OP0161 PREDICTIVE VALUE OF MUSCULOSKELETAL ULTRASOUND IN PATIENTS WITH JUVENILE IDIOPATHIC ARTHRITIS IN CLINICAL REMISSION

Background:

The accurate assessment of remission status in JIA patients is of utmost relevance to taper medications and prevent side effects from their long-term administration. In RA patients in clinical remission (CR), musculoskeletal ultrasound (MSUS) allows to detect persistent joint inflammation (subclinical synovitis), which predicts disease flare and structural damage progression. Although subclinical synovitis has been reported in a substantial proportion of JIA patients with inactive disease, its prognostic value is still being defined.

Objectives:

1) to investigate the prevalence of MSUS-detected subclinical synovitis in JIA patients in CR; 2) to establish which and how many joints should be scanned to reliably assess remission; 3) to evaluate the persistence of subclinical synovitis over the time; 4) to investigate whether subclinical synovitis entails a risk of disease flare and whether it should affect the therapeutic strategy.

Methods:

135 consecutive JIA patients who met the Wallace criteria for CR were included in this 3-years prospective study. All patients underwent MSUS assessment of 56 joints at study entry and at 6 months follow-up visit. Joints were scanned for synovial hyperplasia, joint effusion and Power Doppler (PD) signal by two independent ultrasonographers. Patients were followed clinically for 3 years. A flare of synovitis was defined as a recurrence of clinically active arthritis. The association between clinical and MSUS variables with flare, was evaluated by adjusted logistic regression models.

Results:

135 patients (78.5% F; median age 11.3 y; median disease duration 5.7 y; median CR duration 1.4 y) were included. Fifty-seven/135 (42.2%) patients had persistent oligoarthiritis; 41/135 (30.4%) extended oligoarthiritis; 32/135 (23.7%) polyarthiritis; 5/135 (3.7%) systemic arthritis. Seventy-eight/135 (57.7%) patients were in CR on medication. Subclinical synovitis was detected in 32/135 (23.7%) patients and in 53/7560 (0.7%) joints. Subclinical tenosynovitis was present in 20/135 (14.8%) patients. Subclinical synovitis was found more frequently in the ankle and wrist joints. 58.6% of patients showed persistent subclinical synovitis at 6 month follow up MSUS examination. During the 3-year follow up 45/135 (33.3%) patients experienced a disease flare (median survival time 2.2 y). PD positivity in tendons was the stronger independent risk factor of flare on multivariable regression analysis (HR: 4.8; P=0.04). Other predictors of flare were the JIA subtype (oligo-extended form: HR: 2.3; P=0.031) and the status of CR on medication (HR: 3.7; P=0.002).

Conclusion:

our results confirm that MSUS is more sensitive than clinical evaluation in the assessment of persistent synovial inflammation in JIA patients. Subclinical tenosynovitis was the best predictor of disease flare. To date, the role of tenosynovitis in the diagnosis and prognosis of JIA has been poorly investigated. Our results further support the role of MSUS, especially of the wrist and the ankle, in monitoring JIA patients in clinical remission and to predict disease flare.

References:

[1]De Lucia O, et al. Baseline ultrasound examination as possible predictor of relapse in patients affected by juvenile idiopathic arthritis (JIA). Ann Rheum Dis. 2018 Oct;77(10):1426-1431.

[2]Filippou G, et al. The predictive role of ultrasound-detected tenosynovitis and joint synovitis for flare in patients with rheumatoid arthritis in stable remission. Results of an Italian multicentre study of the Italian Society for Rheumatology Group for Ultrasound: the STARTER study. Ann Rheum Dis 2018;77:1283-9.

Disclosure of Interests:

None declared

Continuous monitoring of noise levels in the Gulf of Catania (Ionian Sea). Study of correlation with ship traffic

Acoustic noise levels were measured in the Gulf of Catania (Ionian Sea) from July 2012 to May 2013 by a low frequency (<1000Hz) hydrophone, installed on board the NEMO-SN1 multidisciplinary observatory. NEMO-SN1 is a cabled node of EMSO-ERIC, which was deployed at a water depth of 2100m, 25km off Catania. The study area is characterized by the proximity of mid-size harbors and shipping lanes. Measured noise levels were correlated with the passage of ships tracked with a dedicated AIS antenna. Noise power was measured in the frequency range between 10Hz and 1000Hz. Experimental data were compared with the results of a fast numerical model based on AIS data to evaluate the contribution of shipping noise in six consecutive 1/3 octave frequency bands, including the 1/3 octave frequency bands centered at 63Hz and 125Hz, indicated by the Marine Strategy Framework Directive (2008/56/EC).

