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Purpose and principlesIndications, selection and cautionsMethod and interpretationClinical next stepsRisks, monitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
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Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.

Differentiating primary psychiatric, neurological and medical causes

Psychiatry · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

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Possible acute brain or systemic disease

Hours-to-days onset, fluctuating arousal, new seizure, focal deficit, fever, hypoxia, severe headache, rigidity, autonomic instability or rapidly progressive psychosis requires urgent medical and neurological evaluation.

Action: Use ABCDE and glucose, obtain observations, identify medicines, substances and last known baseline, examine neurologically and treat immediate causes. Escalate promptly for acute imaging, EEG, lumbar puncture or toxicology support when the clinical syndrome indicates them, while maintaining proportionate psychiatric safety measures.

Purpose and principles

Distinguish primary psychiatric syndromes from delirium, neurological disease, systemic illness, medicine effects and substance states using chronology, attention, physiology, examination and targeted tests while recognising that causes can coexist.

The first discriminator is chronology. Delirium usually develops over hours or days with fluctuation, inattention and altered arousal. Intoxication and withdrawal follow exposure timing and may include autonomic or neurological signs. Seizure-related states can be brief, stereotyped or followed by confusion. Primary psychotic, mood and anxiety disorders more often evolve over days to months with relatively preserved alertness, although severe illness, catatonia and medicines blur that distinction. Dementia is usually progressive over months or years but predisposes strongly to superimposed delirium.

Characterise the phenotype rather than relying on one symptom. Hallucinations require modality, timing, insight, associated consciousness and neurological features. Beliefs require cultural context, conviction, preoccupation and behaviour. Cognitive testing should begin with attention and orientation; effort is not reliably inferred from performance alone. Examine cranial nerves, eye movements, motor system, reflexes, coordination, gait, meningism and abnormal movements as indicated. Catatonic signs can arise in mood, psychotic, neurological and medical disorders and require urgent cause assessment.

Indications, selection and cautions

Delirium pattern

Acute fluctuating attention and arousal with cognitive or perceptual change indicates delirium and prompts an urgent search for one or more causes.

Primary psychiatric pattern

A sustained syndrome with clearer consciousness, compatible longitudinal course and no explanatory physical findings supports a primary diagnosis but never proves exclusion.

Seizure-related state

Stereotyped episodes, automatisms, loss of awareness, lateral tongue injury, post-event confusion or focal onset raises suspicion for epileptic phenomena.

Encephalitic syndrome

Rapid behavioural change with seizures, cognitive decline, dyskinesia, autonomic instability or catatonia requires urgent neurological and infectious or autoimmune evaluation.

Medicine or substance effect

Symptoms track initiation, dose change, intoxication or withdrawal and may be accompanied by pupil, autonomic, movement, sleep or gastrointestinal signs.

Red flags requiring action

  • Inattention and altered arousal are core clues to delirium and should not be explained by agitation, depression or dementia without a cause assessment.
  • New psychosis with seizures, dyskinesia, autonomic instability, catatonia or rapid cognitive decline raises concern for encephalitis or another neurological disorder.
  • Visual loss, field defect, headache, vomiting, focal weakness, gait change or personality change can accompany intracranial disease and needs neurological evaluation.

Method and interpretation

  1. 01
    Chronology, baseline and exposure historyFirst step
    Why
    Establish speed, fluctuation, previous episodes, illness, medicines, substances, trauma and functional change.
    Interpretation and limitations
    Temporal relationships rank hypotheses but do not prove causation. Collateral evidence is especially valuable when attention, memory or insight is impaired.
  2. 02
    Attention, arousal and delirium assessment
    Why
    Identify acute brain dysfunction before interpreting psychiatric symptoms or cognitive scores.
    Interpretation and limitations
    Fluctuating alertness and inability to sustain attention strongly support delirium; identify and treat causes rather than stopping at the syndrome label.
  3. 03
    Physical and neurological examination
    Why
    Detect systemic illness, toxidrome, focal deficits, movement disorder, meningism, seizure evidence and catatonia.
    Interpretation and limitations
    Positive signs redirect urgency and testing. If the examination is limited, document uncertainty and repeat rather than recording normal findings.
  4. 04
    Cause-directed blood tests and ECG
    Why
    Assess metabolic, endocrine, infectious, organ-function, pregnancy, cardiac and treatment-safety questions.
    Interpretation and limitations
    Test choice depends on phenotype. Normal common tests reduce selected probabilities but cannot exclude neurological, autoimmune or intermittent toxic causes.

