Synopsis
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.
- 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.
- Communicate a working differential with red flags and review triggers, especially when early tests are normal but the trajectory remains atypical.
Key red flags
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.
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.
Investigation priorities
Establish speed, fluctuation, previous episodes, illness, medicines, substances, trauma and functional change.
Management branches
A patient presents with new behavioural, perceptual, cognitive or mood change.
- Establish last known baseline, onset, fluctuation, exposures and physical symptoms using collateral evidence where needed.
- Assess observations, glucose, attention, arousal and immediate neurological or systemic warning signs before assigning a psychiatric label.
- Treat urgent abnormalities and decide whether the patient requires acute medical, neurological, toxicological or psychiatric containment.