This chapter first discusses the definition of Alzheimerās disease (AD) and distinguishes it from other dementias. It illustrates how problematic it is to diagnose, in part because it is often mixed with other types, but also because it varies among individuals. The chapter next examines the stages of AD and the symptoms at each stage. It then turns attention to competing theories of the causes of AD and to the range of frameworks for studying it. Although the disease model is predominant in the literature, it remains highly controversial. The chapter then analyzes testing possibilities for AD, other forms of cognitive impairment, and asymptomatic individuals at high risk of developing AD. Finally, it presents data on AD prevalence and deaths in the United States and globally.
Defining Alzheimerās Disease and Other Dementias
AD is a progressive and irreversible brain disorder which gradually damages a personās memory, judgment, reasoning skills, personality, autonomy, and, ultimately, bodily functions. Although the cause remains unknown, it results in a measured loss of neurons, damage to neurons so they no longer function properly, and a loss of synapses (neural connections) through which messages are passed from neuron to neuron. Also known as late-onset Alzheimerās disease, AD is primarily a disease of older adults, although the first noticeable symptoms can occur much earlier. Early-onset AD (EAD) sometimes can affect people as young as 30 but this is rare. When AD runs in families, it is called familial Alzheimerās disease (FAD). The underlying causes and specific risk factors for AD remain unclear despite considerable research investment.
In 2011, the National Institute on Aging (NIA) and the Alzheimerās Association (AA) proposed revised criteria and guidelines for diagnosing AD. These criteria and guidelines update those published in 1984 by the National Institute of Neurological Disorders and Stroke and the AA. The criteria of the earlier version were based chiefly on a doctorās clinical judgment about the cause of an individualās symptoms as described by the individual and family members, the results of cognitive tests, and a general neurological assessment. The new criteria incorporate three notable changes. First, they identify two stages of AD: mild cognitive impairment (MCI) due to AD and dementia due to AD. This contrasts with the earlier criteria where dementia was already apparent. Second, they propose for research purposes criteria for a pre-clinical phase before symptoms such as memory loss develop. And third, they incorporate biomarker tests that can indicate the presence or absence of AD, or the risk of developing it. Although finding a simple and inexpensive test, such as a blood test, would be ideal for patients and physicians, to date no test has shown the accuracy and reliability needed to diagnose AD based on that criterion alone.
MCI is a condition in which a person has mild but measurable changes in thinking abilities that are noticeable to oneself and close others, but do not affect the capacity for everyday activities. Approximately 15ā20 percent of people aged 65 or older have MCI. People with MCI, especially that involving memory problems, are more likely to develop AD than those without it. A recent review found that 32 percent of individuals with MCI developed AD within five years (Ward et al. 2013). Identifying which individuals with MCI are most likely to develop AD is a major research goal, according to Livingston et al. (2017).
Similarly, in 1988, the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimerās and Related Disorders Association defined three diagnostic categories: possible, probable, and definite AD. Definite AD can be confirmed only at autopsy after a brain biopsy confirms the presence of characteristic neurofibrillary tangles. Probable AD is diagnosed clinically when the patient meets core clinical criteria which can be supplemented by tests, including cerebrospinal fluid levels of tau protein or beta-amyloid precursor protein (Alonso Vilatela et al. 2012). In 2012, the NIA and the AA developed guidelines to help pathologists describe and categorize the brain changes associated with AD and other dementias on autopsy.
The Mini Mental State Examination (MMSE) is the most commonly used test to help clinicians diagnose AD and to assess its progression and severity. The MMSE tests numerous mental abilities, including a personās memory, attention, and language. Clinicians consider a personās score along with their history, symptoms, a physical exam, and the results of other tests including brain scans. The MMSE can also be used to assess changes in a person who has already been diagnosed with AD and help clarify how severe a personās symptoms are and how quickly they are progressing (see Box 1.1). Some commentators have been critical of the current diagnostic criteria. For instance, Sabat (2018) contends that while the conventional formulaic questionnaire (usually MMSE) is useful in medical diagnostics, it is vulnerable to the pitfalls of stereotyping and labeling. For Sabat it is important to understand the subjective experience of AD: how does the person react to those losses imposed by AD, how do caregivers react, and what do their reactions mean to the person diagnosed? In an extensive review of cognitive screening trials by the US Preventive Services Task Force, Lin et al. (2013) found no trial that examined the effect of screening on patient, caregiver, or clinician decision making or important individual or societal outcomes.