Clinical next steps

01First discriminationUse time, attention and physiologyFirst stepA patient presents with new behavioural, perceptual, cognitive or mood change.+
  1. 1Establish last known baseline, onset, fluctuation, exposures and physical symptoms using collateral evidence where needed.
  2. 2Assess observations, glucose, attention, arousal and immediate neurological or systemic warning signs before assigning a psychiatric label.
  3. 3Treat urgent abnormalities and decide whether the patient requires acute medical, neurological, toxicological or psychiatric containment.
02Targeted work-upTest the ranked hypothesesInitial assessment leaves medical, neurological, substance and primary psychiatric explanations plausible.+
  1. 1Rank diagnoses by likelihood and consequence, stating which features support or challenge each possibility.
  2. 2

Risks, monitoring and follow-up

  • Trend attention, arousal, observations and neurological findings during acute illness because fluctuation can be more diagnostic than one examination.
  • Review culture, imaging, EEG and specialist results promptly, ensuring that transfer between services does not leave ownership unclear.
  • Track symptoms against medicine doses, substance access, abstinence and sleep while avoiding withdrawal or rechallenge experiments that create harm.
  • Reassess the differential when the rate or pattern of change differs from the expected course of the working psychiatric diagnosis.
  • Maintain suicide, violence, vulnerability and capacity review throughout medical investigation; physical causation does not neutralise behavioural risk.

Special situations

Phenomenology overlaps

Visual hallucinations, paranoia, agitation and withdrawal occur across psychiatric, toxic, neurological and systemic disease, so context carries diagnostic weight.

Attention comes early

Cognitive and psychotic interpretations are less reliable when the person cannot sustain attention because an acute brain syndrome may dominate performance.

Comorbidity is common

A known psychiatric diagnosis neither protects against physical disease nor ensures that every new symptom belongs to the established disorder.

Trajectory tests hypotheses

Unexpected speed, fluctuation, neurological evolution or treatment failure is evidence that should trigger diagnostic revision rather than stronger certainty.

Normal EEG is limited

Interictal recordings can be normal in epilepsy, making witness description, semiology and specialist review essential when suspicion persists.

Common pitfalls

  1. 01

    Diagnosing primary psychosis solely because hallucinations or unusual beliefs are present.

  2. 02

    Calling acute confusion behavioural disturbance without testing attention, arousal and physiological causes.

  3. 03

    Letting one positive toxicology result end assessment for trauma, infection or coexisting illness.

  4. 04

    Treating normal basic blood tests as exclusion of encephalitis, epilepsy or structural disease.

  5. 05

    Attributing every new symptom to an established psychiatric diagnosis.

  6. 06

    Failing to repeat examination when new neurological signs or unexpected deterioration develops.

  7. 07

    Using advanced tests indiscriminately without a hypothesis or plan for interpretation.

Practice

Two practice questions

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Question 1 of 2
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PsychiatryOriginal SBA

Key delirium discriminator

A hospital inpatient reports frightening voices after surgery. Which additional finding most strongly favours delirium over an uncomplicated primary psychotic disorder?

Choose one answer, then submit
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Sources and review status4 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • NICE CG103 delirium recommendationsClinical indicators, assessment and management of delirium.
  • NICE NG97 dementia recommendationsAssessment of cognitive decline, reversible causes and imaging.
  • NICE CG178 psychosis and schizophrenia recommendationsMultidomain assessment of psychotic presentations.
  • NICE NG10 violence and aggression recommendationsSafe assessment of acute behavioural disturbance and medical contributors.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom

Medical causes include hypoxia, infection, endocrine disturbance, glucose or electrolyte disorder, hepatic or renal dysfunction, nutritional deficiency, autoimmune disease and porphyria in selected phenotypes. Neurological causes include epilepsy, encephalitis, tumour, stroke, traumatic injury, demyelination and neurodegeneration. Medicines such as corticosteroids, anticholinergics, dopamine agonists and some anticonvulsants can alter mood, perception or behaviour; abrupt withdrawal from alcohol, benzodiazepines or other sedatives can be dangerous. Temporal association increases suspicion but adverse-reaction and causality assessment remain necessary.