Congenital esophageal stenosis diagnosed in an infant at 9 month of age

Esophageal stenosis is a relatively uncommon condition in pediatrics and requires an accurate diagnostic approach. Here we report the case of a 9-month old female infant who presented intermittent vomiting, dysphagia and refusal of solid foods starting after weaning. She was treated for gastroesophageal reflux. At first, radiological investigation suggested achalasia, while esophagoscopy revelaed a severe congenital esophageal stenosis at the distal third of the esophagus. She underwent four endoscopic balloon dilatations that then allowed her to swallow solid food with intermittent mild dysphagia. After 17 months of esomeprazole treatment off therapy impedance-pH monitoring was normal. At 29 months of follow-up the child is asymptomatic and eats without problems.Infants with dysphagia and refusal of solid foods may have undiagnosed medical conditions that need treatment. Many disorders can cause esophageal luminal stricture; in the pediatric age the most common are peptic or congenital. Careful assessment with endoscopy is needed to diagnose these conditions early and referral to a pediatric gastroenterologic unit may be necessary.

Transcranial brain photoplethysmography to study the venules of cerebral cortex in patients with multiple sclerosis

Objective

To evaluate the utility of a transcranial brain photoplethysmography parameter as a potential marker for patients with multiple sclerosis.

Methods

We investigated 38 patients affected by multiple sclerosis, according to the revised McDonald criteria (12 males and 26 females, mean age 41.1 ± 8.5 years, Expanded Disability Status Scale mean value 2.6 ± 2.1) and compared them with 20 age- and sex-matched healthy controls. By means of transcranial brain photoplethysmography, a safe and non-invasive technology, we measured the increase in cerebral blood volume during compression for 10 s of the internal jugular veins of both sides in sitting position.

Results

The cerebral blood volume increase was significantly smaller in the multiple sclerosis patients (left frontal cortex: −58%, p < 0.0001; right frontal cortex: −59%, p < 0.0001) compared with the controls.

Conclusions

Our study reveals that a minor increase in cerebral blood volume on the frontal cortex of both sides in sitting position is associated with a diagnosis of multiple sclerosis and might be a new marker.

Creatin-kinase elevation after accidental ingestion of almotriptan in an 18-month-old girl

Few studies have been published to demonstrate tolerability and efficacy of almotriptan in adolescents and children with migraine, particularly in the first years of life, though preliminary results are favorable. We report the case of an 18-month-old infant with elevation of serum levels of creatin-kinase after the accidental ingestion of almotriptan. A previously healthy 18-month-old girl (weight: 13 kg) was admitted to our Department four hours after the accidental ingestion of 6.25 mg of almotriptan (0.48 mg/kg), without any specific symptom. The performed investigations showed high serum levels of creatin-kinase (CK) (527 IU/L; normal values: 24-170 IU/L). Transaminase, creatinine, aldolase, myoglobin and troponin T serum levels were normal. The electrocardiogram proved negative. Initial management consisted of parenteral rehydration with saline solution. CK levels lowered significantly at 12 hours (455 IU/L) and at 65 hours (188 IU/L) after the ingestion. No symptoms were observed before discharge and on follow-up.

Tolerability of donkey's milk in 92 highly-problematic cow's milk allergic children

OBJECTIVE

Not exclusively breastfed children with cow's milk allergy (CMA) require a formula or other alternative food, but past and present guidelines differ concerning the best choice. Our aim was to investigate the clinical tolerability, palatability and nutritional adequacy of donkey's milk (DM) in children with proven CMA. It was important to identify a CM replacement for these children, highly problematic from the feeding standpoint, in spite of their age.

STUDY DESIGN

A prospective study was conducted on 92 children with CMA, diagnosed through a CM elimination diet, followed by double-blind, placebo-controlled food challenge (DBPCFC) unless contraindicated. Maternal milk was unavailable and current CM substitutes could not be used. Moreover, 89 percent were affected by multiple FA, and subjected to very restricted diets. Within 3 months after the last CM challenge, DBPCFC for DM was performed. CM or DM skin prick test and sIgE determination preceded the CM or DM challenge, respectively. Native electrophoresis and immunoblotting were used to identify CM and DM cross-reactive proteins. Z-scores of weight and length/stature for age were calculated at DM food challenge (T0) and during DM assumption.

