Diagnosis of autonomic neuropathy

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Gustatory sweating is the only symptom which is almost pathognomic of diabetic autonomic neuropathy. Peripheral neuropathy (at least absent ankle jerks) must be present before the diagnosis can be made. A resting tachycardia, postural hypotension, or a gastric splash may be present.

Bedside cardiovascular tests for autonomic neuropathy are now well established: their most important role is probably in the exclusion of autonomic neuropathy. Normal and abnormal values are shown the table.

The loss of heart rate variability during deep breathing due to vagal impairment is the most reliable and simplest test of autonomic neuropathy. It is best assessed using a cardiotachograph during deep respirations (six breaths per minute) taking average readings during six breaths; it can also be performed using an ordinary electrocardiograph during a single deep breath (five seconds in, five seconds out). The heart rate difference (maximum rate during inspiration minus minimum rate during expiration) in those under 55 years old is always greater than ten. Heart rate increase on standing up should be assessed, and there should normally be an overshoot as well.

The Valsalva manoeuvre can be included among the tests: a mercury sphygmomanometer is used, the patient blowing hard into the empty barrel of a 20 ml syringe to maintain the mercury column at 40 mm Hg for 10 seconds. Maximum heart rate during blowing, followed by minimum heart rate after cessation, are recorded. There should be a bradycardia after cessation of blowing; the ratio of maximum: minimum heart rate is normally greater than 1-21 and clearly abnormal when less than 1-10. The Valsalva test should not be performed in those with proliferative retinopathy. Many other sophisticated tests need special equipment.

The figure showing the intracavernosal injection of alprostadil is from Tomlinson J, ed. ABC of Sexual Health. London: BMJ Publishing Group, 1999.

Normal values for autonomic function tests*

Normal

Abnormal

Heart rate variation (deep breathing) (beats/min)

> 15

< 10

Increase in heart rate on standing

(at 15 seconds)

(beats/min)

> 15

< 12

Heart rate on standing 30 : 15 ratio

> 1-04

< h00

Valsalva ratio

> h21

< h20

Postural systolic pressure fall at 2 min

< 10 mmHg

> 30 mm Hg

*These test results decline with age. The figures apply generally in those less than 60 years old.

Normal

Autonomic neuropathy

40 0

Normal

Autonomic neuropathy

Deep breathing

Deep breathing

Minutes

Minutes

Normal

Autonomic neuropathy

Normal

Standing

Standing

Autonomic neuropathy

Standing

Standing

Minutes

Heart rate changes in a normal subject (left) and a patient with autonomic neuropathy (right) showing loss of heart rate variation in autonomic neuropathy during deep breathing, at six breaths a minute (top), and loss of "overshoot" cardiac acceleration on standing (bottom)

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