Aporia 03: Four psychosis codes, one clinical question

ICD-10 criteria, operational

A characteristic psychotic syndrome present most of the time for at least one month, with at least one of (a) thought echo, insertion, withdrawal, broadcast; (b) delusions of control, influence, passivity; (c) hallucinatory voices commenting or discussing; (d) persistent bizarre delusions; or at least two of (e) persistent hallucinations of any modality; (f) breaks in train of thought, neologisms; (g) catatonic phenomena; (h) negative symptoms; (i) significant change in personal behaviour. Organic, substance and mood-confined causes excluded.

ICD-11 criteria, what changed

The subtypes (paranoid, hebephrenic, catatonic, simple, residual) are removed in ICD-11. The Schneiderian first-rank symptoms are de-emphasised but not excluded; they are folded into the broader symptom requirement. Six symptom dimensions are added as specifiers (positive, negative, depressive, manic, psychomotor, cognitive). Course specifiers (first episode, multiple episodes, continuous) replace the older course modifiers.² The duration concept is retained at one month.

DSM-5 criteria, what differs

Two or more of (1) delusions, (2) hallucinations, (3) disorganised speech, (4) grossly disorganised or catatonic behaviour, (5) negative symptoms, with at least one being delusions, hallucinations or disorganised speech. Marked functional decline is Criterion B (an explicit requirement, unlike ICD). Duration is six months total disturbance, with at least one month of active-phase symptoms (or less if successfully treated). Schizoaffective and mood-with-psychosis exclusion, substance and medical exclusion. If autism spectrum or communication disorder is present, schizophrenia requires prominent delusions or hallucinations for at least one month.

What the IPS CPG 2026 emphasises

The Indian Psychiatric Society 2026 Clinical Practice Guideline for schizophrenia management positions phase-based care: acute (symptom remission and safety), continuation (six to twelve months of maintenance at minimum effective dose), and maintenance (relapse prevention). Second-generation antipsychotic monotherapy is first line in first-episode psychosis. Clozapine consideration follows two adequate antipsychotic trials. LAI is recommended proactively, not as a last resort, where nonadherence or relapse pattern indicate. ECT remains relevant for catatonia, severe affective psychosis, and selected treatment-resistant cases.⁴

What discriminates F20 from F22 at the bedside

• Formal thought disorder on examination, present in F20, absent in F22.
• Negative symptoms: blunted affect, alogia, avolition. Present in F20, absent in F22.
• Functional decline outside the delusional theme: broad in F20, narrowly preserved in F22.
• Multiple delusional themes or shifting content: F20. A single encapsulated theme, F22.
• Persistent hallucinations not tied to the delusional content: F20. Tactile or olfactory hallucinations congruent with the delusion (worms in delusional infestation, smell in olfactory reference), F22.
• Age of onset: F20 typically late teens to early thirties; F22 typically thirties to seventies with a late-onset peak in women.

What discriminates F20 from F28

The honest answer is duration and atypicality. A patient with clear primary psychotic phenomena that do not meet the schizophrenia syndrome (atypical, subthreshold, mixed) is coded F28 if the clinician can articulate what the pattern is and why it does not fit F20. The trap is using F28 as a hedge for early F20 when the duration threshold has not yet been crossed; F23 is the correct hedge for short-duration cases, not F28.

Schneiderian first-rank symptoms, where they sit now

Schneider's 1959 list (thought insertion, withdrawal, broadcast; voices commenting, voices arguing; somatic passivity; delusional perception; made feelings, impulses, acts) was historically treated as pathognomonic of schizophrenia. Subsequent work showed prevalence of one or more FRSs at 8 to 28 percent in bipolar disorder, 10 to 25 percent in major depression with psychotic features, and 15 to 30 percent in schizophrenia.⁵ ICD-11 and DSM-5 both de-emphasise FRSs as a diagnostic shortcut. They retain viva relevance and remain useful as a phenomenological vocabulary, but they are not a diagnostic test.

ICD-10 criteria, operational

One delusion or a set of related delusions dominates the clinical picture, is usually long-standing (at least three months in clinical guideline tradition), and is not better explained by organic disease, substance use, schizophrenia, or persistent mood disorder.⁷ The delusion is the disorder. Hallucinations may be present but not persistent and not schizophrenia-like (typically content-congruent: tactile in infestation, olfactory in reference). No formal thought disorder. No negative symptoms. No persistent affective syndrome. F22.0 is delusional disorder proper; F22.8 includes involutional paranoid state and paraphrenia (late); F22.9 is unspecified.

