Mechanisms of tension-type headache

Response Predictors of Repetitive Neuromuscular Magnetic Stimulation in the Preventive Treatment of Episodic Migraine

Background

Repetitive neuromuscular magnetic stimulation (rNMS) of the trapezius muscles showed beneficial effects in preventing episodic migraine. However, clinical characteristics that predict a favorable response to rNMS are unknown. The objective of this analysis is to identify such predictors.

Methods

Thirty participants with a diagnosis of episodic migraine (mean age: 24.8 ± 4.0 years, 29 females), who were prospectively enrolled in two non-sham-controlled studies evaluating the effects of rNMS were analyzed. In these studies, the interventional stimulation of the bilateral trapezius muscles was applied in six sessions and distributed over two consecutive weeks. Baseline and follow-up assessments included the continuous documentation of a headache calendar over 30 days before and after the stimulation period, the Migraine Disability Assessment Score (MIDAS) questionnaire (before stimulation and 90 days after stimulation), and measurements of pain pressure thresholds (PPTs) above the trapezius muscles by algometry (before and after each stimulation session). Participants were classified as responders based on a ≥25% reduction in the variable of interest (headache frequency, headache intensity, days with analgesic intake, MIDAS score, left-sided PPTs, right-sided PPTs). Post-hoc univariate and multivariate binary logistic regression analyses were performed.

Results

Lower headache frequency (P = 0.016) and intensity at baseline (P = 0.015) and a migraine diagnosis without a concurrent tension-type headache component (P = 0.011) were significantly related to a ≥25% reduction in headache frequency. Higher headache frequency (P = 0.052) and intensity at baseline (P = 0.014) were significantly associated with a ≥25% reduction in monthly days with analgesic intake. Lower right-sided PPTs at baseline were significantly related to a ≥25% increase in right-sided PPTs (P = 0.0.015) and left-sided PPTs (P =0.030). Performance of rNMS with higher stimulation intensities was significantly associated with a ≥25% reduction in headache intensity (P = 0.046).

Conclusions

Clinical headache characteristics at baseline, the level of muscular hyperalgesia, and stimulation intensity may inform about how well an individual patient responds to rNMS. These factors may allow an early identification of patients that would most likely benefit from rNMS.

Quantitative analysis of the retinal nerve fiber layer, ganglion cell layer and optic disc parameters by the swept source optical coherence tomography in patients with migraine and patients with tension-type headache

The aim of the study was to measure the thicknesses of the inner retinal segments and optic nerve head (ONH) parameters in migraineurs and patients with tension-type headache (TTH) in headache-free period using swept source optical coherence tomography (SS-OCT) and to compare the outcomes with each other and those of healthy subjects. The study population consisted of 23 migraineurs, 22 TTH patients, and 25 controls with a best-corrected visual acuity of 20/20 and without a history of systemic or ocular disease. Macular ganglion cell inner plexiform layer (mGCIPL), macular ganglion cell complex (mGCC), circumpapillary retinal nerve fiber layer (cpRNFL), and ONH parameters were evaluated using SS-OCT, and the areas under the receiver-operating characteristic (ROC) curves were calculated to determine the ability of these parameters to distinguish between the patient and normal eyes. There were not statistically significant differences between the measurements acquired from migraineurs, TTH patients, and the controls. The outcomes of the patients with TTH were very similar to those of the normal participants. The areas under the ROC curves (AUC) correlated highly with the measurements obtained from the same subfields for the mGCC, MGCIPL, cpRNFL, and ONH parameters. In conclusion, SS-OCT presented reproducible and reliable measurements of posterior segment layers of the eyes, especially in sectoral configuration, and the parameters did not show significant difference between the groups.

THE EFFECT OF BODY AWARENESS THERAPY AND AEROBIC EXERCISES ON PAIN AND QUALITY OF LIFE IN THE PATIENTS WITH TENSION TYPE HEADACHE

BACKGROUND

This study is to investigate the effect of Body Awareness Therapy (BAT) and Aerobic Exercises on pain and quality of life in patients with Tension-Type Headache (TTH).

MATERIALS AND METHOD

Sixty individuals with TTH diagnosis who referred Neurologist were incorporated into study. The individuals were randomly grouped into 3 as BAT (n=20), aerobic exercise (n=20) and control group (n=20). Pain severity of the individuals was evaluated by Visual Analog Scale (VAS) and pain diary, disability with ache; by Pain Disability Index (PDI) and Headache Impact Tests (HIT) and quality of life was evaluated by SF-36. Subsequent to first assessments, 3 sessions of 60 minutes per week throughout 6 weeks totally.

