Loss of Smell May Be an Early Sign of Cognitive Impairment and Alzheimer's Dementia

Many scientific studies seek to identify causes of dementia and related cognitive dysfunction, which cause serious disability worldwide, recognize early indicators, prevent or delay onset and provide treatment. JAMA Neurology reported that a declining ability to smell and distinguish odors may herald dementia, in other words, serve as an early marker . Before describing the study, explaining cognitive functions, mild cognitive impairment and dementia will help clarify the subject.

What Are Cognitive Functions?

Cognitive functions are a group of mental activities our brain performs consciously. For example;

• Communicating with people bidirectionally, verbally or nonverbally
• Understanding others and expressing oneself appropriately through language (speaking, reading or writing)
• Knowing the time, location and identity of people around oneself
• Focusing and maintaining attention
• Recording and recalling immediate, recent and distant learning and memories
• Noticing, recognizing and identifying letters, numbers, shapes, objects, faces, etc.
• Planning, organizing, solving problems, deciding, distinguishing right from wrong and judging
• Planning and performing simple daily activities such as self-care, meal preparation, dressing and shopping, and much more complex activities

Deterioration in one or more of these functions for various reasons is referred to as cognitive impairment in terminology.

What Is Mild Cognitive Impairment (HBB)?

Mild deterioration in cognitive functions is normal during healthy aging. In mild cognitive impairment, functions detailed above, such as attention, memory, decisions, problem-solving, daily activities and complex tasks, are worse than expected in healthy older adults, but milder than dementia and do not impair daily life or functioning. It can therefore be considered a transition between normal aging and dementia, but not every case eventually becomes dementia. Generally detected after age 65 at a rate of %5-20, it has 4 subtypes: amnestic with prominent forgetfulness, non-amnestic without prominent forgetfulness, single-domain and multi-domain types. Multi-domain amnestic impairment is known to progress to dementia most frequently.

What Is Dementia?

Although commonly understood as an illness involving forgetfulness, dementia is an umbrella term describing a symptom group. Attention and memory problems may be central, accompanied by personality and behavioral changes, impaired judgment and declining daily activity skills. Alzheimer's dementia is the most common type. Vascular dementia ranks second, caused by inadequate function or complete damage in brain regions affected by vascular problems. Other types include Lewy Body Dementia, Parkinson's Dementia and Frontal Lobe Dementia.

Declining Smell over Time, Olfactory Deficits and Dementia

Autopsy studies found plaque- and tangle-like deposits in olfactory nerve fibers (olfactory bulb) and pathways to the brain's smell center in people with smell loss. Consistently, studies associated loss of smell or odor discrimination with poor cognitive performance, mild cognitive impairment or Alzheimer's dementia. This raised the question of whether smell or identification loss could mark risk for mild cognitive impairment or Alzheimer's, or indicate progression from impairment to Alzheimer's dementia. This study investigated associations between olfactory deficits, cognitive impairment subtypes and progression to dementia.

Method

Participants meeting The Mayo Clinic Study of Aging criteria between 2004-2010, aged 70-89, were included. Each underwent psychiatric and neurological examination and neuropsychiatric tests, then was classified as amnestic mild cognitive impairment (HBBa), non-amnestic impairment (HBB na), single-domain, multi-domain, Alzheimer's dementia or normal cognition. Smell was assessed using the Brief Smell Identification Test (B-SIT). With 6 food and 6 nonfood odors, higher scores indicated healthy smell function. Participants were followed at 15-month intervals, repeating all baseline tests.

Results

Of 1630 initially cognitively normal patients, 1430 were followed, and 250 developed mild cognitive impairment. These 250 also showed declining smell scores. Similarly, of 317 initially impaired people, 221 were followed (HBBa: 185, HBBna: 36); during an average 3-year follow-up, 64 new Alzheimer's dementia cases emerged, also with declining odor discrimination scores. The most marked deterioration involved memory, executive functions and language-related cognition.

The results associated olfactory deficits with onset of mild cognitive impairment, particularly amnestic HHB, and worsening cognitive function. They also linked smell deficits to progression from mild impairment to Alzheimer's dementia. Nerve cell damage was considered the underlying mechanism. Damage arising for various reasons in certain brain regions can cause smell deficits as well as memory problems.

In conclusion, easy, inexpensive, noninvasive smell tests may be important markers for early detection of cognitive impairment and Alzheimer's dementia and predicting progression in at-risk individuals.