The causes of schizophrenia have been the subject of much debate over many decades with various factors proposed and discounted. Studies suggest that genetics, prenatal development, early environment, neurobiology and psychological and social processes are important contributory factors. Current psychiatric research into the development of the disorder is often based on a neurodevelopmental model. In the absence of a confirmed specific pathology underlying the diagnosis, some question the legitimacy of schizophrenia's status as a disease. Furthermore, some propose that the perceptions and feelings involved are meaningful and do not necessarily involve impairment.
Although no common cause of schizophrenia has been identified in all individuals diagnosed with the condition, currently most researchers and clinicians believe it results from a combination of both brain vulnerabilities (either inherited or acquired) and life events. This widely-adopted approach is known as the 'stress-vulnerability' model, and much scientific debate now focuses on how much each of these factors contributes to the development and maintenance of schizophrenia. Schizophrenia is most commonly first diagnosed during late adolescence or early adulthood suggesting it is often the end process of childhood and adolescent development. There is on average a somewhat earlier onset for men than women, with the possible influence of the female hormone oestrogen being one hypothesis and sociocultural influences another.
Both individual twin studies and meta-analyses of twin studies estimate the heritability of risk for schizophrenia to be approximately 80% (this refers to the proportion of variation between individuals in a population that is influenced by genetic factors, not the degree of genetic determination of individual risk). Adoption studies have also indicated a somewhat increased risk in those with a parent with schizophrenia even when raised apart. Studies suggest that the phenotype is genetically influenced but not genetically determined; that the variants in genes are generally within the range of normal human variation and have low risk associated with them each individually; and that some interact with each other and with environmental risk factors; and that they may not be specific to schizophrenia. Some twin studies have found rates as low as 11.0%–13.8% among monozygotic twins, and 1.8%–4.1% among dizygotic twins, however. In addition, some scientists criticize the methodology of the twin studies, and have argued that the genetic basis of schizophrenia is still largely unknown or open to different interpretations.
There is currently a great deal of effort being put into molecular genetic studies of schizophrenia, which attempt to identify specific genes which may increase risk. Because of this, the genes that are thought to be most involved can change as new evidence is gathered. A 2003 review of linkage studies listed seven genes as likely to increase risk for a later diagnosis of the disorder. Two more recent reviews have suggested that the evidence is currently strongest for two genes known as dysbindin (DTNBP1) and neuregulin (NRG1), with a number of other genes (such as COMT, RGS4, PPP3CC, ZDHHC8, DISC1, and AKT1'') showing some early promising results that have not yet been fully replicated. Variations near the gene FXYD6 have been associated with schizophrenia in the UK but not in Japan. In 2008, rs7341475 SNP of the reelin gene was associated with an increased risk of schizophrenia in women, but not in men. This female-specific association was replicated in several populations.
Recent research has suggested that a greater than average number of rare deletions or duplications of tiny DNA sequences within genes (known as copy number variants) are linked to increased risk for schizophrenia, especially in those "sporadic" cases not linked to family history of schizophrenia, and that the genetic factors and developmental pathways can thus be different in different individuals.
The largest most comprehensive genetic study of its kind, involving tests of several hundred single nucleotide polymorphisms (SNPs) in nearly 1,900 individuals with schizophrenia or schizoaffective disorder and 2,000 comparison subjects, has reported no significant association between the disorders and any of 14 previously identified candidate genes (RGS4, DISC1, DTNBP1, STX7, TAAR6, PPP3CC, NRG1, DRD2, HTR2A, DAOA, AKT1, CHRNA7, COMT, and ARVCF). The statistical distributions suggested nothing more than chance variation. The authors concluded that the findings make it unlikely that common SNPs in these genes account for a substantial proportion of the genetic risk for schizophrenia, although small effects could not be ruled out.
It is well established that obstetric complications or events are associated with an increased chance of the child later developing schizophrenia, although overall they constitute a non-specific risk factor with a relatively small effect. Obstetric complications occur in approximately 25 to 30% of the general population and the vast majority do not develop schizophrenia, and likewise the majority of individuals with schizophrenia have not had a detectable obstetric event. Nevertheless, the increased average risk is well-replicated, and such events may moderate the effects of genetic or other environmental risk factors. The specific complications or events most linked to schizophrenia, and the mechanisms of their effects, are still under examination.