ICD-11 criteria, what changed

Renamed to plain Delusional disorder (6A24): "persistent" is dropped from the name but the duration requirement is now explicit at three months. Bizarre delusions are permitted if no other schizophrenia features are present, closing a long-standing F22/F20 grey zone where bizarre content automatically pushed coding to schizophrenia. Hallucinations are allowed only when content-congruent with the delusion. Symptom-themed subtypes (persecutory, grandiose, jealous, somatic, erotomanic) drop as formal codes and are described in the text. Course specifiers are added. Paraphrenia and induced delusional disorder are no longer separate codes, they collapse into 6A24 or 6A2Y.²

DSM-5 criteria, what differs

Criterion A: one or more delusions for at least one month (shorter than ICD). Criterion B: Criterion A for schizophrenia never met. Criterion C: functioning not markedly impaired and behaviour not obviously bizarre. Criterion D: any mood episodes brief relative to the delusional period. Criterion E: substance, medical and better-explanation exclusion. Specify subtype: erotomanic, grandiose, jealous, persecutory, somatic, mixed, unspecified. Specify with bizarre content if delusions are clearly implausible (the bizarre/non-bizarre specifier was retained from DSM-IV but broadened, bizarre delusions in the absence of other schizophrenia symptoms still qualify for delusional disorder).⁸

The subtypes, with their clinical signatures

• Persecutory: the most common subtype. Believes others are plotting, poisoning, harassing, defaming. Often litigious; legal correspondence to police, courts, regulators. Functioning preserved outside the persecutory frame.
• Grandiose: special talent, identity, relationship to a deity or famous figure. Religious avatars common in Indian samples; differentiate from culturally sanctioned bhakti using conviction, distress, preoccupation, and functional impact.
• Jealous (Othello syndrome): partner is unfaithful. Often comorbid alcohol use disorder, frontal pathology, stimulant use. High homicide risk. Indian forensic samples repeatedly link to alcohol dependence.
• Somatic: bodily function, sensation, appearance. Includes delusional infestation (Ekbom), delusional halitosis, delusional dysmorphia. Patients present first to dermatology, ENT, or cosmetic surgery. The matchbox sign (specimens brought to the consultation) is classic for Ekbom.
• Erotomanic (de Clerambault): another, often higher-status person is in love with the patient. Three phases: hope, spite, resentment. Forensic risk concentrates in spite and resentment.
• Mixed: two or more themes coexist without one dominating.

The eponymic delusional syndromes

• Capgras (1923): l'illusion des sosies; familiar person replaced by impostor. Right frontotemporal dysfunction; 73% co-occur with schizophrenia, but also Lewy-body dementia, post-TBI, and primary delusional disorder.⁹ The case in the lede of this issue is a Capgras presentation.
• Cotard (1880): delire des negations; nihilistic delusion of being dead, decaying, organs missing. Strong association with psychotic depression; responds to ECT. Less commonly seen in F22 proper than in mood-with-psychosis.
• Ekbom (1938): delusional infestation. Coded under 6A24 in ICD-11 as a somatic-theme variant. The matchbox sign is pathognomonic. Best evidence: risperidone or olanzapine first line; pimozide historical first line, now reserved where ECG monitoring and access permit.¹⁰
• Folie a deux / induced delusional disorder: F24 in ICD-10, dropped as separate category in ICD-11 (now coded as 6A24 or 6A2Y in each affected individual). Inducer and induced. Separation of the induced person often resolves the induced delusion within weeks.
• Paraphrenia (Kraepelin, 1913): chronic systematised delusions and hallucinations with preserved personality and affect, no deterioration. Kraepelin separated it from dementia praecox on that basis. Modern Indian usage tends to follow this Kraepelinian sense, often applied to middle-to-late life cases that do not fit F20 or F22 cleanly.
• Late paraphrenia (Roth, 1955): onset after forty-five, mostly after sixty; paranoid delusions with or without auditory hallucinations; no negative symptoms or personality decline; female predominance; sensory deprivation (deafness) and social isolation as risk factors. The international consensus retired the term in 2000 in favour of Late-Onset Schizophrenia (onset 40-60) and Very-Late-Onset Schizophrenia-Like Psychosis (over 60), with the latter responding to lower antipsychotic doses (twenty-five to fifty percent of adult dose).¹¹

Treatment, anchored to current evidence

There are no RCTs in delusional disorder. The treatment literature is observational, case-series-heavy, and built around Alistair Munro's foundational work on monosymptomatic hypochondriacal psychosis at Queen's University, Kingston.¹² Munro established that delusional disorder is a treatable condition, not a chronic hopeless state, and that pimozide produced striking response in delusional infestation and dysmorphic variants. The Munoz-Negro and Cervilla 2016 PRISMA systematic review of 385 cases reported overall good response in approximately 33.6%, with FGAs marginally outperforming SGAs (39% vs 28%); selection bias is heavy and the figures should be read as approximate.¹³ The strongest real-world data is the Lahteenvuo Swedish national cohort of 9,076 patients with delusional disorder, which showed any antipsychotic reduced psychiatric hospitalisation (adjusted hazard ratio 0.4 to 0.6), with LAIs and clozapine producing the largest reductions; no clear class advantage for FGAs over SGAs in hard outcomes.¹⁴ Practical Indian first-line: risperidone 2 to 6 mg or olanzapine 5 to 15 mg; escalate to LAI risperidone or paliperidone when adherence is poor (the norm in this group, who deny illness). Aripiprazole is a reasonable second-line option, particularly for somatic-type and for prolactin-sensitive patients. Pimozide remains a defensible choice in dermatology-referred Ekbom cases where ECG monitoring is available, but is no longer first line.

