June 2, 2009 — The increased risk for violent crime associated with schizophrenia is largely due to the subgroup of patients who are also substance abusers, according to a study in the May 20 issue of the Journal of the American Medical Association.

Seena Fazel, MD, from the University of Oxford, Warneford Hospital, in the United Kingdom, and colleagues report that schizophrenia without substance abuse was associated with a slightly increased rate of violent crime compared with the general population (odds ratio, 1.2), but persons who had schizophrenia with comorbid substance abuse were more than 4 times as likely as the general population to have a violent-crime conviction.

"The other main finding is that there is familial confounding of the relationship between violent crime and schizophrenia, and that occurs through substance abuse," Dr. Fazel told Medscape Psychiatry. "In other words, unaffected siblings had increased rates of violent crime compared with the general population, and this appeared to be mediated through higher rates of substance abuse than general population controls."

Contrary Findings

Other reports have found a 4- to 6-fold increased risk for violence associated with schizophrenia, which has led to the view that schizophrenia and other major mental disorders are preventable causes of violence and violent crime, according to the investigators.

To examine this relationship, the investigators analyzed data from nationwide Swedish registers of hospital admissions and criminal convictions from 1973 to 2006. Risk for violent crime in patients after diagnosis of schizophrenia (n = 8003) was compared with that among general population controls (n = 80,025). Potential confounders, including age, sex, income, and marital and immigrant status as well as substance abuse, were measured at the beginning of the study. To study familial confounding, the researchers also investigated risk of violence among unaffected siblings (n = 8123) of patients with schizophrenia.

The researchers found that among patients with schizophrenia, 1504 (13.2%) had at least 1 violent offense compared with 4276 (5.3%) general population controls (adjusted odds ratio, 2.0).

The rate of violent crime in individuals diagnosed as having schizophrenia and substance abuse (27.6%) was significantly higher than in those without substance abuse (8.5%), which resulted in adjusted odds ratios of 4.4 for violent crime in schizophrenia with substance abuse and 1.2 in schizophrenia without substance abuse.

The risk increase among those with substance abuse was significantly less pronounced when unaffected siblings were used as controls — 28.3% of those with schizophrenia had a violent offense compared with 17.9% of their unaffected siblings — suggesting significant familial, either genetic or early environmental, confounding of the association between schizophrenia and violence.

More Routine Attention to Substance Abuse Needed

This is "an important question that needs further research and synthesis of the evidence. In terms of managing the risk, it highlights the importance of reviewing the substance-abuse problems regularly and treating them," Dr. Fazel said.

Coauthor Niklas Långström, MD, PhD, who is head of the Center for Violence Prevention at the Karolinska Institutet, in Stockholm, Sweden, advised routine assessment and treatment of substance misuse in individuals with schizophrenia. "This might require modifications of already-existing treatment modalities — pharmacological, psychosocial, and psychotherapeutic," he said.

Dr. Långström also noted that these data, in addition to those from studies by forensic psychologist Eric B. Elbogen, PhD, from the University of North Carolina School of Medicine, in Chapel Hill, suggest that the American Psychiatric Association guideline recommending violence risk assessment for all patients with schizophrenia should be reconsidered.

"We should focus on the individuals with additional problems. Address risk factors that increase the risk for violent behavior among all individuals, independent of whether they suffer from schizophrenia or not," Dr. Långström said. "These include substance misuse, violence proneness reflected also in family history, negative affect, impulsivity, and cognitions that support the use of violence."

Asked to comment on the study findings, Dr. Elbogen said the large sample size gives the study findings weight.

Treatment Not Optimal

"This is an important study because it includes over 8000 patients with schizophrenia, which affects only 1% of the population," he said. Most important, he added, it highlights the additional violence risk associated with comorbid schizophrenia and substance abuse and the importance of addressing both issues.

"The main weakness is that it is not clear that the people with schizophrenia in this study are representative of all people with schizophrenia, which affects how generalizable the conclusions are," he said. The inclusion criteria required hospitalization with discharge diagnoses of schizophrenia on at least 2 separate occasions.

"These strict criteria might have selected for a relatively sicker group of patients with schizophrenia, and the impact might be that the risk of violence associated with schizophrenia in most patients without comorbid substance abuse might be even smaller than these researchers found.