RESULTS

When the groups were compared at the end of the study, a significant decrease was observed in VAS, PDI and HIT values in the individuals in the BAT and aerobic exercise groups. With the individuals in group BAT and aerobic exercise all parameters of quality of life were observed to be increased significantly.

CONCLUSION

BAT and aerobic exercise programs to be applied on TTH patients were concluded to be important in decreasing the pain, in increasing the quality of life and in reducing pain-related daily constraints of the individuals.

Visual Field Losses in Patients with Migraine without Aura and Tension-Type Headache

The aim of the study was to compare the visual fields during pain attacks in the patients with migraine without aura and tension-type headache using automated perimetry. In this study 25 patients with migraine and 25 patients with tension-type headache were evaluated.The optic disc, macula and retina were assessed and patients with normal values were enrolled into the study. Intraocular pressure of all patients were measured. Furthermore, visual field test was applied to both groups using Humphrey field analyzer (Carl-Zeiss Meditec, model-745 i, Dublin, CA, USA). Both mean deviation and pattern standart deviation values of these two goups were not statistically significant.

Abnormal Modulatory Influence of Diffuse Noxious Inhibitory Controls in Migraine and Chronic Tension-Type Headache Patients

The aim of this study was to evaluate the function of pain modulating systems subserving diffuse noxious inhibitory controls (DNICs) in primary headaches. DNICs were examined in 24 migraineurs, 17 patients with chronic tension-type headache (CTTH) and 20 healthy subjects by means of nociceptive flexion RIII reflex and the cold pressor test (CPT) as heterotopic noxious conditioning stimulation (HNCS). The subjective pain thresholds (Tp) and the RIII reflex threshold (Tr) were significantly lower in CTTH vs. controls. In controls a significant inhibition of the RIII reflex was observed during the CPT (-30±, P < 0.05). Conversely, migraine and CTTH patients showed facilitation (+31±, P < 0.05 and +40±, P < 0.01, respectively) of the RIII reflex during the HNCS. This study demonstrates a dysfunction in systems subserving DNICs in both migraine and CTTH. Impairment of endogenous supraspinal pain modulation systems may contribute to the development and/or maintenance of central sensitization in primary headaches.

Outcome of Headache Frequency in Adolescence

Only a few studies have been published of the outcome of adolescent headache (HA). The aim of this study was to examine the predictors of the outcome of headache frequency. A population-based sample of 13-year-olds with or without HA ( N = 228) was followed to the age of 16 years. HA was classified on the basis of a face-to-face interview and clinical examination. The outcomes of monthly HA (>1/month) and non-frequent HA (0-1/month) were studied. Frequent use of analgesics, female gender and multiple non-headache pain predicted the persistence of monthly HA (>1/month). Significant predictors for worsening non-frequent HA (from 0 to 1/month to >1/month) were female gender, consistent migraine and high basic educational level of one parent. Adolescents frequently using analgesics constitute a risk group for a poor outcome of HA. Especially girls meeting this criterion should be considered a target group in the planning and implementing of preventive measures.

Comparison of masticatory muscle myofascial pain in patients with and without a chief complaint of headache

OBJECTIVES

Headaches are a common complaint in temporomandibular disorder (TMD) patients. However, few studies have compared the symptom characteristics between TMD patients with and without a complaint of headache. The aim of this study was to compare subjective symptoms and treatment outcomes between myofascial TMD patients who had a chief complaint of headache and those who did not.

METHODS

One hundred sixty one patients underwent comprehensive examinations and scored their pre- and post-treatment symptoms on a form (4 items assessing pain intensity and 1 assessing sleeping difficulty). On the basis of the primary diagnosis, patients were divided into two groups: myofascial pain with and without a chief complaint of headache (MPH and MP).

RESULTS

Before treatment, patients in the MPH group scored significantly higher with respect to pain intensity and level of sleeping difficulty than did patients in the MP group (jaw/face pain, P = 0.015; headache, P < 0.001; neck pain, P < 0.001, and difficulty in sleeping, P = 0.005; Mann-Whitney U-test). Patients in the two groups demonstrated similar treatment outcomes, except for neck pain.

CONCLUSION

Significant differences in symptom characteristics and outcomes were observed between primarily myofascial TMD patients with and without a chief complaint of headache.