RESULTS

83 children (90.2 percent) liked and tolerated DM, at challenge and during follow-up, with increased Z-score for weight and length/stature and improved nutritional parameters. Bovine beta-lactoglobulin was identified as the cross-reacting protein among the DM allergic patients.

CONCLUSIONS

DM was found to be a valid alternative foodstuff, in terms of clinical tolerability, palatability and nutritional adequacy, in subjects with CMA who were highly problematic from the feeding standpoint.

Correlation between the arterial pulse wave of the cerebral microcirculation and CBF during breath holding and hyperventilation in human

OBJECTIVE

To evaluate if relative changes in the amplitude of the arterial pulse wave of the cerebral microcirculation (APWCM) measured by near-infrared spectroscopy (NIRS) may provide information about relative changes of cerebral blood flow (CBF) in cerebral cortex.

METHODS

In 10 healthy human volunteers, through simultaneous recording of the APWCM amplitude by means of NIRS and the mean blood flow velocity (MBFV) of middle cerebral artery by means of transcranial Doppler (TCD) at rest and during breath holding and hyperventilation, we evaluate a possible correlation between relative changes of the mean APWCM amplitude and relative changes of MBFV.

RESULTS

We found a significant linear correlation: breath holding: R(2) 0.84, p < 0.001, hyperventilation: R(2) 0.81, p<0.001.

CONCLUSION

The relative changes of the mean APWCM amplitude seem able to provide information about relative changes of CBF of cerebral cortex in healthy adult humans during breath holding and hyperventilation.

SIGNIFICANCE

APWCM detected by NIRS, a safe, repeatable, inexpensive technology and at the bedside may improve the study of cerebral cortex microcirculation in neurological diseases.

A case of severe allergic reaction to cooked potato

White potato is a very common ingredient in the diet of infants in Mediterranean countries, and in its cooked form, it is one of the first solid foods introduced, usually around the age of 4-6 months. Allergy to potato is uncommon, and allergic reactions to cooked potato have been reported only in children. We report a case of severe potato-induced allergic reaction in an 8-month-old infant with atopic dermatitis and multiple food allergies that raises questions about differential diagnosis between anaphylaxis and food protein-induced enterocolitis syndrome (FPIES).

CONCLUSION

Allergy to cooked potatoes could be a cause of severe although rare allergic reactions; it could be very difficult, in some cases, to make a differential diagnosis between anaphylaxis and FPIES. Moreover, the diagnosis appears to be very important for the choice of therapy and long-term allergologic management.

Does removal of aids/devices and help make a difference in the Childhood Health Assessment Questionnaire disability index?

Objective:

To assess whether the removal of aids/devices and/or help from another person in the Childhood Health Assessment Questionnaire (C-HAQ) leads to a significant change in the disability index (DI) score and responsiveness in juvenile idiopathic arthritis (JIA).

Methods:

Changes in the C-HAQ DI score in a cross-sectional sample of 2663 children with JIA and in 530 active patients with JIA in a trial of methotrexate (MTX) were compared.

Results:

Patients in the MTX trial had higher disease activity and disability than the cross-sectional sample. The frequency of aids/devices (range 1.2–10.2%) was similar between the two samples, while help (range 5.3–38.1%) was more frequently used in the MTX group. Correlation between disease severity variables and the two different C-HAQ DI scoring methods did not change substantially. There was a decrease in the C-HAQ DI score for both the cross-sectional (mean score from 0.64 with the original method to 0.54 without aids/devices and help, p<0.0001) and the MTX sample (mean score from 1.23 to 1.07, p<0.0001). A linear regression analysis of the original C-HAQ DI score versus the score without aids/devices and help demonstrated the substantial overlap of the different scoring methods. Responsiveness in the responders to MTX treatment did not change with the different C-HAQ DI scoring methods (range 0.86–0.82).

Conclusion:

The removal of aids/devices and help from the C-HAQ does not alter the interpretation of disability at a group level. The simplified C-HAQ is a more feasible and valid alternative for the evaluation of disability in patients with JIA.

[Malnutrition in children with chronic bronchitis]

OBJECTIVE

To study nutritional status in children with chronic bronchitis (CB) in relation with lung function.