"In clinical practice, asking a patient with schizophrenia if he or she has a history of substance abuse or dependence is an important screen, because it alerts you to be concerned about the patient's potential risk for violent behavior," he added.

"Unfortunately," he added, "because of how mental-health services are typically structured, most patients get either mental-health services or substance-abuse services, but not both. The result is that those at highest risk are getting only half of what they need."

The authors have disclosed no relevant financial relationships.

JAMA. 2009;301:2016-2023. Abstract Clinical Context

Many epidemiologic studies have reported on the association between major mental disorder, specifically schizophrenia, and violence. Patients with schizophrenia are thought to be at increased risk of committing a violent crime 4 to 6 times the level of people in the general population without this disorder.

Conceptual models propose that patients with schizophrenia are violent because of the psychopathologic symptoms of the disorder or secondary comorbid substance use (an established risk factor for violence). Another model suggests that schizophrenia and violent behavior co-occur because of familial factors that are related to both (eg, personality traits such as irritability, poor anger management, or inadequate coping with stress). However, risk estimates vary substantially across studies, and considerable uncertainty exists as to what mediates this elevated risk. The aim of this study was to determine the risk for violent crime in patients diagnosed as having schizophrenia and the role of substance abuse in mediating this risk.

Study Highlights

* In this study, the risk for violent crime in patients after a diagnosis of schizophrenia (n = 8003) was compared vs control subjects in the general population (n = 80,025).
* Patients with schizophrenia were identified by 2 criteria.
First, they had been discharged from hospitals beginning in January 1, 1973, and had discharge diagnoses of schizophrenia on at least 2 separate inpatient hospitalizations. Second, they were born between 1958 and 1989, so they were at least 15 years old (the age of criminal responsibility) at the start of the study in 1973.

* Longitudinal designs were used to link data from nationwide Swedish registers of hospital admissions and criminal convictions from 1973 to 2006.
* Potential confounders (age, sex, income, and marital and immigrant status) and mediators (substance abuse comorbidity) were measured at baseline.
* To study familial confounding, the investigators also examined the risk for violence in unaffected siblings (n = 8123) of patients with schizophrenia.
* The outcome measure was violent crime (any criminal conviction for homicide, assault, robbery, arson, any sexual offense, illegal threats, or intimidation).
* The results demonstrated that patients with schizophrenia, 1054 (13.2%) had at least 1 violent offense
vs 4276 (5.3%) of control subjects in the general population (adjusted odds ratio, 2.0; 95% confidence interval, 1.8 - 2.2). * The risk was mostly confined to patients with substance abuse comorbidity
(of whom 27.6% committed an offense), yielding an increased risk for violent crime in such patients (adjusted odds ratio, 4.4; 95% confidence interval, 3.9 - 5.0).
* However, the risk increase was small in patients with schizophrenia without substance abuse
comorbidity (8.5% of whom had at least 1 violent offense; adjusted odds ratio, 1.2; 95% confidence interval, 1.1 - 1.4; P < .001 for interaction).
* The risk increase in those with substance abuse comorbidity was significantly less pronounced when unaffected siblings
were used as control subjects (28.3% of those with schizophrenia had a violent offense vs 17.9% of their unaffected siblings;
adjusted odds ratio, 1.8; 95% confidence interval, 1.4 - 2.4; P < .001 for interaction), suggesting significant familial confounding of the association between schizophrenia and violence.
* The risk for violent outcomes in schizophrenia was significantly increased vs unaffected control subjects
when a more severe definition of violent crime was used,
when individuals who died or emigrated during follow-up were excluded,
when case ascertainment was based on only 1 hospital diagnosis of schizophrenia,
and when the follow-up period was extended.

* Weaknesses of the study included the reliance on hospital data for case ascertainment and comorbidity, leading to a possible underestimate of the effects of substance abuse, and no data were available regarding whether treatment was received and the nature of such treatment.

Clinical Implications

* Current conceptual models suggest that patients with schizophrenia are violent as a consequence of the psychopathologic symptoms of the disorder itself,
secondary to comorbid substance abuse, or because of familial factors related to both.
* Schizophrenia was associated with an increased risk for violent crime,
and this association was further attenuated by adjustment for substance
abuse.

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