Nerve growth factor and chronic daily headache: a potential implication for therapy

The pivotal role of nerve growth factor in inducing hyperalgesia and central sensitization has been emphasized in experimental pain models. Higher nerve growth factor levels have recently been found in the cerebrospinal fluid of patients with chronic daily headache. These levels were significantly correlated with the cerebrospinal fluid levels of substance P and calcitonin gene-related peptide, supporting the involvement of this neurotrophin in enhancing the production of the two sensory neuropeptides of the trigemino-vascular system in chronic daily headache. This may, in part, account for the long-lasting sensitization and activation of this system, which could contribute to headache chronicity. More recent research has shown a significant correlation between the higher cerebrospinal fluid levels of nerve growth factor and those of another neurotrophin, the brain-derived neurotrophic factor, as well as glutamate in chronic daily headache patients. These findings suggest the potential involvement of nerve growth factor-mediated upregulation of brain-derived neurotrophic factor in persistent head pain. Therefore, nerve growth factor appears to indirectly exert its effect through enhancing glutamatergic transmission involved in the processing of head pain via brain-derived neurotrophic factor. Based on these data, a potential application can be hypothesized for novel strategies targeting neurotrophins (nerve growth factor and brain-derived neurotrophic factor) and their receptors to chronic daily headache. To date, the majority of the molecules discovered in this regard have been scarcely or never proved in animal pain models and are far from clinical use in chronic pain, including chronic daily headache. If this approach is to be developed in the near future, research should be focused on identifying strategies with few central side effects and specific selective action on central sites involved in chronic head pain and more generally in chronic pain conditions. This will represent a very difficult challenge, taking into account the pleiotropic effect of nerve growth factor and the wide range of intracellular signalling pathways activated by this neurotrophin which are not limited to the nociceptive system.

Impaired postural control in patients affected by tension-type headache

Sixteen subjects, affected by chronic tension-type headache (TTH) accordingly to the International Headache Society Classification (1988) criteria, in presence of tenderness in pericranial muscles,with a mean age of 37+/-11.8 years, and ten healthy volunteer subjects, age and sex matched, were submitted to postural analysis by Static Posturography (S.Ve.P. Amplaid). Aim of the study was to evaluate whether patients with TTH have disturbed postural control, as compared to normal subjects. Postural analysis considered all posturographic variables but focused on spectral frequency analysis of body sway. In both open (OE) and closed eyes (CE) condition, spectral frequency analysis showed a significantly increased body sway at low (OE= p < or = 0.01; CE= p < or = 0.01) and middle (OE= p < or = 0.01; CE= p < or = 0.01) frequencies on the antero-posterior (y) plane and at low frequencies (OE= p < or = 0.05; CE= p < or = 0.05) on the lateral (x) plane. Statistical analysis was performed using the Student's t test for unpaired data, p value 0.05 defined significant. The proprioceptive input seems to be predominant at middle and high frequencies in maintaining posture, our results seem then to suggest a proprioceptive disturbance in TTH patients. The disturbance is likely related to chronic pericranial muscle contraction and tenderness. Posturography and spectral analysis may help not only in the diagnosis of a postural disturbance but even more in the follow-up of TTH patients, during and after a medical and/or a rehabilitative treatment.

Tension-type headache with medication overuse: pathophysiology and clinical implications

Tension-type headache (TTH) is the most prevalent primary headache disorder. An important factor in the long-term prognosis of TTH is the overuse of acute medications used to treat headache. There are many reasons why patients with TTH overuse acute medications, including biobehavioral influences, dependency, and a lack of patient education. Chronic daily headache occurs in 4.1% of the general population, and chronic tension-type headache and medication overuse headache (MOH) occur in approximately 2.2% and 1.5%, respectively. A proper diagnosis is essential for the treatment of these patients. Treatment should include pathological considerations concerning TTH and MOH, which include peripheral and central mechanisms. Because TTH with MOH carries the worst prognosis, more clinical studies focusing on the complex interaction and treatments of TTH and MOH are needed.

