The Nosology of Dementia Kenneth Rockwood

INTRODUCTION

Nosology is the classification of disease. How the syndrome of dementia has been classified has changed over the years. Recognizing that textbooks are commonly consulted because readers want pragmatic answers to questions, and that understanding nuances first requires understanding the basic facts, this chapter will begin with the current classification of dementia. A second section will provide some historical context. The third section will suggest some areas that remain to be understood or at least, settled on. The focus will be on understanding the basis for the definition of the syndrome of dementia, which historically has been rooted in the diagnosis of Alzheimer’s disease. Separate chapters for many of the other dementing disorders will deal with these in separate detail; here we focus on diagnostic criteria for Alzheimer’s disease, and how these have influenced the way in which dementia is understood.

CURRENT NOSOLOGY OF DEMENTIA

At present, three references – DSM-IV-TR¹, ICD-10² and the NINCDS-ADRDA³ criteria – provide authoritative approaches to the nosology of dementia. Each is rooted in the formulation codified in an influential textbook by Lishman⁴, which proposed that dementia was a global cognitive impairment (memory plus other cognitive features) that was sufficiently severe to interfere with social or occupational function. The newly developed 10/66 criteria, discussed below, have drawn attention to difficulties in the operationalization of the idea that the cognitive impairment must be severe enough to interfere with function, and it may be that their approach will become increasingly influential⁵. Similarly, although largely technically infeasible in the low-and middle-income countries being studied by the 10/66 group, another influential proposal suggests that clinical features beyond demonstrable memory impairment count for little, compared with biomarkers⁶. How these newer trends will play out is not clear.

As a syndrome, dementia must be differentiated from other syndromes in which cognition is (or can be) impaired, including delirium, depression and age-associated cognitive decline. This last is variably conceptualized. Each of the syndromes in which cognition is impaired, but which the cognitive impairment does not meet criteria for dementia, forms part of what has been referred to as ‘cognitive impairment, no dementia’ (CIND). The prevalence of the heterogeneous group of disorders that is CIND is estimated to be about twice that of the prevalence of dementia⁷.

DSM-IV-TR

The Text Revision of the Fourth Edition of the Diagnostic and Statistical Manual of the American Psychiatric Association (DSM-IV-TR) was published in 2000¹. As with DSM-IV, it gives a general (syndromic) definition of dementia, but the actual dementia diagnostic codes are presented by subtype, based on presumed aetiology. These include Alzheimer’s disease, vascular dementia, and separate categories for dementia due to HIV disease, head trauma, Parkinson’s disease, Huntington’s disease, Pick’s disease, Creutzfeldt-Jakob disease, other general medical conditions, substance-induced persisting dementia, dementia of multiple aetiologies, delirium and dementia not otherwise specified.

Within the classification of other general medical conditions are listed a variety of other problems. Some, such as structural brain lesions, including subdural haematomas, and brain tumours, are not infrequently encountered, as are dementia due to multiple sclerosis or to systemic lupus erythematosis. Others are less common, such as dementia due to endocrine disorders (e.g. hypothyroidism or hyperparathyroidism) and CNS infection (neurosyphillis, cryptococcus). Still others are often obscure disorders, such as Kuf’s disease and adrenoleukodystrophy.

Note, too, that dementia due to Lewy bodies can be classified in DSM-IV-TR as due to ‘other general medical conditions’ (or as Parkinson’s disease dementia; DSM-IV-TR is in this way up to date in noting controversy over whether Lewy body dementia and Parkinson’s disease dementia are aspects of the same disorder, or usefully separable)^8-10. Frontotemporal dementia, a broader classification than the not unproblematic Pick’s disease, can also be recorded under dementia to other general medical conditions.

Although dementia is diagnosed by aetiology, the syndromic aspects said to be ‘essentialbe’ are repeated for each type. In the criteria for ‘Dementia of the Alzheimer Type’ Type’ (Table 37.1) these are items A, B, C and E. Item D is adapted to be more disease-specific for each separate dementia classification. Note that DSM-IV-TR maintains the development of DSM-IV in that memory impairment is not further delineated, whereas in DSM-IIIR, impairment had to be demonstrated in both so-called shortand long-term memory.

ICD-10

Chapter V of the 10th revision of the International Classification of Diseases (ICD-10) is the World Health Organization’s codification of

Table 37.1 DSM-IV-TR diagnostic criteria for dementia of the Alzheimer’s type

A. The development of multiple cognitive deficits manifested by both (1) memory impairment (impaired ability to learn new information or to recall previously learned information) (2) one (or more) of the following cognitive disturbances:

(a) aphasia (language disturbance)

(b) apraxia (impaired ability to carry out motor activities despite intact motor function)

(d) disturbance in executive functioning (i.e., planning, organizing, sequencing, abstracting)

B. The cognitive deficits in Criteria A1 and A2 each cause significant impairment in social or occupational functioning and represent a significant decline from a previous level of functioning.