One epidemiological finding is that people diagnosed with schizophrenia are more likely to have been born in winter or spring (at least in the northern hemisphere). However, the effect is not large. Explanations have included a greater prevalence of viral infections at that time, or a greater likelihood of vitamin D deficiency. A similar effect (increased likelihood of being born in winter and spring) has also been found with other, healthy populations, such as chess players. Women who were pregnant during the Dutch famine of 1944, where many people were close to starvation (experiencing malnutrition) had a higher chance of having a child who would later develop schizophrenia. Studies of Finnish mothers who were pregnant when they found out that their husbands had been killed during the Winter War of 1939–1940 have shown that their children were significantly more likely to develop schizophrenia when compared with mothers who found out about their husbands' death after pregnancy, suggesting that maternal stress may have an effect.
Lower than average birth weight has been one of the most consistent findings, indicating slowed fetal growth possibly mediated by genetic effects. Almost any factor adversely affecting the fetus will affect growth rate, however, so the association has been described as not particularly informative regarding causation. In addition, the majority of birth cohort studies have failed to find a link between schizophrenia and low birth weight or other signs of growth retardation.
Animal models have suggested links between intrauterine growth restriction and specific neurological abnormalities similar to those that may be involved in the development of schizophrenia, including ventricular enlargement and reduced hippocampal volume in guinea pigs.
It has been hypothesized since the 1970s that brain hypoxia (low oxygen levels) before, at or immediately after birth may be a risk factor for the development of schizophrenia. This has been recently described as one of the most important of the external factors that influence susceptability, although studies have been mainly epidemiological. Fetal hypoxia, in the presence of certain unidentified genes, has been correlated with reduced volume of the hippocampus, which is in turn correlated with schizophrenia. Although most studies have interpreted hypoxia as causing some form of neuronal dysfunction or even subtle damage, it has been suggested that the physiological hypoxia that prevails in normal embryonic and fetal development, or pathological hypoxia or ischemia, may exert an effect by regulating or dysregulating genes involved in neurodevelopment. A literature review judged that over 50% of the candidate genes for susceptibility to schizophrenia met criteria for "ischemia–hypoxia regulation and/or vascular expression".
A longitudinal study found that obstetric complications involving hypoxia were one factor associated with neurodevelopmental impairments in childhood and with the later development of schizophreniform disorders. Fetal hypoxia has been found to predict unusual movements at age 4 (but not age 7) among children who go on to develop schizophrenia, suggesting that its effects are specific to the stage of neurodevelopment. A Japanese case study of monozygotic twins discordant for schizophrenia (one has the diagnosis while the other does not) draws attention to their different weights at birth and concludes hypoxia may be the differentiating factor. The unusual functional laterality in speech production (e.g. right hemisphere auditory processing) found in some individuals with schizophrenia could be due to aberrant neural networks established as a compensation for left temporal lobe damage induced by pre- or perinatal hypoxia. Prenatal and perinatal hypoxia appears to be important as one factor in the neurodevelopmental model, with the important implication that some forms of schizophrenia may thus be preventable.
Research on rodents seeking to understand the possible role of prenatal hypoxia in disorders such as schizophrenia has indicated that it can lead to a range of sensorimotor and learning/memory abnormalities. Impairments in motor function and coordination, evident on challenging tasks when the hypoxia was severe enough to cause brain damage, were long-lasting and described as a "hallmark of prenatal hypoxia". Several animal studies have indicated that fetal hypoxia can affect many of the same neural substrates implicated in schizophrenia, depending on the severity and duration of the hypoxic event as well as the period of gestation, and in humans moderate or severe (but not mild) fetal hypoxia has been linked to a series of motor, language and cognitive deficits in children, regardless of genetic liability to schizophrenia.
