[1] The 2017 Fourth Consensus Report of the DLB Consortium determined these to be core features based on the availability of high-quality evidence indicating they are highly specific to the condition.

[25] Individuals with DLB may be easily distracted, have a hard time focusing on tasks,[35] or appear to be "delirium-like", "zoning out", or in states of altered consciousness[25][36] with spells of confusion, agitation or incoherent speech.

[9] Deficits can manifest in impaired job performance, inability to follow conversations, difficulties with multitasking, or mistakes in driving, such as misjudging distances or becoming lost.

[7] These disorders include daytime sleepiness, drowsiness or napping more than two hours a day, insomnia, periodic limb movements, restless legs syndrome and sleep apnea.

[42] Abnormal sleep behaviors may begin before cognitive decline is observed,[25] and may appear decades before any other symptoms, often as the first clinical indication of DLB and an early sign of a synucleinopathy.

[48] RBD behaviors may include yelling, screaming, laughing, crying, unintelligible talking, nonviolent flailing, or more violent punching, kicking, choking, or scratching.

[5] Examples of visual hallucinations "vary from 'little people' who casually walk around the house, 'ghosts' of dead parents who sit quietly at the bedside, to 'bicycles' that hang off of trees in the back yard".

The supportive features are:[1] Partly because of loss of cells that release the neurotransmitter dopamine, people with DLB may have neuroleptic malignant syndrome, impairments in cognition or alertness, or irreversible exacerbation of parkinsonism including severe rigidity,[51] and dysautonomia from the use of antipsychotics.

[74] "Degeneration of the cardiac sympathetic nerves is a neuropathological feature" of the Lewy body dementias, according to Yamada et al.[75] Almost all people with synucleinopathies have cardiovascular dysfunction, although most are asymptomatic.

[5] Capgras delusion may occur, in which the person with DLB loses knowledge of the spouse, caregiver, or partner's face,[85] and is convinced that an imposter has replaced them.

[49] Dementia with Lewy bodies can only be definitively diagnosed after death with an autopsy of the brain (or in rare familial cases, via a genetic test),[2] so diagnosis of the living is referred to as probable or possible.

[1] Known as the one-year rule, the distinction is acknowledged to be arbitrary; it recognizes overlap between the conditions along with key differences, while allowing for variations in treatment and prognosis and providing a framework for research.

[25] Prompt evaluation and treatment of RBD is indicated when a prior history of violence or injury is present as it may increase the likelihood of future violent dream enactment behaviors.

[25][53] The REM Sleep Behavior Disorder Single-Question Screen offers diagnostic sensitivity and specificity in the absence of polysomnography with one question:[52] "Have you ever been told, or suspected yourself, that you seem to 'act out your dreams' while asleep (for example, punching, flailing your arms in the air, making running movements, etc.)?

[85] AD pathology frequently co-occurs in DLB and is associated with more rapid decline; cerebrospinal fluid (CSF) testing may reveal an "Alzheimer's pattern" of higher tau and lower amyloid beta.

[42] In multiple system atrophy, autonomic dysfunction appears earlier and is more severe,[39] and is accompanied by uncoordinated movements, while visual hallucinations and fluctuating cognition are less common than in DLB.

"Pharmacological management of DLB is complex because of adverse effects of medications[49] and the wide range of symptoms to be treated (cognitive, motor, neuropsychiatric, autonomic, and sleep).

[86] Medications (including tricyclic antidepressants and treatments for urinary incontinence) with anticholinergic properties that cross the blood–brain barrier can cause memory loss.

[170] To improve daytime alertness, there is mixed evidence for the use of stimulants such as methylphenidate and dextroamphetamine; although worsening of neuropsychiatric symptoms is not common, they can increase the risk of psychosis.

[56] If such medications are needed for motor symptoms, cautious introduction with slow increases to the lowest possible dose may help avoid psychosis.

[6] Although clozapine has been shown effective in Parkinson's disease, there is very low evidence for its use to treat visual hallucinations in DLB, and its use requires regular blood monitoring.

[163] Contributing factors to the caregiver burden in DLB are emotional fluctuations,[163] apathy,[186] psychosis, aggression, agitation, and night-time behaviors such as parasomnias,[139] that lead to a loss of independence earlier than in AD.

[56] Failure to thrive[8] and aspiration pneumonia, a complication of dysphagia (difficulty swallowing) that results from dysautonomia, commonly cause death among people with the Lewy body dementias.

[222][223] In 1912, studying Parkinson's disease (paralysis agitans),[224] he described findings of these inclusion bodies in the vagus nerve, the nucleus basalis of Meynert and other brain regions.

[235] DLB was included in the fourth text revision of the DSM (DSM-IV-TR, published in 2000) under "Dementia due to other general medical conditions".

[236] The British author and poet Mervyn Peake died in 1968 and was diagnosed posthumously as a probable case of DLB in a 2003 study published in JAMA Neurology.

"[241] Ian G. McKeith, professor and researcher of Lewy body dementias, commented that Williams' symptoms and autopsy findings were explained by DLB.

[242] The identification of prodromal biomarkers for DLB will enable treatments to begin sooner,[243] improve the ability to select subjects and measure efficacy in clinical trials,[244] and help families and clinicians plan for early interventions and awareness of potential adverse effects from the use of antipsychotics.

[247] Nonetheless, severe late-onset psychiatric disorders can be an indication to consider Lewy body dementia,[248] and unexplained delirium raises the possibility of prodromal DLB.

[243] Four clinical trials for treating parkinsonian symptoms in DLB have been completed as of 2021, but more studies are needed to assess risk vs. benefits, adverse effects, and longer-term therapeutic protocols.