CANNABIS CONSUMPTION AT AGE 18

AND LATER RISK OF SCHIZOPHRENIA

Number of reported occasions

Figure 2.12 Substance misuse can contribute to later development of schizophrenia. This study following army recruits found that those who admitted taking cannabis on more than 50 occasions had a sixfold risk of schizophrenia compared with non-users. Figure reproduced with permission from Andreasson S, Allebeck P, Engstrom A, Rydberg U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts. Lancet 1987;2:1483–6

patients as a group may be of lower birth weight than the general population.

Neurodevelopmental abnormality

The morphological abnormalities that have been reported in schizophrenia are consistent with a neurodevelopmental event occurring in fetal or early development. Some believe that the onset of frank psychotic symptoms reflects the delayed sequelae of an earlier developmental aberration, which is then expressed as the brain continues to develop in adolescence and adult life – sometimes termed the ‘doomed from the womb’ view. Delayed emergence of abnormal behavior following lesions sustained during early development is a well recognized phenomenon, and is seen, for example, in animal models where ventral hippocampal lesions, initially ‘silent’, are followed by hyperactivity and increased responsiveness to stressful stimuli and to dopamine blockade as the animal matures. The inherited neurodevelopmental disease metachromatic leukodystrophy is more likely to be associated with schizophreniform symptoms if the clinical onset is in adolescence. In this case, as in schizophrenia, late maturational events, such as myelination of

prefrontal nerve tracts and perforant pathway, or abnormal synaptic plasticity, may reveal earlier developmental abnormalities.

Later environmental factors

Substance misuse

The relationship between substance misuse and schizophrenia is complex. Many drugs of abuse, such as ketamine, amphetamine, cocaine, and LSD, are psychotomimetic and can induce an acute schizophrenia-like psychosis. Psychoactive substance misuse both precedes and follows the onset of psychotic symptoms. Some patients state that they receive transient symptom relief and are using the drugs as a form of ‘self-medication’. However, it is also clear that abuse of certain drugs can increase the risk of schizophrenia. Evidence concerning cannabis comes from the Swedish army study19 in which army recruits were interviewed about their drug consumption and then followed-up for a decade and a half. Those who admitted taking cannabis on more than 50 occasions had a risk of later schizophrenia some six times that of non-abusers (Figure 2.12).