Whereas most studies find only a modest effect of hypoxia in schizophrenia, a longitudinal study using a combination of indicators to detect possible fetal hypoxia, such as early equivalents of Neurological Soft Signs or obstetric complications, reported that the risk of schizophrenia and other nonaffective psychoses was "strikingly elevated" (5.75% versus 0.39%).
There is an emerging literature on a wide range of prenatal risk factors, such as prenatal stress, intrauterine (in the womb) malnutrition, and prenatal infection. Increased parental age has been linked, possibly due to prenatal complications increasing the risk of genetic mutations. Maternal-fetal rhesus or genotype incompatibility has also been linked, via increasing the risk of an adverse prenatal environment. And, in mothers with schizophrenia, an increased risk has been identified via a complex interaction between maternal genotype, maternal behavior, prenatal environment and possibly medication and socioeconomic factors.
Influenza has long been studied as a possible factor. A 1988 study found that individuals who were exposed to the Asian flu as second trimester fetuses were at increased risk of eventually developing schizophrenia. This result was corroborated by a later British study of the same pandemic,, but not by a 1994 study of the pandemic in Croatia. A Japanese study also found no support for a link between schizophrenia and birth after an influenza epidemic.
Polio, measles, varicella-zoster, rubella, herpes simplex virus type 2, maternal genital infections, and more recently Toxoplasma gondii, have been correlated with the later development of schizophrenia. However, studies of postmortem brain tissue have reported equivocal or negative results, including no evidence of herpesvirus or T. Gondii DNA in schizophrenia in a recent study.
Psychiatrists E. Fuller Torrey and R.H. Yolken have hypothesized that the common parasite Toxoplasma gondii contributes to some if not many cases of schizophrenia. Studies have tended to find moderately higher levels of Toxoplasma antibodies in those with schizophrenia and possibly higher rates of prenatal or early postnatal exposure to Toxoplasma gondii, but not to acute infection. Causal mechanisms remain speculative.
There is some evidence for the role of autoimmunity in the development of some cases of schizophrenia. A statistical correlation has been reported with various autoimmune diseases and direct studies have linked dysfunctional immune status to some of the clinical features of schizophrenia.
In general, the antecedents of schizophrenia are subtle and those who will go on to develop schizophrenia do not form a readily identifiable subgroup. Average group differences from the norm may be in the direction of superior as well as inferior performance. Overall, birth cohort studies have indicated subtle nonspecific behavioral features, some evidence for psychotic-like experiences (particularly hallucinations), and various cognitive antecedents. There have been some inconsistencies in the particular domains of functioning identified and whether they continue through childhood and whether they are specific to schizophrenia.
A prospective study found average diffferences across a range of developmental domains, including reaching milestones of motor development at a later age, having more speech problems, lower educational test results, solitary play preferences at ages four and six, and being more socially anxious at age 13. Lower ratings of the mother's skills and understanding of the child at age 4 were also related.
Some of the early developmental differences were identified in the first year of life in a study in Finland, although generally related to psychotic disorders rather than schizophrenia in particular. The early subtle motor signs persisted to some extent, showing a small link to later school performance in adolescence. An earlier Finnish study found that childhood performance of 400 individuals diagnosed with schizophrenia was significantly worse than controls on subjects involving motor co-ordination (sports and handcrafts) between ages 7 and 9, but there were no differences on academic subjects (contrary to some other IQ findings). However, reanalysis of the data from the later Finnish study, on older children (14 to 16) in a changed school system, using narrower diagnostic criteria and with less cases but more controls, did not support a significant difference on sports and handicraft performance. However, another study found that unusual motor coordination scores at 7 years of age were associated in adulthood with both those with schizophrenia and their unaffected siblings, while unusual movements at ages 4 and 7 predicted adult schizophrenia but not unaffected sibling status.
A birth cohort study in New Zealand found that children who went on to develop schizophreniform disorder had, as well as emotional problems and interpersonal difficulties linked to all adult psychiatric outcomes measured, significant impairments in neuromotor, receptive language, and cognitive development. A retrospective study found that adults with schizophrenia had performed better than average in artistic subjects at ages 12 and 15, and in linguistic and religious subjects at age 12, but worse than average in gymnastics at age 15.