Box 1.1 Cognitive Quotients (QuoCo)
The MMSE continues to be used to screen for cognitive impairment, but it remains unclear how to interpret changes in its score over time to distinguish age associated-cognitive decline from early degenerative process. To rectify this, Canadian researchers devised a tool, the QuoCo, to enable doctors to track an individualās cognitive performance over time (Bernier et al. 2017). They found that QuoCo was 80 percent accurate in identifying those with dementia and 89 percent for those without it. Although QuoCo constitutes only part of diagnostic process and must be interpreted by a physician with a full clinical history and physical examination to eliminate other causes of cognitive changes, it offers a longitudinal evaluation to facilitate initiation of further investigation and treatment when appropriate.
The Diagnostic and Statistical Manual of Mental Disorders (DSM-5) stopped using the word ādementiaā and instead uses the phrase āmajor neurocognitive disorders,ā or illnesses with demonstrable neural substrate abnormalities together with cognitive symptoms that occur in people who have had normal brain development (Sachdev et al. 2014). In some cases, individuals with symptoms of dementia do not actually have it, but rather a condition that mimics those of dementia. Causes of dementia-like symptoms can include depression, medication side effects, thyroid problems, certain vitamin deficiencies, and alcohol abuse, many of which can be mitigated with treatment. Medication side effects are especially prominent among older people prescribed multiple drugs for chronic physical conditions. One study reported that 9 percent of people with dementia-like symptoms had other conditions that were potentially reversible (Clarfield 2003).
AD is the most common form of dementia, constituting an estimated 60ā80 percent. Symptoms vary among individuals, but the most common initial symptom is a worsening ability to remember new information. This happens because the first neurons to be damaged and destroyed with AD are those in brain regions involved in forming new memories. As neurons in other parts of the brain are destroyed, broader cognitive and functional abilities deteriorate. The pace at which symptoms advance from mild to moderate to severe varies from person to person. The characteristic pathologies of AD are the progressive accumulation of the protein fragment beta-amyloid (plaques) outside neurons in the brain, twisted strands of the protein tau (tangles) inside neurons, lesions of cholinergic neurons together with synaptic alterations in cerebral cortex, hippocampus, and other brain regions, eventually accompanied by the damage and death of neurons (Wu et al. 2011).
Vascular dementia (VD), previously known as multi-infarct or post-stroke dementia, as a sole cause of dementia accounts for about 10 percent, but around 50 percent of AD patients also display pathologic evidence of vascular dementia (see Bowler and Hachinski 2003). Unlike the memory loss associated with AD, the initial symptoms of VD are more likely to be impaired judgment or inability to make decisions, plan, or organize. People with VD also often have difficulty with motor function, particularly slow gait and poor balance. VD commonly follows blood vessel blockage or damage from strokes or bleeding in the brain. The location, number, and size of the brain injuries determine whether dementia will follow and how much the individualās thinking and physical functioning will be affected.
Parkinsonās disease (PD) is most associated with early problems with movement including slowness, rigidity, tremor, and changes in gait. In PD, alpha-synuclein aggregates appear deep in the brain in the substantia nigra. These aggregates are thought to cause degeneration of the nerve cells that produce dopamine. The incidence of PD is about one-tenth that of AD. As PD progresses, it often produces dementia secondary to the accumulation of Lewy bodies in the cortex or the accumulation of beta-amyloid clumps and tau tangles like that of found in AD.
Dementia with Lewy bodies (DLB) shares some symptoms with AD, but early sufferers are more likely to have sleep disturbances, visual hallucinations, slowness, gait imbalance, or other Parkinsonian movement features, as well as visuospatial impairment, that may occur before significant memory impairment. Lewy bodies are abnormal aggregations of the protein alpha-synuclein in neurons. When they develop in the cortex, dementia can result. Although people with DLB and PD both have Lewy bodies, the onset of PD is marked by motor impairment and LBD by cognitive impairment.
Frontotemporal lobar degeneration (FTLD) includes dementias such as behavioral-variant FTLD, primary progressive aphasia, Pickās disease, cortico-basal degeneration, and progressive supranuclear palsy that collectively account for under 10 percent of dementia cases. Typical early symptoms include marked changes in personality and behavior and/or difficulty producing or comprehending language. Unlike AD, memory is normally spared in the initial stages and most people develop symptoms at a younger age, generally 45ā60.
Mixed dementia is characterized by the hallmark abnormalities of more than one cause of dementia. Most commonly this constitutes AD combined with VD, followed by AD with DLB, and AD with both VD and DLB. Vascular dementia with DLB is much less common. Recent studies suggest that mixed dementia is more common than previously recognized, with about half of ...