Geriatrics

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11 Geriatrics

There has been a steady increase in the number of elderly people, defined as those over 65 years of age, since the beginning of the twentieth century. They formed only 4.8% of the population in 1901, increasing to 15.2% in 1981 and about 18% in 2001. In 2008, there were 4.8 million people over the age of 75 years in the UK. Their number is projected to increase to 5.8 million by 2018 and to 8.7 million by 2033 – a rise of 81% over 25 years. In addition, the number of people over 85 years of age is also projected to increase from 1.3 million in 2008 to 3.3 million in 2033. The number of centenarians is projected to increase from 11,000 in 2008 to 80,000 in 2033 (Office for National Statistics, 2010). The significant increase in the number of very elderly people will have important social, financial and health care planning implications.

The elderly have multiple and often chronic diseases. It is not surprising, therefore, that they are the major consumers of drugs. Elderly people receive about one-third of National Health Service (NHS) prescriptions in the UK. In most developed countries, the elderly now account for 25–40% of drug expenditure.

A survey of drug usage in 778 elderly people in the UK showed that 70% had been on prescribed medication and 40% had taken one or more prescribed drugs within the previous 24 h; 32% were taking cardiovascular drugs, and the other therapeutic categories used in decreasing order of frequency were for disorders of the central nervous system (24%), musculoskeletal system (10%), gastro-intestinal system (8%) and respiratory system (7%). The most commonly used drugs were diuretics; analgesics; hypnotics, sedatives and anxiolytics; antirheumatic drugs; and β-blockers.

Institutionalised patients tend to be on larger numbers of drugs compared with patients in the community. One study has shown that patients in long-term care facilities are likely to be receiving, on average, eight drugs. Psychotropic drugs are used widely in nursing or residential homes.

For optimal drug therapy in the elderly, a knowledge of age-related physiological and pathological changes that might affect handling of and response to drugs is essential. This chapter discusses the age-related pharmacokinetic and pharmacodynamic changes which might affect drug therapy and the general principles of drug use in the elderly.

Pharmacokinetics

Ageing results in many physiological changes that could theoretically affect absorption, first-pass metabolism, protein binding, distribution and elimination of drugs. Age-related changes in the gastro-intestinal tract, liver and kidneys are

Hepatic clearance

Hepatic clearance (ClH) of a drug is dependent on hepatic blood flow (Q) and the steady state extraction ratio (E), as can be seen in the following formula:

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where Ca and Cv are arterial and venous concentrations of the drug, respectively. It is obvious from the above formula that when E approaches unity, ClH will be proportional to and limited by Q. Drugs which are cleared by this mechanism have a rapid rate of metabolism, and the rate of extraction by the liver is very high. The rate-limiting step, as mentioned earlier, is hepatic blood flow, and therefore drugs cleared by this mechanism are called ‘flow limited’. On the other hand, when E is small, ClH will vary according to the hepatic uptake and enzyme activity, and will be relatively independent of hepatic blood flow. The drugs which are cleared by this mechanism are termed ‘capacity limited’.

Hepatic extraction is dependent upon liver size, liver blood flow, uptake into hepatocytes, and the affinity and activity of hepatic enzymes. Liver size falls with ageing and there is a decrease in hepatic mass of 20% and 40% between the third and tenth decade. Hepatic blood flow falls equally with declining liver size. Although it is recognised that the microsomal mono-oxygenase enzyme systems are significantly reduced in ageing male rodents, evidence suggests that this is not the case in ageing humans. Conjugation reactions have been reported to be unaffected in the elderly by some investigators, but a small decline with increasing age has been described by others.

Impaired clearance of many hepatically eliminated drugs has been demonstrated in the elderly. Morphological changes rather than impaired enzymatic activity appear to be the main cause of impaired elimination of these drugs. In frail debilitated elderly patients, however, the activities of drug-metabolising enzymes such as plasma esterases and hepatic glucuronyltransferases may well be impaired.

Pharmacodynamics

Molecular and cellular changes that occur with ageing may alter the response to drugs in the elderly. There is, however, limited information about these alterations because of the technical difficulties and ethical problems involved in measuring them. It is not surprising, therefore, that there is relatively little information about the effect of age on pharmacodynamics.

Changes in pharmacodynamics in the elderly may be considered under two headings:

Reduced homeostatic reserve

Age-related changes in specific receptors and target sites

Many drugs exert their effect via specific receptors. Response to such drugs may be altered by the number (density) of receptors, the affinity of the receptor, postreceptor events within cells resulting in impaired enzyme activation and signal amplification, or altered response of the target tissue itself. Ageing is associated with some of these changes.