Some small studies on offspring of individuals with schizophrenia have identified various neurobehavioral deficits, a poorer family environment and disruptive school behaviour, poor peer engagement, immaturity or unpopularity or poorer social competence and increasing schizophrenic symptomology emerging during adolescence.
A minority "deficit syndrome" subtype of schizophrenia is proposed to be more marked by early poor adjustment and behavioral problems, as compared to non-deficit subtypes.
The rate of substance use is known to be particularly high in this group. In a recent study, 60% of people with schizophrenia were found to use substances and 37% would be diagnosable with a substance use disorder.
Drugs such as ketamine, PCP, and LSD have been used to mimic schizophrenia for research purposes. Using LSD and other psychedelics as a model has now fallen out of favor with the scientific research community, as the differences between the drug induced states and the typical presentation of schizophrenia have become clear. The dissociatives ketamine and PCP are still considered to produce states that are remarkably similar however.
Despite the higher prevalence of tobacco smoking, people diagnosed with schizophrenia have a much lower than average chance of developing and dying from lung cancer. While the reason for this is unknown, it may be because of a genetic resistance to the cancer, a side-effect of drugs being taken, or a statistical effect of increased likelihood of dying from causes other than lung cancer.
A recent study of over 50,000 Swedish conscripts found that there was a small but significant protective effect of smoking cigarettes on the risk of developing schizophrenia later in life. While the authors of the study stressed that the risks of smoking far outweigh these minor benefits, this study provides further evidence for the 'self-medication' theory of smoking in schizophrenia and may give clues as to how schizophrenia might develop at the molecular level. Furthermore, many people with schizophrenia have smoked tobacco products long before they are diagnosed with the illness, and some groups advocate that the chemicals in tobacco have actually contributed to the onset of the illness and have no benefit of any kind.
It is of interest that cigarette smoking affects liver function such that the antipsychotic drugs used to treat schizophrenia are broken down in the blood stream more quickly. This means that smokers with schizophrenia need slightly higher doses of antipsychotic drugs in order for them to be effective than do their non-smoking counterparts.
The increased rate of smoking in schizophrenia may be due to a desire to self-medicate with nicotine. One possible reason is that smoking produces a short term effect to improve alertness and cognitive functioning in persons who suffer this illness. It has been postulated that the mechanism of this effect is that people with schizophrenia have a disturbance of nicotinic receptor functioning which is temporarily abated by tobacco use.
The chance of developing schizophrenia has been found to increase with the number of adverse social factors (e.g. indicators of socioeconomic disadvantage or social exclusion) present in childhood. Stressful life events generally precede the onset of schizophrenia. A personal or recent family history of migration is a considerable risk factor for schizophrenia, which has been linked to psychosocial adversity, social defeat from being an outsider, racial discrimination, family dysfunction, unemployment and poor housing conditions. Childhood experiences of abuse or trauma are risk factors for a diagnosis of schizophrenia later in life. Recent large-scale general population studies indicate the relationship is a causal one, with an increasing risk with additional experiences of maltreatment, although a critical review suggests conceptual and methodological issues require further research. There is some evidence that adversities may lead to cognitive biases and/or altered dopamine neurotransmission, a process that has been termed "sensitization". Specific social experiences have been linked to specific psychological mechanisms and psychotic experiences in schizophrenia. In addition, structural neuroimaging studies of victims of sexual abuse and other traumas have sometimes reported findings similar to those sometimes found in psychotic patients, such as thinning of the corpus callosum, loss of volume in the anterior cingulate cortex, and reduced hippocampal volume.
Most schizophrenia studies have found average reduced volume of the left medial temporal lobe and left superior temporal gyrus, and half of studies have revealed deficits in certain areas of the frontal gyrus, parahippocampal gyrus and temporal gyrus. However, at variance with some findings in individuals with chronic schizophrenia (where use of antipsychotics and other factors may have a confounding effect), significant group differences of temporal lobe and amygdala volumes are not shown in first-episode patients on average. The neurobiological abnormalities are so varied that no single abnormality is observed across the entire group of people with DSM-IV–defined schizophrenia. In addition, it remains unclear whether the structural differences are unique to schizophrenia or cut across the traditional diagnostic boundaries between schizophrenia and affective disorders - though perhaps being unique to conditions with psychotic features.
