Tests of autonomic function

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BLOOD PRESSURE CONTROL

1. Maintenance of blood pressure with alteration in posture - is normally dependent upon reflex baroreceptor function. A fall in BP occurs with efferent or afferent lesions - postural (orthostatic) hypotension.

2. Exposure to cold induces vasoconstriction and a rise in BP - cold pressor test. Stress will produce a similar pressor response, e.g. ask patient to do mental arithmetic.

Both central and peripheral lesions affect these tests.

3. Valsalva manoeuvre:

The patient exhales against a closed glottis, increases intrathoracic pressure and thus reduces venous return and systemic BP. The heart rate accelerates to maintain BP. On opening the glottis, venous return increases and an overshoot of BP with cardiac slowing occurs. An impaired response occurs with afferent or efferent autonomic lesions.

4. Noradrenaline infusion test:

A postganglionic sympathetic lesion results in 'supersensitivity' of denervated smooth muscle to adrenaline, with a marked rise in BP following infusion.

HEART RATE

1. Massage of the carotid sinus should stimulate the baroreceptors, increase vagal parasympathetic discharge and slow the heart rate. Either efferent or afferent lesions abolish this response.

2. Atropine test:

Intravenous atropine 'blocks' vagal action and with intact sympathetic innervation results in an increase in heart rate.

SWEATING

A rise in body temperature causes increased sweating, detectable on the skin surface with starch-iodide paper. Any lesion from the central to the postganglionic sympathetic system impairs sweating.

SKIN TEMPERATURE

Skin temperature is a function of the sympathetic supply to blood vessels. With pre- or postganglionic lesions the skin becomes warm and red. With chronic postganglionic lesions the skin may become cold and blue (denervation hypersensitivity.) Compare the temperature of various regions.

PUPILLARY FUNCTION

Check the response to light and accommodation. Pharmacological tests arc important:

1. Atropine - blocks parasympathetic system - dilates pupil.

2. Cocaine - stimulates adrenergic receptors - dilates pupil.

In highly specialised units detailed neurophysiology (e.g. thermal threshold measurements) and plasma concentrations of neurotransmitters and hormones at rest and in response to baroreceptor stimulation are employed to characterize the site and selectivity of the autonomic lesion.

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