Cognitive decline: initial assessment and dementia diagnosis algorithm

Initial approach and clinical framing

When assessing a patient with memory complaints, difficulties with attention or changes in reasoning, we are dealing with an extremely common yet clinically demanding problem. The presentation may be subtle, often described only as “becoming more forgetful”, but it may reflect very different realities: from benign and stable cognitive changes to early mild cognitive impairment or dementia with functional impact.

The key is to avoid two equally dangerous responses: on the one hand, automatically attributing symptoms to age and reassuring without assessment; on the other, assuming dementia too early and overlooking acute or potentially reversible causes. The approach should be systematic, because the correct diagnosis depends more on the clinical process than on a single test.

Temporal pattern and exclusion of acute causes

The first axis of assessment is the temporal pattern. It is essential to define when symptoms began, the speed of progression and whether there is fluctuation throughout the day. Decline developing over weeks or a few months, particularly if described as alternating between “good days and bad days”, should immediately raise the possibility of delirium or a secondary cause.

Delirium is a critical entity because it may mimic dementia and often reflects an underlying clinical emergency. The central feature is impaired attention, expressed as difficulty maintaining focus, marked distractibility and inability to follow a conversation. It is often associated with altered level of consciousness, reversal of the sleep–wake cycle and fluctuation in mental state. In these cases, the priority is to identify common precipitants, such as infection, urinary retention, constipation, pain, dehydration, metabolic disturbances and medication.

Depression and iatrogenesis

In parallel, depression and iatrogenesis must be considered. Depression may present with significant cognitive complaints, often accompanied by low mood, anhedonia, fatigue and psychomotor slowing. In some cases, the patient emphasises the failures, while the informant describes them as less prominent; in other situations, depression and neurodegenerative disease coexist, making assessment more complex.

Medication review is also central, particularly in older adults. Anticholinergics, benzodiazepines, hypnotics, opioids and some antipsychotics, as well as polypharmacy, may reduce attention, processing speed and working memory. Recent changes in treatment may explain apparent deterioration.

Functional assessment and distinction between MCI and dementia

Once acute situations and reversible causes have been excluded, the next step is to objectify cognitive impairment and, above all, determine whether there is functional impact. This is the key element for distinguishing mild cognitive impairment from dementia.

In practice, early dementia often manifests as loss of autonomy in instrumental activities of daily living: medication management, finances, telephone use, transport, shopping, safe meal preparation and organisation of complex tasks. Caregiver information is essential, because the patient may minimise difficulties. The central clinical question is not only “does the patient forget?”, but whether that forgetfulness compromises independence.

Brief cognitive tests and interpretation

Brief cognitive tests should be used as screening and documentation tools. The aim is not to diagnose based on an isolated score, but to obtain objective evidence of impairment and allow comparisons over time.

The choice of test depends on the context. When functional impact is evident, global tests such as the MMSE or Mini-Cog may be sufficient to document significant impairment. When there is no clear functional loss and mild cognitive impairment is suspected, the MoCA has greater sensitivity, particularly for executive and attentional deficits.

Interpretation should always take age, education and clinical context into account. An abnormal result increases the likelihood of cognitive impairment, but a normal result does not exclude early stages, especially in people with high cognitive reserve.

Persistent clinical suspicion

A frequently overlooked element is persistent clinical suspicion despite an apparently normal test. The overall consistency of the history and information from the informant are decisive. A common error is to close the investigation after an “acceptable” test result when the history suggests progressive decline.

The reverse may also occur: low scores may result from limited education, sensory deficits, language barriers or anxiety, requiring critical and individualised interpretation.

Neurological examination and red flags

The third axis is the neurological examination and the search for red flags. Focal deficits may indicate vascular pathology or structural lesions. Early parkinsonism with cognitive fluctuation and hallucinations suggests the Lewy body spectrum. Early gait disturbance may indicate subcortical vascular disease or normal pressure hydrocephalus. Seizures and rapid progression should raise suspicion of potentially treatable secondary aetiologies.

Complementary investigation

Complementary investigation aims to exclude reversible causes and support aetiological definition. Minimum laboratory assessment includes full blood count, electrolytes, renal and liver function, glucose, TSH and vitamin B12. Additional tests may be required in specific contexts.

Structural neuroimaging is recommended in the initial assessment of dementia to exclude treatable lesions, characterise vascular disease and identify patterns suggestive of aetiology.

Syndromic decision and follow-up

The central clinical decision is syndromic: mild cognitive impairment, probable dementia or insufficient evidence of significant impairment. The diagnosis of dementia requires documented acquired cognitive impairment, functional impact and exclusion of delirium or other psychiatric conditions.

Regardless of aetiology, it is essential to intervene in modifiable factors, including cardiovascular risk control, physical activity, sleep optimisation, alcohol reduction and medication review.

Summary of the approach

In summary, clinical interpretation follows a logical and flexible sequence: clarify temporality, exclude reversible causes, document cognitive impairment, assess functional impact, identify red flags and complete the investigation with laboratory tests and neuroimaging. This structured approach makes it possible to transform a common and often ambiguous complaint into a robust clinical process, promoting consistent and safe decisions.