Studies of the rare childhood-onset schizophrenia (before age 13) indicate a greater-than-normal loss of grey matter over several years, progressing from the back of the brain to the front, levelling out in early adulthood. Such a pattern of "pruning" occurs as part of normal brain development but appears to be exaggerated in childhood-onset psychotic disorders, particularly in schizophrenia. Abnormalities in the volume of the ventricles or frontal lobes have also been found in some studies, but not in others. Volume changes are most likely glial and vascular rather than purely neuronal, and reduction in grey matter may primarily reflect a reduction of neuropil rather than a deficit in the total number of neurons. Other studies, especially some computational studies, have shown that a reduction in the number of neurons can cause psychotic symptoms . Studies to date have been based on small numbers of the most severe and treatment-resistant patients taking antipsychotics.
Electroencephalograph (EEG) recordings of persons with schizophrenia performing perception oriented tasks showed an absence of gamma band activity in the brain, indicating weak integration of critical neural networks in the brain. Those who experienced intense hallucinations, delusions and disorganized thinking showed the lowest frequency synchronization. None of the drugs taken by the persons scanned had moved neural synchrony back into the gamma frequency range. Gamma band and working memory alterations may be related to alterations in interneurons that produce the neurotransmitter GABA.
However, there was controversy and conflicting findings over whether post-mortem findings resulted from chronic antipsychotic treatment. Studies using SPET and PET methods in drug naive patients have generally failed to find any difference in dopamine D2 receptor density compared to controls. Recent findings from meta-analyses suggest that there may be a small elevation in dopamine D2 receptors in drug-free patients with schizophrenia, but the degree of overlap between patients and controls makes it unlikely that this is clinically meaningful. In addition, newer antipsychotic medication (called atypical antipsychotic medication) can be as potent as older medication (called typical antipsychotic medication) while also affecting serotonin function and having somewhat less of a dopamine blocking effect. In addition, dopamine pathway dysfunction has not been reliably shown to correlate with symptom onset or severity.
It is still thought that dopamine mesolimbic pathways may be hyperactive, resulting in hyperstimulation of D2 receptors and positive symptoms. There is also growing evidence that, conversely, mesocortical pathway dopamine projections to the prefrontal cortex might be hypoactive (underactive), resulting in hypostimulation of D1 receptors, which may be related to negative symptoms and cognitive impairment. The overactivity and underactivity in these different regions may be linked, and may not be due to a primary dysfunction of dopamine systems but to more general neurodevelopmental issues that precede them. Increased dopamine sensitivity may be a common final pathway.
Another reliable finding is that there is an excess of binding sites insensitive to a certain testing agent (raclopride)
Another one of Philip Seeman's findings was that the dopamine D2 receptor protein looked abnormal in schizophrenia. Proteins change states by flexing. The activating of the protein by folding could be permanent or fluctuating, just like the courses of patients' illnesses waxes and wanes. Increased folding of a protein leads to increased risk of 'additional fragments' forming The schizophrenic d2 receptor has a unique additional fragment when digested by papain in the test-tube in the FASEB experiment above, but none of the controls exhibited the same fragment. The D2 receptor in schizophrenia are thus in a highly active state as found by Philip Seeman et al.
Dyregulation of neural calcium homeostasis has been hypothesized to be a link between the glutamate and dopaminergic abnormalities and some small studies have indicated that calcium channel blocking agents can lead to improvements on some measures in schizophrenia with tardive dyskinesia.
There is evidence of irregular cellular metabolism and oxidative stress in the prefrontal cortex in schizophrenia, involving increased glucose demand and/or cellular hypoxia.
Andrew Sims has said that the top two 'Factors mainly concerned in the germination of delusions' are:1. Disorder of brain functioning and 2. background influences of temperament and personality.