METHODS

In this cohort of study, 46 patients aged 6.0 to 17.5 years (mean: 11.9 years) with chronic bronchitis were recruited. None had cystic fibrosis. Body weight, height, skinfold thicknesses, percentage of ideal body weight-for-height (percentage of IBW), body mass index (BMI), BMI Z-score, fat mass and fat-free mass were used to evaluate nutritional status. Arterial blood gases, vital capacity (VC), forced expiratory volume in one s (FEV1), functional residual capacity (FRC) and maximum inspiratory (Pi(max)) and expiratory (Pe(max)) pressures at the mouth were used to evaluate respiratory function.

RESULTS

Thirteen children (28%) had malnutrition defined as percentage of IBW lower than 90%, with a predominant fat mass depletion. VC (65+/-13% versus 79+/-15%; p=0.006) and FEV1 (59+/-16% versus 69+/-14%; p=0.03) were significantly lower in children with malnutrition than in children without malnutrition, but no significant differences were observed with regard to the FEV1/VC ratio and blood gases. Pi(max) (56+/-11% versus 88+/-37%, p=0,02) and Pe(max) (46+/-12% versus 58+/-19%, p=0,3) were also lower in children with malnutrition as compared to than without malnutrition.

CONCLUSION

Malnutrition can be observed in children with CB and is associated with significant lower lung function parameters. This could be explained by decrease in respiratory muscle strength.

The Paediatric Rheumatology International Trials Organization (PRINTO)

In this review we describe an international project, conducted by the Paediatric Rheumatology International Trials Organization (PRINTO) that was aimed to identify, validate and promulgate core sets of measures and a definition of improvement for the evaluation of response to therapy in clinical trials and in daily clinical practice in patients with juvenile systemic lupus erythematosus (JSLE). The following clinical measures were included in the PRINTO core set of outcome measure for the evaluation of response to therapy: 1) physician's global assessment of disease activity; 2) global disease activity measure; 3) 24-hour proteinuria; 4) parent's global assessment of the overall patient's well-being; 5) health-related quality of life assessment. The measures included in the core set were found to be feasible and not redundant, to have good construct validity, discriminative ability, internal consistency, with fair responsiveness to clinically important change in disease activity, and to be associated strongly with treatment outcome. In order to be classified as responder to a given treatment, a patient should demonstrate at least 50% improvement from baseline in any two of the five core set measures with no more than one of the remaining worsening by more than 30%. This definition is now known as the 'PRINTO/American College of Rheumatology (ACR) provisional criteria for JSLE'. The proposed core set and definition of improvement incorporate clinically meaningful change in a composite endpoint for the evaluation of global response to therapy in JSLE. The definition is now proposed for use in JSLE clinical trials, and may help physicians to decide if a child with SLE has responded adequately to therapy.

The magnitude of early response to methotrexate therapy predicts long-term outcome of patients with juvenile idiopathic arthritis

OBJECTIVE

To investigate the relationship between the magnitude of clinical response in the first 6 months of methotrexate (MTX) therapy and long-term outcome in children with juvenile idiopathic arthritis (JIA).

METHODS

The clinical charts of 125 JIA patients who were started with MTX and then followed for at least 5 years were reviewed. Based on the level of American College of Rheumatology (ACR) Pediatric response at 6 months, patients were divided in four mutually exclusive groups: (1) non-responders, (2) responders at 30%, (3) responders at 50%, and (4) responders at 70%. The long-term outcome in each response group was evaluated by calculating the percentage change in active and restricted joint counts from baseline to 1, 2 and 5 years and the frequency of inactive disease at 5 years.

RESULTS

At 6 months, 42 patients were classified as non-responders, 24 as 30% responders, 26 as 50% responders, and 33 as 70% responders. Patients who had achieved a 70% response showed a significantly greater percentage improvement in active joint count between baseline to 5 years compared with non-responders and 30% responders, and a significantly greater percentage improvement in restricted joint count between baseline to 5 years compared with 30% responders. The 70% responders also had a greater frequency of inactive disease at 5 years compared with 30% responders,

CONCLUSIONS

Our results show that the achievement of an ACR Pediatric 70 response at 6 months after start of MTX therapy predicts a more favorable long-term outcome of patients with JIA.

Discordance between physician's and parent's global assessments in juvenile idiopathic arthritis

OBJECTIVE

To investigate the discrepancy between physician's and parent's global assessments of disease status and the factors explaining discordance in patients with juvenile idiopathic arthritis (JIA).

