abnormalities in microcirculation (85, 86), controlled
and blinded studies failed to find firm evidence
of peripheral abnormalities (87) in patients with

chronic myofascial pain (88). Using

133

xenon clear-

ance technique, Langemark and co-authors (89)
found normal resting blood flow and relative flow
increase during isometric work in temporal muscle
in patients with chronic tension-type headache.
However, in that study muscle blood flow was mea-
sured from a large muscle area and not in a tender
point. Thus, firm evidence of peripheral muscle
pathology as a cause of muscle pain and chronic
headache is still lacking. More sensitive techniques
are needed to answer the question of whether ten-
sion-type headache and other myofascial pain disor-
ders are associated with peripheral pathology in
tender points.

Muscle blood flow in chronic tension-type headache

Microdialysis is a unique technique for investigating
and monitoring local muscle blood flow and metab-
olism

in vivo

within a tissue volume of

<

1 cm

3

(90).

Using the microdialysis technique, Ashina and col-
laborators (91) estimated blood flow and interstitial
lactate concentrations in the trapezius muscle at rest
and in response to static exercise in patients with
chronic tension-type headache. The major finding of
that study was a decreased blood flow in response
to static exercise in a tender point in patients. Thus,
the increase in muscle blood flow from baseline to
exercise and post-exercise periods was significantly
lower in patients than in controls (Fig. 6) (91). There
was no difference in resting blood flow between
patients and controls.

Figure 6

Mean nutritive muscle blood flow in 16 patients

with chronic tension-type headache and in 17 healthy control
subjects. The figure shows that the increase in muscle blood
flow from baseline (Rest) to exercise (Ex 1 and Ex 2) and post-
exercise periods (Pex 1 and Pex 2) was significantly lower in
patients (



) than in controls (



) (

P

=

0.03). The plots represent

mean

±

SEM scores. (Reproduced from Ashina et al. 2002, by

permission of Oxford University Press.)

0

0.02

0.04

0.06

0.08

0.1

Out/inflow ratio

Rest

Ex 1

Ex 2 Pex 1 Pex 2

168

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2004,

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, 161–172

The question is, how do we explain the reduced

blood flow response to exercise in the tender point?
Difference in the static load between patients and
controls is unlikely because there was no difference
between groups, in either absolute force or relative
to maximal voluntary force. It could be suggested
that patients develop a relative ischaemia in the
tender point during static exercise. If so, one would
expect that the increase of interstitial lactate concen-
tration would be higher in patients than in controls.
However, Ashina and colleagues (91) observed no
difference in local increase of interstitial lactate
between patients and controls (Fig. 7). This seems to
rule out the presence of ischaemia in the tender point
of patients with chronic tension-type headache dur-
ing rest and static exercise.

The altered blood flow might be secondary to

chronic muscle pain. As mentioned in previous sec-
tions, chronic tension-type headache may be caused
by prolonged painful input from pericranial myofas-
cial tissues, e.g. tender points, resulting in central
sensitization (36, 38). The pathophysiological basis
for the painful input from the periphery is still
unknown. Once the central sensitization had been
established, chronic tension-type headache might be
an entirely central process without further or only
minimal input from the periphery (9). Because of
central sensitization the central interpretation and
response to normal sensory input are altered, possi-
bly mainly when input is increased, as during exer-
cise. This may lead to enhanced sympathetically
mediated vasoconstriction and thereby a decreased

blood flow in response to static exercise (91). This is
supported by studies in animals and humans show-
ing that static exercise produces a one-to-one syn-
chronization of activation of the muscle nociceptors
(group IV) and muscle nerve sympathetic activity
(92). Furthermore, it has been shown that in patients
with fibromyalgia a complete sympathetic blockade
produced a marked reduction of the number of ten-
der points, suggesting an improvement in microcir-
culation (93). Moreover, it has been proposed that
central sensitization may maintain increased efferent
sympathetic outflow that in turn maintains sensiti-
zation of sensory afferents (94). These data suggest
that the sympathetic outflow may influence or main-
tain afferent activity in nociceptors or altered blood
flow regulation in tender points of patients with
chronic tension-type headache.

Summary

A microdialysis study provides

in vivo

evidence of

altered blood flow regulation in tender skeletal mus-
cle during static work in patients with chronic ten-
sion-type headache (91). The lack of any difference
in local increase of interstitial lactate between
patients and controls seems to rule out the presence
of ischaemia in the tender point of patients with
chronic tension-type headache during static exercise.
It is possible that because of increased excitability of
neurons in the CNS the central interpretation and
response to normal sensory input are altered in
patients with chronic tension-type headache. This
may lead to enhanced sympathetically mediated
vasoconstriction and thereby a decreased blood flow
in response to static exercise.

