May 29, 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 disclose no conflicts of interest.

JAMA. 2009;301:2016-2023. Abstract