Differential diagnosis - The 3Ds

At first presentation, the three Ds should be considered when taking into account differential diagnosis in a patient with cognitive impairment:


To meet the diagnostic criteria for dementia the patient must not only have significant memory problems, they must also have impairment in at least one of four cognitive domains:

  • Aphasia: problems with language; this is often heard during the history more than elicited directly as a sign, e.g. being able to identify a watch but being unable to describe its parts.
  • Apraxia: inability to carry out purposeful movements even though there is no sensory or motor impairment; this can present as difficulty with dressing in which there is a loss of detail.
  • Agnosia: failure of recognition, especially people; this often presents as difficulty recognising family members or well known friends – not so much forgetting a name, as recalling that they are known at all
  • Executive dysfunction: impaired planning, sequential organisation and attention; this may present with poor initiation, difficulty with problem-solving and a reluctance to change routines.

Cognitive deficits must be severe enough to interfere with occupational and / or social functioning, represent a decline from a previously higher level of function and not occur exclusively during the course of delirium.[1]

Cognitive tests undertaken during a comprehensive assessment such as the sMMSE and clock drawing tests are useful to assess and document the severity of cognitive impairment and to measure changes in cognitive function over time.

Where cognitive impairment is identified, consulting with family / carers with regard to whether a person's current cognitive state is a departure from the 'usual' status is recommended. In the untreated population with Alzheimer's Disease the sMMSE declines by 4 points a year when the sMMSE is between 24 and 12.

The application of repeated cognitive testing where sudden decline or significant deterioration are identified should be considered. A sudden (in weeks) decline in score on the sMMSE of 2 or more points indicates the need for further assessment of delirium.


Delirium is a disturbance of brain function that causes deterioration in mental functioning. The disturbance develops rapidly, generally hours to days, tending to fluctuate during the course of the day. The cause is usually multifactorial and reversible, and may involve infection, metabolic disturbance, hypoxia, and medication toxicity or withdrawal. Delirium is often under-diagnosed and is a medical emergency. On average it persists for 6 weeks post insult so is frequently seen in the post-hospitalisation group.

Detection is often based on a history of fluctuating alertness with cognitive impairment that has developed over hours to days, and is worse at night. Some patients are predominately hyperaroused with agitation and hallucinations, others are hypoactive with decreased consciousness, somnolence or stupor, and some alternate between agitated and hypoactive forms.[2]

The Confusion Assessment Method (CAM) may be used to screen for delirium. The CAM is a valid and reliable diagnostic tool.

Confusion Assessment Method (CAM) Diagnostic Algorithm PDF


Depression in older people may be unrecognised and untreated. Older people may not report symptoms or may think symptoms are part of ageing or due to physical causes.

Clinical features of depression in older people include:

  • psychological – fluctuating depressed mood, loss of interest in activities, loss of motivation
  • irritability
  • somatic – loss of energy, fatigue, headache, pain and palpitations
  • cognitive – forgetfulness, poor concentration, psychomotor slowing
  • behavioural – social withdrawal, reduction in activity, disinhibition

Use the Geriatric Depression Scale (GDS) to screen for dementia. Obtain a history from family / carer, undertake cognitive assessment, physical examination and investigations to differentiate depression from dementia.

Geriatric Depression Scale (GDS) PDF

The dementia pathway (Yates, M and Pond, D)

RACGP Medical Care of older persons in residential aged care facilities 4th Edition 2006, pg.24