METHODS

The mothers of 197 patients with JIA rated the child's overall well-being on a 10 cm visual analogue scale (VAS) and the attending physician rated the child's overall disease activity on a 10 cm VAS. A discordance score was calculated by subtracting the physician's global assessment from that of the parent's, leading to the definition of three patient groups: (1) no discordance, when physician's and parent's assessments were within 1 cm of each other; (2) negative discordance, when parent's assessment was underrated relative to the physician; and (3) positive discordance, when parent's assessment was over-rated relative to the physician. Negative and positive discordance was defined as 'marked' when the difference between the two assessments was greater than 3 cm.

RESULTS

No discordance was found in 40.6% of the patients. Negative discordance was found in 51.3% of the patients, with 34% showing marked discordance. Positive discordance was found in 8.1% of the patients, with 2% showing marked discordance. Significant differences between groups included a shorter disease duration among patients with a markedly positive discordance (P = 0.02) and a greater frequency of ongoing second-line drug therapy among patients with no discordance or with positive discordance (P = 0.008). Patients with no discordance or with marked positive discordance had a significantly lower joint counts (P = 0.02-0.004).

CONCLUSION

Parents and physicians often perceive the health status of children with JIA differently, with parents providing most frequently lower rating.

Improvement of cardiovascular risk profile in an elderly population of low social level: the ICON (Improving Cardiovascular risk profile in Older Neapolitans) study

Older age and low socio-economic conditions are associated with poor control of cardiovascular risk factors (RFs). We assessed the prevalence and awareness of cardiovascular RFs in 503 elderly outpatients of low social status attending two public Internal Medicine clinics in Naples, and studied the interaction of education and employment level with risk profile. The therapeutic intervention was oriented to improve patients' motivation through a positive patient-physician relationship, in keeping with the current guidelines for hypertension. The effect of treatment was evaluated by comparing retrospectively the level of cardiovascular RFs at baseline and at the last follow-up examination performed within 31 October 2005. Only 33.3% of patients (age=68+/-6 years) had attended primary school. Overall (current or previous) employment level was also low. Obesity, hypertension and dyslipidaemia were present in most patients, diabetes in 17.3% of them. In all 8.0% of hypertensives, 16.1% of diabetics and 24.7% of dyslipidaemiacs were unaware of their diseases. Cardiovascular risk profile was worse at lower educational and employment levels. Odds ratios for the metabolic syndrome were 0.28 (95% confidence interval (CI)=0.15-0.52) and 0.35 (0.20-0.62) in the most qualified of three education and employment groups, respectively, compared to the lowest ones. The level of all cardiovascular RFs was effectively reduced during treatment. Control rate of most RFs improved significantly (for hypertension, from 12.8 to 36.5%, P<0.001). These patients had a high prevalence of cardiovascular RFs, which correlated with their educational and work activity levels. Awareness of their health status was unsatisfactory. Treatment, specifically addressing patient-physician relationship, favourably affected cardiovascular risk profile.

Responsiveness of clinical measures to flare of disease activity in juvenile idiopathic arthritis

OBJECTIVE

To compare the responsiveness of clinical measures in the assessment of disease flare in patients with juvenile idiopathic arthritis (JIA).

METHODS

The clinical records of all consecutive patients with JIA who were diagnosed between 1995 and 2000 were retrospectively reviewed. In each patient, all visits made during follow-up were analyzed and those meeting the criteria for disease flare were recorded. The definition of flare was based on the therapeutic alterations made by the attending physician. Responsiveness of JIA clinical measures to relevant increase in disease activity (a flare) was evaluated by assessing the score change of each measure from a visit made 6 (+/- 3) months before a flare and the flare visit. Responsiveness statistics included the standardized response mean (SRM) and the effect size (ES).

RESULTS

A total of 115 patients, who were followed for 0.5 to 6.2 years (mean 2.8 years), were studied. During follow-up, 51 patients (44%) experienced 1 or more disease flares, with the total number of flares being 75. Strong responsiveness (ES and SRM > or = 0.8) to increase in disease activity was demonstrated by the physician's and parent's global assessments, the global articular severity score, and the morning stiffness. The active, swollen and painful joint counts, the swelling, pain on motion/tenderness and limited range of motion (LROM) scores, and the erythrocyte sedimentation rate revealed moderate responsiveness (ES and SRM > or = 0.5). The poorest performances (ES and/or SRM < 0.5) were provided by the parent's assessment of pain, the functional ability tool, the number of joints with LROM, the LROM score, the C-reactive protein, the white blood cell and platelet count, and the hemoglobin level.