Stretch Reflex and Pressure Pain Thresholds in Chronic Tension-Type Headache Patients and Healthy Controls

To compare the jaw-stretch reflex and pressure pain thresholds (PPT) in chronic tension-type headache (CTTH) patients and healthy controls, 30 patients (15 male and 15 female) and 30 age- and sex-matched healthy subjects were investigated. Stretch reflexes were recorded in the temporalis and masseter muscles and PPT was determined in the anterior temporalis, splenius capitis and masseter muscles. The results showed that the amplitude of the stretch reflex in CTTH patients was higher compared with control subjects ( P < 0.045), and higher in women compared with men in the right and left anterior temporalis muscles ( P < 0.009). There were no differences in the PPT value between CTTH and control subjects ( P > 0.509), whereas women showed significantly lower PPT measurements ( P < 0.046). The results demonstrated a facilitation of the stretch reflex pathways in CTTH patients that is unrelated to measures of pericranial sensitivity.

Effect of serotonergic antidepressant therapy on temperament and character scales in patients with chronic tension-type headache

The purpose of the present study was to assess a group of patients with chronic tension-type headache (CTTH) and control subjects using the personality questionnaire proposed by Cloninger and to determine possible changes in the Temperament and Character Inventory (TCI) patterns of CTTH patients after therapy with serotonergic antidepressants. Forty-five patients with CTTH filled out the TCI and Beck Depression Inventory (BDI) before and after 4-month prophylactic therapy with serotonergic antidepressants. A total of 50 age-, sex- and education level-matched healthy subjects were selected as a control group. During the pretreatment period patients were found to have higher harm avoidance and lower self-directedness scores than healthy comparison subjects. During the post-treatment period, although harm avoidance scores decreased and self-directedness scores increased, harm avoidance scores were still significantly higher and self-directedness scores significantly lower in CTTH patients than in controls. After using BDI score and age as covariates at the post-treatment period, harm avoidance scores were still higher in patients with CTTH while self-directedness scores did not differ between CTTH patients and controls. These findings suggest state and trait dependence of harm avoidance, and strong state dependence of self-directedness scores in CTTH patients. When interpreting data regarding personality measured by the TCI in CTTH patients, the effects of depressive symptomatology should be taken into account.

A comparison of selected osteopathic treatment and relaxation for tension-type headaches

OBJECTIVE

The objective of this study was to compare the effects of osteopathic treatment and progressive muscular relaxation (PMR) exercises on patients with tension-type headache (TTH).

BACKGROUND

Relaxation is generally accepted as a treatment for TTH. Osteopathy is considered by some practitioners to be useful for headache management but there is limited scientific evidence regarding the effectiveness. This study compares relaxation and relaxation plus selected osteopathic techniques in the treatment of people with TTH.

DESIGN

This was a single-blind, randomized, clinical study using an experimental design. Twenty-nine patients with TTH according to the International Headache Classification Subcommittee, 2004, were recruited for this study and randomly placed in either a control or experimental group. Both groups practiced PMR exercises at home while the experimental group also received 3 osteopathic treatments.

METHOD

All participants recorded headache frequency and intensity in a headache diary (HD) for 2 weeks pretreatment, and continued recording during the treatment period until reassessment for a total of 6 to 7 weeks. All tests of significance were set at P</= .05.

RESULTS

Twenty-six people completed the study. Results indicated that the number of Headache Free Days Per Week was significantly improved (P= .016) in the experimental group. Two other measures, the Headache Degree of Improvement (P= .075) and the HD rating (P= .059), which combine headache frequency and intensity, did not meet our criteria for statistical significance but both scores are <.10 indicating a trend toward improvement in the experimental group that is clinically significant. The HD Rating also showed that the experimental group improved 57.5%, while the control group improved 15.6%. The intensity of headache did not show a significant improvement (P= .264).

CONCLUSION

The people in this study who did relaxation exercises and received 3 osteopathy treatments had significantly more days per week without headache than those who did only relaxation exercises.

Botulinum toxin type A in the prophylactic treatment of chronic tension-type headache: a multicentre, double-blind, randomized, placebo-controlled, parallel-group study

We studied the safety and efficacy of 0 U, 50 U, 100 U, 150 U (five sites), 86 Usub and 100 Usub (three sites) botulinum toxin type A (BoNTA; BOTOX); Allergan, Inc., Irvine, CA, USA) for the prophylaxis of chronic tension-type headache (CTTH). Three hundred patients (62.3% female; mean age 42.6 years) enrolled. For the primary endpoint, the mean change from baseline in the number of TTH-free days per month, there was no statistically significant difference between placebo and four BoNTA groups, but a significant difference favouring placebo vs. BoNTA 150 was observed (4.5 vs. 2.8 tension headache-free days/month; P = 0.007). All treatment groups improved at day 60. Although efficacy was not demonstrated for the primary endpoint, at day 90, more patients in three BoNTA groups had >or=50% decrease in tension headache days than did placebo (P <or= 0.024). Most treatment-related adverse events were mild or moderate, and transient. BoNTA was safe and well-tolerated in the study.

