Rare Psychotic Disorders: Schizotypal and Induced Delusions

Not all psychotic disorders fit the well-known profile of schizophrenia. Some conditions sit at the edges of the psychotic spectrum – close enough to raise clinical concern, but distinct in their causes, presentation, and course. Three such conditions coded under the ICD-10 classification system are Schizotypal Disorder (F21), Induced Delusional Disorder (F24), and Unspecified Psychosis (F29). Understanding each of these is essential for anyone studying mental disorders, as they reveal how psychosis can manifest in subtle, socially shaped, and diagnostically uncertain ways.

Schizotypal disorder (F21): eccentric thinking on the edge of psychosis 🔗

Schizotypal disorder occupies a unique position in psychiatric classification. It is not schizophrenia – but it is not entirely separate from it either. According to the ICD-10, schizotypal disorder (F21) is characterized by an enduring pattern of cognitive and perceptual distortions, eccentric behavior and appearance, odd beliefs and speech, and difficulty establishing close relationships – yet these features are not severe enough to meet the criteria for schizophrenia.

Think of it as a persistent pattern where someone’s thinking and experience of reality are consistently unusual, but psychotic episodes – full-blown hallucinations or fixed delusions – are absent or only transient.

Core features of schizotypal disorder 🔗

Research on schizotypal disorder identifies a consistent cluster of symptoms. These include magical thinking (believing one can influence events through thought), paranoid ideation, ideas of reference (feeling that unrelated events have personal significance), odd or vague speech that is digressive or overelaborate, and inappropriate or restricted affect during social interactions. Perceptual distortions – such as sensing a presence that isn’t there – are also common, as is significant social anxiety that does not diminish with familiarity.

People with this disorder often feel pronounced discomfort in close relationships, primarily because they tend to believe others harbor negative thoughts about them. They may react oddly in conversations or talk to themselves. Paranormal and superstitious beliefs are frequently reported.

Classification differences: ICD-10 vs. DSM-5 🔗

There is a notable classification discrepancy worth understanding. In the ICD-10, schizotypal disorder is placed within the psychotic disorders block (F20-F29), treating it as part of the schizophrenia spectrum. In contrast, the DSM-5 lists it as a personality disorder under Cluster A. This dual placement reflects genuine clinical ambiguity: the disorder shares genetic links with schizophrenia, yet its presentation – without clear psychosis – also resembles a personality style.

Studies show that schizotypal disorder can be diagnosed as early as age five, and when it precedes a diagnosis of schizophrenia, the schizotypal diagnosis is maintained but marked as premorbid. Common comorbidities include borderline personality disorder, social anxiety disorder, and ADHD.

Treatment and management 🔗

Treatment typically combines low-dose antipsychotic medication (to address perceptual distortions), cognitive-behavioral therapy to help challenge odd beliefs and reduce social anxiety, and social skills training. Because individuals with this disorder often do not see their thinking as unusual, engaging them in therapy can be a significant clinical challenge. Psychoeducation for the individual and their family plays an important role in long-term management.

Induced delusional disorder (F24): when delusions spread between people 🔗

Perhaps the most intriguing condition in this cluster is induced delusional disorder – better known by its French name, folie à deux, meaning “madness of two.” Described in the psychiatric literature as far back as 1860, this rare condition involves the transmission of delusional beliefs from one person to another within a close, often isolated relationship.

Under the ICD-10, this is coded as F24 – Induced Delusional Disorder. It is classified separately from schizophrenia and delusional disorder because the key mechanism is relational – the delusion in the secondary person arises through their close association with someone who is already psychotic, not independently.

The inducer and the induced 🔗

Research consistently describes a clear dynamic between two roles. The inducer (or “primary”) is the person who originally develops the psychotic delusion – typically from an underlying condition such as schizophrenia or delusional disorder. The induced (or “secondary”) is a non-psychotic individual who gradually adopts the inducer’s delusional beliefs. The inducer usually plays a dominant role in the relationship, and the secondary is often more passive, emotionally dependent, or socially vulnerable.

The delusions shared are most commonly persecutory in nature – for example, both individuals believing they are being monitored, poisoned, or conspired against. A key factor in the development of folie à deux is the dominant partner’s ability to use suggestion – which some researchers have compared to a form of psychological influence – to transfer the delusional framework to the weaker partner. Crucially, this process is greatly accelerated by social isolation: the more cut off the pair is from outside social reality, the more the delusion takes hold.

Subtypes and clinical nuances 🔗

Clinicians have identified four main subtypes of folie à deux:

• Folie imposée – the most common form. Delusions are transferred from the psychotic primary to a mentally sound secondary, and typically disappear once the two are separated.
• Folie communiquée – similar to the above, but the secondary resists the delusions for a prolonged period before eventually adopting them, and may maintain them even after separation.
• Folie simultanée – both individuals independently develop identical psychosis simultaneously due to shared genetic or environmental risk factors.
• Folie induite – the secondary is already psychotic, and the relationship induces new, shared delusional content.

