1.4 AGEING CHANGES
We can make several observations about ageing changes:
- they are progressive
- they are irreversible
- multiple organ systems are involved
- physiological mechanisms ultimately become impaired
- variable expression is shown between individuals.
Ageing changes that may occur in tissues include:
- fatty infiltration
- delayed ability to repair
- reduced number of active cells
- reduced rate of activity
- reduced organ function.
The pigment lipofuscin is deposited in body tissues in increasing amounts with increasing
age, indeed it has been called 'the pigment of ageing'. In the dog lipofuscin is
deposited at five times the rate that it is in humans.
Ageing changes proceed at varying rates in different organ systems of the body, and
may be present in varying degrees of severity between individuals of the same age.
Typically, older animals lose sensitivity of their major senses, e.g. vision, hearing,
taste and smell, and all organ systems may be affected to some degree by age-related
Geriatric screening is helpful in determining whether or not organ function is impaired,
particularly before elective procedures such as minor surgery. Some organ systems
are more likely to be affected than others, for example with increasing age teeth
are likely to demonstrate:
- dental calculus accumulation
- gingival hyperplasia
- gum atrophy and retraction
- enamel wear
- ulcerative lesions
- tooth loss.
By the time they are 7-8 years of age 95% of dogs are said to be affected by periodontal
disease (Harvey 1988).
The occurrence of obesity increases with age probably due to:
- reduced lean body mass (hence reduced basal energy requirement (BER))
- reduced exercise
Obesity can have serious effects on other body systems, e.g. cardiovascular and skeletal
systems, and may have a role in the cause of some diseases, e.g. diabetes mellitus.
Some age-related changes that may be seen in various organ systems are listed below
(after Mosier and others):
- reduced salivary secretion
- impaired oesophageal function (neuronal)
- reduced HCI secretion
- slower rate of renewal of epithelium
- reduced villous size
- impaired nutrient absorption.
- decreased number of hepatocytes
- increased binucleated hepatocytes
- fatty infiltration
- increased fibrous tissue
- decreased hepatic function
- decreased bile formation.
- reduced renal size
- reduced number of nephrons
- reduced glomerular filtration rate (GFR)
- reduced renal plasma flow
- reduced tubular excretion
- reduced tubular reabsorption.
- iris atrophy
- nuclear sclerosis
- loss of rods and cones
- cystoid retinal degeneration
- asteroid hyalosis
- eyelid papillomas
- cysts of the gland of Moll
- increased tear viscosity
- decreased lysozyme activity, increased susceptibility to infection
- corneal pigment changes due chronic irritation.
- hair becomes sparse, dull and lustreless
- patchy alopecia
- white hairs (loss of pigment)
- calluses over pressure points
- skin thickens, reduced pliability
- hyperkeratosis; later, epidermis atrophies
- calcium/pseudoelastin replace elastic fibres
- hyperplasia of apocrine and sebaceous glands.
- fatty infiltration, cyst formation and fibrosis
- reduced hormone production, e.g. thyroid and sex hormones
- reduced response to T3 and T4
- reduced T4 binding capacity of serum proteins
- chronic thyroiditis (15-20% aged beagles)
- reduced response to adrenocorticotrophic hormone (ACTH)
- mammary nodules/tumours in 80% intact bitches at 11 years age.
- loss of muscle mass
- reduced number and size of muscle cells
- fibrosis, atrophy and reduced response to adenosine triphosphate (ATP)
- impaired resynthesis of ATP
- impaired ability to use amino acids as an energy source
- reduced oxygen transport
- reduced neuromuscular functidn
- long bone cortices thin, change in density and become brittle
- reduced number and activity of osteoblasts
- reduced amount of cartilage which splits and fragments
- reduced mucopolysaccharide production
- reduced chondroitin sulphate production
- spondylosis and costochondral calcification
- degenerative joint disease
- synovial fluid thickens with increased globulin content
- increased rheumatoid and antinuclear factors.
Central nervous system
- neurotransmitter changes;
- acetylcholinesterase levels increase, choline acetyltransferase decreases
- monoamine oxidase levels increase, causing decreased neurotransmitter levels;
- reduced serotonin increases sleep, and causes neuromuscular disorders and depression;
- stimulation of interneurones in the brain is longer lasting, leading to short-term
memory loss, impaired learning and delayed response time;
- hypoxia due to reduced respiratory function and cardiovascular changes, e.g. arteriocapillary
- senile behaviour changes, e.g. loss of house training.
Peripheral nervous system
- loss of reflexes;
- cell loss and lipofuscin accumulation in ganglia of sympathetic and parasympathetic
systems cause impaired gut motility (constipation);
- reduced reaction to stimuli and partial loss of senses (taste, vision, hearing
- obstructive lung disease
- reduced ciliary activity
- decreased secretions with increased viscosity
- bronchial constriction due to decreased adenosine monophosphate (cAMP)
- pulmonary fibrosis
- decreased alveolar diffusing capacity
- depressed cough reflex
- chronic bronchitis
- involution of lymph nodes, Peyer's patches and tonsils
- reduced size of cortices in lymph nodes
- reduced immunocompetence despite normal numbers of immunocytes.
- bone marrow becomes pale and fatty
- splenomegaly with hyperplasia, haemosiderosis and haematomas
- decreased red blood cell count and haemoglobin carrying capacity
- relative anaemia (common)
- replenishment of red cells may take longer (2 x).
These types of age-related changes may impair normal physiological activities, reduce
the ability of the animal to respond to stresses, infections or other attacks on
the body, and delay healing processes. Nevertheless advanced age is not regarded
as being a disease state.
Because of reduced hepatic function, renal function and suboptimal metabolic processes
older animals may have an impaired ability to metabolise and excrete drugs. Hence
medications may need to be given at different dose rates from the normal adult dose.
There may also be addirisks associated with general anaesthesia or elective surgery.