Prognosis, anchored to current evidence

The Hui 2023 Hong Kong four-year follow-up cohort is the single most useful prognostic dataset: 65% of first-episode delusional disorder patients still met delusional disorder criteria at four years; 35% converted to schizophrenia. Conversion predictors were younger onset, presence of any hallucination at baseline, and poor premorbid functioning.¹ Functioning is better preserved than in schizophrenia: occupational and social roles are often maintained outside the encapsulated delusional system, and hospitalisation rates are lower. Symptom persistence is the rule rather than the exception; full symptomatic remission is uncommon, but behavioural containment is achievable. Family burden is high and characteristically focal, partners and children of persecutory and jealous types live under chronic accusation. Forensic risk concentrates in jealous (Othello), erotomanic (stalking, post-rejection violence) and persecutory (litigious, sometimes assaultive) types.

Operational definition

A primary, non-organic, non-substance, non-mood-confined psychotic disorder that does not meet criteria for any of the specifically defined disorders in F20-F25. The clinician has enough information to commit to "this is primary psychosis" and to specify what the pattern is, but the pattern fails the rule-set for F20, F22, F23, F24 or F25. The category is informational, not provisional. The provisional code is F29.

When clinicians actually use F28 (high-yield)

• Chronic hallucinatory psychosis: persistent hallucinations as the dominant clinical feature, without the full schizophrenia syndrome, without a focal delusional system meeting F22.
• Autoscopic phenomena: heautoscopy, doppelganger experiences, out-of-body phenomena occurring in clear consciousness and persisting as a primary psychotic syndrome.
• Sensitive Beziehungswahn of Kretschmer: paranoid sensitivity reaction with referential and persecutory features in a sensitive personality, not meeting the duration or systematisation of F22 or the syndrome of F20.
• Monosymptomatic states not captured by F22 (which historically required "persistence"), particularly when the duration is between one and three months in ICD-10 framing.
• Culture-related or atypical psychoses with primary character, where organic and substance causes are excluded, mood primacy is excluded, and the pattern is not captured by the main ICD-10 buckets.
• Atypical mixed psychoses with features of more than one named category, where no single category dominates.

What to document, verbatim

• Which psychotic symptoms are present, in what modality, with what duration.
• What you have ruled out (organic, substance, delirium, mood-primary).
• Why it is not F20 (which schizophrenia criterion is not met).
• Why it is not F22 (single-delusion focality not present, or schizophrenia features present).
• Why it is not F23 (duration exceeds the acute and transient threshold, or onset was not sudden).
• Why it is not F25 (mood syndrome does not meet schizoaffective threshold).
• The specifier for what the pattern actually is.
• The follow-up plan and reassessment timepoint.

Diagnostic stability

Castagnini and colleagues' long-term Danish register follow-up of ICD-10 F23 and adjacent residual codes showed substantial diagnostic shift over five to ten years: roughly one-third of F23 cases converted to schizophrenia, mood disorder, or schizoaffective disorder, with smaller proportions to F22 and persistent F28.¹⁵ Indian longitudinal data is sparser, but consistent: residual codes are intermediate stops on the way to a named diagnosis, not endpoints. The clinician using F28 should expect to revise the code over the next twelve to twenty-four months.

Operational definition

Psychosis is present. The information needed to classify the disorder into a specific category (F20, F22, F23, F25, F28, F30-F33 with psychotic features, F0x organic psychosis, F1x substance-induced psychosis) is not yet available. Use is provisional and should be revised at the earliest opportunity.

When to use F29 (and when not)

• Use: duration is unclear (no reliable timeline; collateral history awaited); substance and organic causes not yet excluded; patient too unwell or uncooperative to elicit detailed phenomenology; first contact or emergency setting where diagnosis is genuinely provisional.
• Do not use: as a hedge for early F20 when you have enough information to commit (use F23 if duration is short, F28 if pattern is specifiable but atypical, or commit to F20 if criteria are met); as a placeholder you forget to revise (every F29 needs a reassessment date written in the chart).

What makes F29 defensible (what to write)

• "Provisional diagnosis: F29 nonorganic psychosis, insufficient information to specify."
• The immediate differentials being considered (typically F20, F22, F23, F25, F31 with psychotic features, F1x substance-induced).
• The medical and substance exclusions being conducted (vitals, neuro exam, CBC, RFT, LFT, electrolytes, glucose, TSH, B12/folate if indicated, urine toxicology where feasible, pregnancy test when relevant, ECG and imaging when indicated).
• The plan for reassessment: timepoint (typically one to seven days inpatient, three to seven days outpatient), what new information is being sought (collateral history, prior records, substance timeline, premorbid functioning, mood timeline), and the code-revision intent.

The trajectory out of F29

The clinical work after coding F29 is to gather the information that allows revision. At one week, substance and medical exclusions plus clearer phenomenology typically permit a move to F20, F22, F23, F28 or another named code. At four to six weeks, the response to an adequate antipsychotic trial provides additional information. At six months (DSM-5 schizophrenia duration threshold), continuous disturbance permits a defensible F20 diagnosis if criteria are met. F29 carried beyond three months should trigger a senior review of why specification has not been possible.