Autogenic training for tension type headaches: A systematic review of controlled trials

OBJECTIVE

To determine from the published evidence whether autogenic training as sole therapy is effective for prevention of tension-type headaches in adults.

METHOD

Systematic review of controlled trials. Literature searches were performed in January 2005 in six major databases, specifically Medline, EMBASE, AMED, CENTRAL, PsychInfo and CINAHL and information was extracted and evaluated in a pre-defined manner.

RESULTS

Seven controlled clinical trials were included in the review. The methodological quality of these studies was low. Patient samples were generally representative of the more severely affected cases. None of the studies show autogenic training to be convincingly superior to other interventions care. Some trials suggested that the effect of autogenic training is no different from hypnosis and inferior to biofeedback.

CONCLUSION

There is no consistent evidence to suggest that autogenic training is superior to other interventions for prevention of tension headaches, or different from other forms of relaxation. Further studies should investigate the use of standard autogenic training in patients with moderate headache.

Pain-relieving factors in migraine and tension-type headache

We performed the present study to compare patients with migraine and tension-type headache (TTH) in their behaviour during the attacks and the manoeuvres to relieve the pain. One hundred thirty consecutive patients with either migraine (n = 75) or TTH (n = 55) were questioned (including the use of a checklist) concerning their usual behaviour during the attacks and non-pharmacological manoeuvres performed to relieve the pain. The results of the two types of headache were compared. Patients with migraine tended to perform more manoeuvres than patients with TTH (mean: 4.3 vs. 3.6). These manoeuvres included pressing and applying cold stimuli to the painful site, trying to sleep, changing posture, sitting or reclining in bed (using more pillows than usual to lay down), isolating themselves, using symptomatic medication, inducing vomiting, changing diet and becoming immobile during the attacks. The only measure predominantly reported by patients with TTH was scalp massage. Migraineurs, compared to patients with TTH, changing eating habits, pressed the pain site; there were no significant differences between the two groups. The behaviour of patients during headache attacks varies with the diagnosis. Measures that do not always result in pain relief are performed in order to prevent its worsening or to improve associated symptoms. These behavioural differences may be because of the different pathogenesis of the attacks or of various styles of dealing with the pain. They can also aid the differential diagnosis between headaches in doubtful cases.

Examination of anxiety, hostility and psychiatric disorders in patients with migraine and tension-type headache

The role of psychological factors related to headache has long been a focus of investigation. The aim of this study was to evaluate anxiety, depression, hostility and psychological symptoms in patients with migraine and tension-type headache (TTH) and to compare the results with healthy controls. Seventy-five subjects with migraine and 55 subjects with TTH (according to the criteria of the International Headache Society classification) and a control group including 73 healthy subjects were studied. The Buss-Durkee Hostility Inventory, Brief Symptom Inventory, State-trait Anxiety Inventory, Trait form were administered to the subjects. Compared with healthy controls, the patients with headache had significantly higher scores on measures of anxiety, depression and hostility and lower scores on psychological symptoms. The present results indicate the need to distinguish the unique dimensions of anxiety, depression and hostility that should be assessed in the population with headaches.

Temperament and character profiles of patients with tension-type headache and migraine

The aim of this present study was to evaluate the temperament and character profiles of persons with tension-type headache (TTH) and migraine, and to compare the results with those of healthy controls. The study population consisted of 81 patients with TTH (60 female, 21 male) and 56 patients with migraine (34 female, 17 male) aged 18-50 years, according to the criteria of the International Headache Society with age and gender - matched healthy control subjects (54 female, 28 male). All participants were instructed to complete a self-administered 240-item temperament and character inventory (TCI) questionnaire and Beck Depression Inventory (BDI). The TCI assesses four dimensions of temperament, namely, novelty seeking (NS), harm avoidance (HA), reward dependence (RD), and persistence (P), and three dimensions of character, being self-directedness (SD), cooperativeness (C) and self-transcendence (ST). According to the TCI biosocial model, the temperament dimension HA is suggested to indicate central serotonergic turnover, which is further correlated with depressive state. It was found that mean BDI scores were significantly higher in patients with TTH and migraine than in those of the controls. The BDI scores were positively correlated with HA scores (r = 0.295, P < 0.001) and negatively correlated with SD (r = -0.386, P < 0.001) and C scores (r = -0.164, P= 0.016). Multivariate analysis showed that BDI scores had significant covariation for HA, SD and C. Despite using the BDI score as a covariate, TTH patients had higher HA scores (P = 0.01) than did the control subjects. No significant differences were found between the groups regarding main NS, RD, P, SD, C and ST scores. Based on the main results of this study, it is suggested that higher serotonergic activity related to HA scores in TTH patients and their relationship with depressive symptomatology supports the role of central serotonergic involvement in TTH.