Although these subtypes are well-described in the research literature, the ICD-10’s F24 criteria primarily address the first two forms – focusing on the secondary patient who did not have psychosis before contact with the primary.

When the disorder extends beyond two people – such as in a family – it may be called folie à famille. About 90% of cases involve people within a nuclear family, with sister-sister and parent-child pairs being the most frequently reported.

Treatment: separation as the first step 🔗

The primary treatment for induced delusional disorder is separating the secondary from the inducer. In cases of folie imposée, the secondary’s delusions often resolve on their own once this separation is achieved. If symptoms persist, short-term antipsychotic medication may be used. The inducer, meanwhile, requires treatment for their underlying psychotic disorder. Clinicians note that this disorder is frequently underdiagnosed – people with shared delusions rarely seek psychiatric help together and tend to reinforce each other’s beliefs rather than question them.

Unspecified psychosis (F29): when the picture isn’t yet clear 🔗

Not every case of psychosis arrives with a neat diagnostic label. In clinical practice, psychiatrists and mental health professionals regularly encounter individuals who clearly have psychotic symptoms – hallucinations, delusions, disorganized thinking – but whose clinical picture is incomplete, contradictory, or still evolving. This is where F29 – Unspecified Psychosis Not Due to a Substance or Known Physiological Condition becomes essential.

ICD-10 code F29 is used when psychotic symptoms are present but cannot yet be attributed to a specific disorder such as schizophrenia (F20), schizoaffective disorder (F25), or brief psychotic disorder (F23). Importantly, it also excludes psychosis caused by substance use or an underlying medical condition – those would receive different codes. F29 is essentially a provisional category, acknowledging clinical reality: sometimes you need to treat before you can fully classify.

When is F29 used? 🔗

F29 applies in several clinical scenarios: when a patient presents for the first time with acute psychotic symptoms and no prior psychiatric history, when diagnostic information is limited or incomplete, or when symptoms do not yet fit the duration or pattern criteria for a more specific disorder. It is particularly common in first-episode psychosis presentations in emergency and inpatient settings.

The code is also used when overlapping features across multiple psychotic disorders make a definitive diagnosis premature. A thorough assessment is still required – clinicians must document specific symptoms, rule out substance-induced causes, and establish that no known physiological condition (such as epilepsy or a brain tumor) is driving the presentation.

Diagnostic stability and long-term outcomes 🔗

One of the key questions about F29 is how stable this diagnosis tends to be over time. A study that followed 138 patients initially diagnosed with F29 found that after a mean follow-up of around 23 months, 43% retained the unspecified psychosis diagnosis, while the remainder were reclassified – most commonly as bipolar disorder (18%), schizophrenia (11%), or major depression (7%). This reflects the inherently transitional nature of F29: it is often a starting point, not a final destination.

When the clinical picture becomes clearer over time – through structured diagnostic interviews, collateral information, symptom tracking, and the exclusion of other conditions – clinicians are expected to refine the diagnosis to a more specific ICD-10 code. F29 should not remain a permanent classification when sufficient evidence for a more precise diagnosis has accumulated.

Clinical and ethical considerations 🔗

Using an unspecified code carries real responsibilities. Clinicians using F29 must clearly justify its use in documentation – detailing the specific symptoms observed, explaining why a more specific diagnosis cannot yet be made, and assessing functional impairment. Vague documentation is both clinically and ethically problematic: it can affect the treatment the patient receives, the services they are eligible for, and how their condition is understood over time.

Why these three disorders matter together 🔗

Schizotypal disorder (F21), induced delusional disorder (F24), and unspecified psychosis (F29) each illuminate a different dimension of how psychosis presents in the real world. F21 shows that psychotic-spectrum disturbances can be persistent, pervasive, and impairing without tipping into full psychosis. F24 demonstrates that social relationships and emotional dependency can shape – and even transmit – delusional thinking. And F29 acknowledges that clinical certainty takes time, and that responsible practice sometimes means sitting with ambiguity while still providing care.

Together, they underscore that psychosis is not a single phenomenon but a spectrum of experiences, each requiring careful assessment, individualized treatment, and – above all – diagnostic honesty.

What do you think? If a person close to someone with a psychotic disorder begins to adopt their delusions, at what point does that become a clinical condition rather than simply loyalty or belief? And given that F29 is often a temporary classification, how should clinicians and patients navigate the uncertainty that comes with an unspecified diagnosis?

References

1. https://www.icd10data.com/ICD10CM/Codes/F01-F99/F20-F29/F21-/F21
2. https://en.wikipedia.org/wiki/Schizotypal_personality_disorder
3. https://www.ncbi.nlm.nih.gov/books/NBK541211/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC11806268/
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC2919794/
6. https://www.sciencedirect.com/topics/medicine-and-dentistry/shared-psychotic-disorder
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC8487833/
8. https://www.blueprint.ai/blog/f29-unspecified-psychosis-and-its-clinical-implications
9. https://www.sprypt.com/behavioral-health-icd-codes/f29
10. https://pmc.ncbi.nlm.nih.gov/articles/PMC6427075/