Common clinical disorders

This section deals in detail only with the most important diseases affecting older people. Other conditions are mentioned primarily to highlight areas where the elderly differ from the young or where modifications of drug therapy are necessary.

Dementia

Dementia is characterised by a gradual deterioration of intellectual capacity. Alzheimer’s disease (AD), vascular dementia (VaD), dementia with Lewy bodies and frontotemporal dementia are the most important diseases of cognitive dysfunction in the elderly. AD has a gradual onset, and it progresses slowly. Forgetfulness is the major initial symptom. The patient has difficulty in dressing and other activities of daily living. He or she tends to get lost in his or her own environment. Eventually, the social graces are lost. VaD is the second most important cause of dementia. It usually occurs in patients in their 60s and 70s, and is more common in those with a previous history of hypertension or stroke. It is commonly associated with mood changes and emotional lability. Physical examination may reveal focal neurological deficits. A number of drugs and other conditions cause confusion in the elderly, and their effects may be mistaken for dementia. These are listed in Box 11.1.

In patients with AD, damage to the cholinergic neurones connecting subcortical nuclei to the cerebral cortex has been consistently observed. Postsynaptic muscarinic cholinergic receptors are usually not affected, but ascending noradrenergic and serotonergic pathways are damaged, especially in younger patients. Based on those abnormalities, several drugs have been investigated for the treatment of AD. Lecithin, which increases acetylcholine concentrations in the brain, 4-aminopyridine, piracetam, oxitacetam and pramiracetam, all of which stimulate acetylcholine release, have been tried, but have produced no, or unimpressive, improvements in cognitive function. Anticholinesterases block the breakdown of acetylcholine and enhance cholinergic transmission. Donepezil, galantamine and rivastigmine are recommended for treatment of patients with AD of moderate severity only (those with a Mini Mental State Examination (MMSE) score of between 10 and 20 points; NICE, 2009). Donepezil is a piperidine-based acetylcholinesterase inhibitor. It has been shown to improve cognitive function in patients with mild to moderately severe AD. However, it does not improve day-to-day functioning, quality-of-life measures or rating scores of overall dementia. Rivastigmine is a non-competitive cholinesterase inhibitor. It has been shown to slow the rate of decline in cognitive and global functioning in AD. Galantamine, a reversible and competitive inhibitor of acetylcholinesterase, has also been shown to improve cognitive function significantly and is well tolerated. Adverse effects of cholinesterase inhibitors include nausea, vomiting, diarrhoea, weight loss, agitation, confusion, insomnia, abnormal dreams, muscle cramps, bradycardia, syncope and fatigue. Treatment with these drugs should only be continued in people with dementia who show an improvement or no deterioration in their minimental score, together with evidence of global (functional and behavioural) improvement after first few months of treatment. The treatment effect should then be reviewed critically every 6 months, before a decision to continue drug therapy is made.

Memantine, an N-methyl-d-aspartate (NMDA) antagonist, has also been used for the treatment of moderate to severe AD. It acts mainly on subtypes of glutamate receptors related to memory (i.e. NMDA), resulting in improvements in cognition. It has also been shown to have some beneficial effects on behaviour and its use is recommended in patients with moderate to severe AD as part of well-designed clinical studies (NICE, 2009).

Deposition of amyloid (in particular the peptide β/A4) derived from the Alzheimer amyloid precursor protein (APP) is an important pathological feature of the familial form of AD that accounts for about 20% of patients. Point mutation of the gene coding for APP (located in the long arm of chromosome 21) is thought to be associated with familial AD. Future treatment strategies, therefore, might involve development of drugs which inhibit amyloidogenesis.

In some studies, donepezil and galantamine have been shown to improve cognition, behaviour and activities of daily living in patients with VaD, and in those with AD and coexistent cerebrovascular disease. Memantine has been reported to stabilise progression of VaD compared with placebo. However, acetylcholinesterase inhibitors and memantine should not be prescribed for the treatment of cognitive decline in patients with VaD, except as part of properly constructed clinical studies (National Collaborating Centre for Mental Health, 2007). Aspirin therapy has also been reported to slow the progression of VaD. The incidence of VaD is likely to decrease with other stroke prevention strategies such as smoking cessation, anticoagulation for atrial fibrillation, control of hypertension and hyperlipidemia.

Parkinsonism

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