Psychiatric symptoms in children with primary headache

OBJECTIVE

To examine the association of psychiatric symptoms with migraine and tension-type headache in children.

METHOD

A questionnaire completed by 1,135 Finnish children in the sixth grade identified 154 children with migraine, 138 with tension-type headache, and 407 children who were headache-free. Seventy children were randomly selected from each group and evaluated by a structured interview to confirm headache type, resulting in a sample of 59 children with migraine, 65 with tension-type headache, and 59 without headache. The children completed the Children's Depression Inventory, and the parents completed the Child Behavior Checklist and General Functioning scale of the McMaster Family Assessment Device.

RESULTS

Children with migraine had significantly higher levels of total, internalizing, and somatic symptoms, as well as social and family problems, than those without headache and had higher levels of somatic symptoms than children with tension-type headache. Children with tension-type headache had significantly higher levels of somatic symptoms and family problems than those without headache.

CONCLUSIONS

The association between psychiatric symptoms and headaches shows differences between different headache types. However, a minority of children with migraine or tension-type headache have high levels of psychiatric symptoms.

Aspirin in episodic tension-type headache: placebo-controlled dose-ranging comparison with paracetamol

Most people with episodic tension-type headache (TTH) treat themselves with over-the-counter analgesics. In the absence of clear evidence of dose-related efficacy of the two most commonly used analgesics, aspirin (acetylsalicylic acid) and paracetamol (acetaminophen), this study compared two doses of each with placebo. In a double-blind, double-dummy, randomized parallel-groups comparative trial, 638 consenting subjects aged 16-65 years with episodic TTH (but not migraine) by IHS criteria were recruited from the UK general population by advertisement. They treated one episode of moderate or severe TTH with a single dose of 500 or 1000 mg aspirin, 500 or 1000 mg paracetamol or placebo. The primary objective was to compare aspirin 1000 mg with placebo, and the primary end-point was subjective pain relief (total or worthwhile) 2 h after treatment ('response'). Additionally, pain intensity on a 100-mm visual analogue scale and functional impairment were monitored regularly for 4 h and at 24 h, although rescue medication was allowed after 2 h. The analysis was of the intention-to-treat population of 542 who took treatment (all providing outcome data). Treatment groups were matched at baseline. Aspirin 1000 mg (75.7% response rate; P = 0.0009) and to a lesser extent aspirin 500 mg (70.3%; P = 0.011) and paracetamol 1000 mg (71.2%; P = 0.007), but not paracetamol 500 mg (63.8%; P = 0.104), were statistically more effective than placebo despite a high placebo-response rate (54.5%). Outcome was not affected by headache intensity at baseline. Secondary end-points including functional recovery (by median times of 4.0-13.5 h) were consistent with these findings, although a minority of subjects recorded long-duration functional impairment (37-54 h). Adverse events reported by 13.4-18.9% of subjects were mild or moderate, and transient. No safety concerns arose.

Chronic daily headache: a scientist's perspective

Certain features of chronic daily headache, namely, increased headache frequency, expansion of headache area, and cutaneous allodynia, may imply sensitization of central nociceptive neurons in the trigeminal pathway. Repetitive activation of the trigeminal nerve can lead to a biologic and functional change in trigeminal nucleus caudalis neurons, characterized by a decrease in nociceptive threshold and receptive field expansion. Suppression of the endogenous pain control system can facilitate the process of central sensitization. Evidence of such suppression in patients with chronic daily headache includes decreased platelet serotonin, up-regulation of 5-HT2A receptors, increased platelet nitric oxide production, and increased levels of substance P and nerve growth factor in the cerebrospinal fluid. Results from a number of animal experiments have indicated that chronic analgesic exposure leads to changes in serotonin content and density of 5-HT2A receptors in the central nervous system. This plasticity of the serotonin-dependent pain control system may accelerate the process of sensitization; a biologic outcome that is expressed clinically by the development of chronic daily headache associated with analgesic overuse.