CONCLUSION

Our analysis suggests that the swollen or painful joint counts are better suited than the count of joints with LROM for the assessment of disease flare in patients with JIA.

Le mégaœsophage de l'enfant : profil clinique et évolution à long terme

Achalasia of the cardia is rare in children. We report our experience in the management of 20 children with achalasia.

PATIENTS

Twenty children (seven girls including two sisters) with achalasia were studied, seven of them had a morbid association. Age at diagnosis ranged from eight months to 18 years (med: 6.4 yrs). Duration of symptoms prior to diagnosis ranged from one to 62 months (med: 8 months). Regurgitations, weight loss and recurrent pneumonias were the most common presenting symptoms. Diagnosis was established using esophageal manometry, which showed aperistalsis throughout the esophageal body with impaired relaxation of lower esophageal sphincter in all children, and chest x-ray and barium esophagram, which were abnormal in 11 and 18 children respectively.

OUTCOME

Nifedipine used in nine children was ineffective. Two balloon dilatations were performed in one child with a poor result. Surgery with Heller's myotomy combined with an antireflux procedure was performed in 19 children. One child died six months later due to an hypoglycemic coma. During the follow-up (6 months-21 years) symptoms reappeared in five children. Seventy-two percent of the children had no symptoms one year after the surgery and 45%, five years after. Esophageal manometry performed after surgery in seven children showed a normal lower esophageal sphincter pressure but with impaired relaxation, and an aperistalsis throughout the esophageal body. These results justify prolonged follow-up of children with achalasia of the esophagus.

Relative responsiveness of condition specific and generic health status measures in juvenile idiopathic arthritis

OBJECTIVES

To compare the relative responsiveness of condition specific measures with that of a generic health status instrument for outcome assessment of intra-articular corticosteroid (IAC) injection in patients with juvenile idiopathic arthritis (JIA).

METHODS

We examined 44 consecutive patients with oligoarticular JIA before an IAC injection and after 6 months. Condition specific measures included physician's and parent's global assessments, the Childhood Health Assessment Questionnaire (CHAQ), the articular indices, and laboratory indicators of systemic inflammation. The generic health status instrument was the Child Health Questionnaire (CHQ), which was divided into two parts: the physical score (PhS) and the psychosocial score (PsS). Responsiveness statistics were the standardised response mean, the effect size, and Guyatt's method. The discriminative ability of the clinical measures in distinguishing improved from non-improved patients was evaluated with the correlation and the receiver operating characteristic methods, using the physician's and the parent's judgements of the treatment outcome as external criteria.

RESULTS

All responsiveness statistics and discriminative ability assessments consistently ranked the physician's global assessment of the disease activity as the most responsive measure. The CHQ-PhS revealed superior ability in detecting baseline versus 6 month change compared with the CHAQ and the CHQ-PsS; both summary scales of the CHQ revealed better discriminative ability than the CHAQ.

CONCLUSIONS

The physician's global assessment of the disease activity proved the most responsive outcome measure in our patients with JIA. The relative evaluative properties of the generic health status instrument and the CHAQ should be further investigated.

Self-organization in a simple model of adaptive agents playing 2 x 2 games with arbitrary payoff matrices

We analyze, both analytically and numerically, the self-organization of a system of "selfish" adaptive agents playing an arbitrary iterated pairwise game (defined by a 2 x 2 payoff matrix). Examples of possible games to play are the prisoner's dilemma (PD) game, the chicken game, the hero game, etc. The agents have no memory, use strategies not based on direct reciprocity nor "tags" and are chosen at random, i.e., geographical vicinity is neglected. They can play two possible strategies: cooperate (C) or defect (D). The players measure their success by comparing their utilities with an estimate for the expected benefits and update their strategy following a simple rule. Two versions of the model are studied: (1) the deterministic version (the agents are either in definite states C or D) and (2) the stochastic version (the agents have a probability c of playing C). Using a general master equation we compute the equilibrium states into which the system self-organizes, characterized by their average probability of cooperation c(eq). Depending on the payoff matrix, we show that c(eq) can take five different values. We also consider the mixing of agents using two different payoff matrices and show that any value of c(eq) can be reached by tuning the proportions of agents using each payoff matrix. In particular, this can be used as a way to simulate the effect of a fraction d of "antisocial" individuals--incapable of realizing any value to cooperation--on the cooperative regime hold by a population of neutral or "normal" agents.