The Autonomic Nervous System
BACKGROUND
The autonomic nervous system is made up of the sympathetic and parasympathetic nervous systems.
Sympathetic system: ALARM system
Stimulation produces: tachycardia, dilatation of the bronchi, release of adrenaline and noradrenaline (maintains blood pressure), decrease in bowel motility, inhibition of micturition (stimulates internal urethral sphincter, relaxes detrusor muscle), increase in sweating and dilatation of the pupils. (Remember what happens when you go into an exam.)
Parasympathetic system: HOLIDAY system
Stimulation produces: bradycardia, constriction of the bronchi, increase in salivation and lacrimation, increase in bowel motility, erections, initiation of micturition (relaxes the internal urethral sphincter, contraction of detrusor) and constriction of the pupils.
Outflow
Bedside testing of the autonomic nervous system is limited.
Patterns of bladder and bowel disturbance are outlined separately (see below).
WHAT TO DO
Examine the pupils (see Chapter 7).
Take the resting pulse.
• Check pulse when asking patients to take 10 breaths per minute.
• Estimate difference between maximum and minimum rate (ideally done using ECG monitoring).
Check pulse response to standing (for 15 beats) (Table 26.1).
Table 26.1
| Test | Normal | Reflex |
| Resting pulse | 60–100 | Tachycardia: parasympathetic abnormal |
| HR response to respiration 10/min | Max–min > 15/min | Loss of variation: parasympathetic abnormal |
| HR response to standing (1st 15 beats) | > 11/min increase | Loss of response: parasympathetic abnormal |
| BP response to standing | Fall < 30/15 | Increased drop: sympathetic abnormal |
| HR response to Valsalva | HR up during | HR stable during: sympathetic abnormal |
| HR down after | HR stable after: parasympathetic abnormal |
Ask the patient to take a deep breath and exhale against a closed glottis: a Valsalva manœuvre (you will probably have to demonstrate this), and then ask him to breathe normally. Note the effect the Valsalva and release have on the pulse.
Take the lying and standing blood pressure (Table 26.1).
Look at the colour of the skin and note any sweating.
Feel the skin temperature.
WHAT YOU FIND
Pupils
WHAT IT MEANS
• Horner’s syndrome: see Chapter 7
• Autonomic neuropathy. Common cause: diabetes mellitus. Rare causes: Guillain–Barré syndrome, amyloidosis, multisystem atrophy (also called Shy–Drager syndrome: see Chapter 24), orthostatic hypotension, congenital autonomic failure (Riley– Day syndrome).