Higher levels of dopamine qualify as a symptom of 'disorders of brain function'. They are needed to sustain certain delusions is confirmed by a study on delusional disorder (another psychotic syndrome). "...pHVA [homovanillic acid] was employed as a "state marker" of the [delusional] disorder...": Delusions of persecution and delusions of jealousy were able to be differentiated by the amount of acid that was released when the dopamine was digested."... From these findings, it seems that pHVA varies depending upon the subtype and course of schizophrenia, and it is also strongly influenced by neuroleptic treatment. This is confirmed in that very high recreational use of amphetamine produces Gender Identity Disorder delusions (which the DSM says are rare), while medical doses of amphetamine can produce morbid jealousy The subject of the large recreational amphetamine dose had an unconfirmed zero sperm count. One journal says amphetamine affects overall fertility in mice only at very high doses - showing how the state of dopamine must be at a rare high.
Sims briefly examines delusional content, but in a separate section, and not elaborating on personalities that are commonly associated with each delusion; but says "Jaspers considered there is a subtle change in personality due to the illness itself; and this creates the condition for the development of the delusional atmosphere in which the delusional intuition arises"
Cultural factors have "a decisive factor in shaping delusions". For example, delusions of guilt and punishment are frequesnt in a Western, Christian country like Austria, but not in Pakistan - where it is more likely persecution. It says cultural factors have a decisive influence in shaping delusions. . Frequent delusions of guilt and punishment are shown in Austria as well and this is with Parkinson's patients treated with l-dopa.
A number of cognitive biases or deficits have been found in people diagnosed with schizophrenia. These include jumping to conclusions when faced with limited or contradictory information; specific biases in reasoning about social situations, for example assuming other people cause things that go wrong (external attribution); difficulty distinguishing inner speech from speech from an external source (source monitoring); difficulty in adjusting speech to the needs of the hearer, related to theory of mind difficulties; difficulties in the very earliest stages of processing visual information (including reduced latent inhibition); difficulty with attention e.g. being more easily distracted, attentional bias towards threat. Some of these tendencies have been shown to worsen or appear when under emotional stress or in confusing situations. As with the related neurological findings, they are not shown by all individuals with a diagnosis of schizophrenia and it is not clear how specific they are to schizophrenia or to particular symptoms. However, the findings regarding cognitive difficulties in schizophrenia are reliable and consistent enough for some researchers to argue that they are diagnostic. Impaired capacity to appreciate one's own and others' mental states has been reported to be the single-best predictor of poor social competence in schizophrenia. Similar cognitive features have been identified in close relatives of people diagnosed with schizophrenia.
A number of emotional factors have been implicated in schizophrenia, with some models putting them at the core of the disorder. It was thought that the appearance of blunted affect meant that sufferers did not experience strong emotions, but more recent studies indicate there is often a normal or even heightened level of emotionality, particularly in response to negative events or stressful social situations. Some theories suggest positive symptoms of schizophrenia can result from or be worsened by negative emotions, including depressed feelings and low self-esteem and feelings of vulnerability, inferiority or loneliness. Chronic negative feelings and maladaptive coping skills may explain some of the association between psychosocial stressors and symptomology. Critical and controlling behaviour by significant others (high expressed emotion) causes increased emotional arousal and lowered self-esteem and a subsequent increase in positive symptoms such as unusual thoughts. Countries or cultures where schizotypal personalities or schizophrenia symptoms are more accepted or valued appear to be associated with reduced onset of, or increased recovery from, schizophrenia.
Related studies suggest that the content of delusional and psychotic beliefs in schizophrenia can be meaningful and play a causal or mediating role in reflecting the life history or social circumstances of the individual. Holding minority or poorly understood sociocultural beliefs, for example due to ethnic background, has been linked to increased diagnosis of schizophrenia. The way an individual personally understands and attributes their delusions or hallucinations (e.g. as threatening or as potentially positive) has also been found to influence functioning and recovery.
Information processing disorders - backed up by Andrzej Jakubik "Personality disorders" (ISBN 83-200-2087-5). pages 218 and following, suggests that in paranoid schizophrenia short term memory organization levels and data processing structures related are impaired, especially selection, structuralisation and recall related functional circuits. His references are :
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