Investigations follow the leading risks. Observations, glucose, blood tests and ECG cover common acute threats and prescribing safety. Neuroimaging is indicated by trauma, focal signs, new seizure, severe headache, rapid progression or atypical late onset. EEG may support seizure or encephalopathy but a routine normal tracing does not exclude intermittent epilepsy. Lumbar puncture and paired infection or autoimmune studies may be necessary after appropriate imaging and specialist input when encephalitis or meningitis is plausible. Toxicology screens do not encompass every substance or establish causation.

Keep diagnostic uncertainty active. A young person with established schizophrenia can still develop meningitis; a patient with encephalitis may also be suicidal; depression can accompany hypothyroidism without being fully explained by it. Treat immediate abnormalities, seek cross-specialty advice and define what evidence would change the working diagnosis. If the course is faster, more fluctuating or more neurological than expected, reopen the differential even after an initial psychiatric formulation.

Key points

  • Start with time course: abrupt and fluctuating change suggests delirium, intoxication, withdrawal, seizure or other acute disease more than an uncomplicated primary psychiatric syndrome.
  • Establish attention and arousal before interpreting unusual beliefs or perceptions; impaired attention makes routine psychiatric and cognitive conclusions less reliable.
  • Compare with baseline using collateral information and identify recent illness, pain, surgery, sleep loss, medicine changes, substances and head injury.
  • Use the nature of symptoms cautiously: visual hallucinations, paranoia or agitation occur in both medical and psychiatric disorders and are not diagnostic by themselves.
  • Perform physical and neurological examination, including movement, gait, eye signs and focal findings, and repeat when intoxication or distress initially limits reliability.
  • Select laboratory tests, ECG, toxicology, imaging, EEG and cerebrospinal-fluid studies to test explicit hypotheses rather than to produce generic medical clearance.
  • Consider endocrine, metabolic, infectious, autoimmune, epileptic, structural, degenerative, toxic and medicine-related causes according to age, course and phenotype.
  • Assess capacity and immediate risk in parallel; an organic cause does not remove suicide, violence, vulnerability or safeguarding needs.
  • Recent corticosteroids, dopaminergic agents, anticholinergics, sedatives, stimulants, alcohol cessation or recreational drugs may precipitate psychiatric and cognitive syndromes.
  • Apparently primary psychiatric illness can coexist with infection, metabolic disturbance or epilepsy; finding one explanation does not cancel a second.
  • 05
    Brain imaging and electroencephalography
    Why
    Investigate structural pathology, stroke, trauma, hydrocephalus, seizure or diffuse cerebral dysfunction.
    Interpretation and limitations
    CT supports urgent structural questions; MRI provides greater detail when indicated. EEG sensitivity is limited between events and requires clinical correlation.
  • 06
    Cerebrospinal fluid and specialist assays
    Why
    Assess infection, inflammation or autoimmune encephalitis in a compatible rapidly progressive syndrome.
    Interpretation and limitations
    Coordinate safe sampling and paired studies with specialists. Results can evolve, so ongoing deterioration may warrant repeat or expanded investigation.
    • Select physical examination, laboratory, ECG, imaging, EEG or cerebrospinal-fluid investigations that could change management.
  • 3Integrate results with course and function, obtain specialist advice and avoid treating a negative screening test as universal exclusion.
    • 03Diagnostic revisionRespond to discordant courseProgression, new signs or non-response conflicts with the working explanation.+
    1. 1Repeat history, collateral, observations and examination and audit medicine, substance and withdrawal timelines.
    2. 2Reconsider delirium, seizure, infection, autoimmune, endocrine, structural and mixed causes with appropriate cross-specialty review.
    3. 3EscalationUpdate the patient and team about revised uncertainty, immediate risks, further tests and the signs that require emergency escalation.
    Communicate a working differential with red flags and review triggers, especially when early tests are normal but the trajectory remains atypical.