C. The course is characterized by gradual onset and continuing cognitive decline.

D. The cognitive deficits in Criteria A1 and A2 are not due to any of the following:

(1) other central nervous system conditions that cause progressive deficits in memory and cognition (e.g., cerebrovascular disease, Parkinson’s disease, Huntington’s disease, subdural hematoma, normal-pressure hydrocephalus, brain tumor)

(2) systemic conditions that are known to cause dementia (e.g. hypothyroidism, vitamin B or folic acid deficiency, niacin deficiency, hypercalcemia, neurosyphilis, HIV infection)

(3) substance-induced conditions

E. The deficits do not occur exclusively during the course of a delirium.

F. The disturbance is not better accounted for by another Axis I disorder (e.g. Major Depressive Episode, Schizophrenia). Additional coding, based on presence or absence of a clinically significant behavioral disturbance:

294.10 Without Behavioral Disturbance: if the cognitive disturbance is not accompanied by any clinically significant behavioral disturbance.

294.11 With Behavioral Disturbance: if the cognitive disturbance is accompanied by a clinically significant behavioral disturbance. (e.g. wandering, agitation)

Specification of subtype:

With Early Onset: if onset is at age 65 years or below With Late Onset: if onset is after age 65 years

Source: DSM-IV-TR: Text Revision of the fourth edition of the Diagnostic and Statistical Manual of the American Psychiatric Association.

diagnoses in relation to ‘mental and behavioural disorders’. Dementia is included as a so-called ‘organic’ mental disorder (Table 37.2). The ICD-10 manual states that ‘Dementia (F00-F03) is a syndrome due to disease of the brain, usually of a chronic or progressive nature, in which there is disturbance of multiple higher cortical functions, including memory, thinking, orientation, comprehension, calculation, learning capacity, language, and judgment. Consciousness is not clouded. The impairments of cognitive function are commonly accompanied, and occasionally preceded, by deterioration in emotional control, social behaviour, or motivation’ (Table 37.2). Its emphasis on impaired behaviour is noteworthy, although in practice this might not be operationalized as distinct from the impaired ‘social functioning’ seen with DSM. Like DSM-IV-TR, the ICD-10 classifies dementia into an early onset, before age 65 years, and a late onset, after age 65 years. ICD-10 suggests that the early-onset form should have a more rapid onset and progression than the late-onset form. In general, however, in both DSM-IV and ICD-10, the validity of subgroups of dementia has often been found lacking, at least as employed in routine clinical use¹¹.

The differences in how dementia is operationalized are not inconsequential. Even the minor differences between standard criteria can importantly influence prevalence rates¹²,¹³, especially at very late ages¹⁴. In general, ICD-10 has been reported to be less sensitive than earlier versions of the DSM criteria, although that is not always the case^15,16.

NINCDS-ADRDA

Even though the diagnostic classification proposed in this 1984 report focuses on Alzheimer’s disease, the consensus diagnostic criteria proposed by the National Institute of Neurological and Communicative

Table 37.2 ICD-10 criteria for dementia. Definition of dementia in the ICD-10 (adapted)

G1 There is evidence of each of the following:

(1) A decline in memory (at least) sufficient to interfere with everyday activities, though not so severe as to be incompatible with independent living

(2) A decline in other cognitive abilities characterized by deterioration in judgement and thinking, such as planning and organizing, and in the general processing of information (at least) sufficient to cause impaired performance in daily living, but not to a degree that makes the individual dependent on others

G2 Awareness of the environment (i.e. absence of clouding of consciousness)

G3 There is a decline in emotional control or motivation, or a change in social behaviour manifest as at least one of emotional liability, irritability, apathy or coarsening of social behaviour

G4 The symptoms in criterion GI should have been present for at least 6 months

Disorders and Stroke, and the Alzheimer’s Disease and Related Disorders Association (NINCDS-ADRDA) has served for many years as the standard for a diagnosis of Alzheimer’s disease, and its definition of dementia was also normative³. It still heavily influences how research on dementia is conducted, especially as it makes up the usual inclusion criteria for drug trials on Alzheimer’s disease (Table 37.3). The original criteria have been adapted somewhat – dementia can be diagnosed with the aid of cognitive screening instruments other than

Table 37.3 NINCDS/ADRDA criteria for Alzheimer’s disease

Criteria for clinical diagnosis of Probable AD include:

Dementia established by clinical exam and documented by MMSE or Blessed Dementia scale, confirmed by further neuropsychological tests.

Deficits in two or more areas of cognition. Progressive worsening of memory and other cognitive functions. No disturbance of consciousness. Onset between the ages of 40 and 90.

Absence of systemic diseases or other brain diseases that could explain the cognitive changes. The diagnosis of Probable AD is supported by:

Progressive deterioration of specific cognitive functions such as language, motor skills, and perception (aphasia, apraxia, agnosia, respectively). Impaired activities of daily living.

Positive family history, particularly if documented neuropathologically.

Lab results: Normal lumbar puncture, EEG, and evidence of cerebral atrophy on CT or MRI.

Clinical features consistent with diagnosis of Probable AD, after exclusion of other causes of dementia:

Plateaus in clinical course.

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