Concluding remarks and future perspectives

Advances made in basic pain research have
improved our knowledge of the neurobiology of
chronic head pain. Particularly, understanding of
molecular mechanisms involved in the process
of central sensitization contributes to the develop-
ment of novel therapeutic approaches for chronic
headache. Studies of NO mechanisms suggest that
NO may play a key role in the pathophysiology of
this disorder and that the anti-nociceptive effect of
NOS inhibitor, previously demonstrated only in ani-
mal models, may become a novel principle in future
treatment of chronic headache. The anti-nociceptive
effect of NOS inhibition is probably due to reduction
of central sensitization at the level of the spinal dor-
sal horn or trigeminal nucleus, or both. However, the
exact site of action and type of NOS involved in

Figure 7

Mean interstitial concentration of lactate in 16

patients with chronic tension-type headache and in 17 healthy
control subjects. There was no difference in change in
interstitial concentration of lactate from baseline (Rest) to
exercise (Ex 1 and Ex 2) and post-exercise (Pex 1 and Pex 2)
periods between patients (



) and controls (



) (

P

=

0.38).

(Reproduced from Ashina et al. 2002, by permission of Oxford
University Press.)

0

1

2

3

4

5

6

Interstitial lactate (mmol/l)

Ex 1

Rest

Ex 2

Pex 1

Pex 2

Neurobiology of chronic tension-type headache

169

© Blackwell Publishing Ltd

Cephalalgia,

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, 161–172

chronic tension-type headache is far from fully
understood. Future studies with selective NOS
inhibitors are needed to determine which type of
NOS is involved and its exact site of action. Whether
the anti-nociceptive effect observed in patients was
clinically relevant is also debatable, but the impor-
tance of the results lies in the proof of the concept
and not in the magnitude of the effect.

In the future, chronic dosing should be tested

when a safe inhibitor of NOS with a longer half-life
becomes available. Other important questions are
whether inhibition of NOS would have analgesic
effects in other chronic pain conditions, or whether
the effect of inhibition of NOS in tension-type head-
ache is due to interaction with specific headache
mechanisms. These important issues should be
addressed in the future studies in patients with
other, non-headache, types of chronic pain. Data
from the GTN model of experimental headache in
chronic tension-type headache indicate that NO con-
tributes to mechanisms of several types of primary
headaches and that NO-related central sensitization
may be an important common denominator in the
pain mechanisms of primary headaches. However,
additional research is needed to prove the role of
central sensitization during NO-induced delayed
headache. It would be interesting to detect possible
sensitization of second-order neurons during the
delayed headache by quantitative sensory testing
techniques. Studies of the role of neuropeptides in
chronic tension-type headache indicate that plasma
levels of CGRP, SP, NPY and VIP are normal in
patients and largely unrelated to headache state.
Investigation of neuropeptides in other compart-
ments, such as CSF or pericranial muscles, with new
sensitive methods of analysis is necessary to clarify
their role in chronic tension-type headache. Further-
more, CGRP studies in chronic tension-type head-
ache sufferers with pulsating pain quality and in
migraine patients with pressing pain quality are
required to determine whether increased CGRP lev-
els are associated with pulsating pain quality.

The results of microdialysis study indicate that

central sensitization may lead to enhanced sympa-
thetically mediated vasoconstriction which may be
responsible for decreased blood flow in tender skel-
etal muscle during static work in patients with
chronic tension-type headache. Thus, it is possible
that the central neuroplastic changes may affect the
regulation of peripheral mechanisms and thereby
lead to increased tenderness and chronic headache.
The study also shows that the microdialysis tech-
nique offers unique possibilities to study mecha-
nisms responsible for not only head pain but also a

wide range of myofascial pain conditions. Using this
technique, it would be interesting to measure

in vivo

concentrations of inflammatory substances or neu-
ropeptides in the tender muscles of patients with
chronic tension-type headache.

In summary, the present study contributes to our

understanding of the complex mechanisms leading
to chronic tension-type headache and provides data
that will hopefully lead to new treatment modalities.

Acknowledgements

The studies in this thesis were supported by grants from the
University of Copenhagen, the Danish Headache Society, the
Danish Medical Association Research Fund, the Danish Hos-
pital Foundation for Medical Research, Region of Copen-
hagen, the Faroe Islands and Greenland, the Foundation for
Research in Neurology, the Novo Nordisk Foundation, the
Gerda and Aage Haensch’s Foundation, the Mauritzen La
Fontane’s Foundation, and the Foundation of Jacob Madsen
and his wife OIga Madsen.

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