Autonomic Function Tests

INDICATIONS

Suspected diabetic autonomic neuropathy.
Shy-Drager Syndrome.
Suspected autonomic failure from other causes.

CONTRAINDICATIONS

Patients with proliferative retinopathy should not perform the Valsalva manoeuvre because of the risk of retinal haemorrhage.
Atrial fibrillation (tests uninterpretable, except postural hypotension and handgrip tests).

SIDE EFFECTS

None

PREPARATION

Sphygmomanometer.
Mouthpiece to attach to sphygmomanometer (5ml syringe minus plunger).
ECG machine (old fashioned type as long rhythm strips recorded).
Tests of cardiac parasympathetic damage

1. Heart rate response to the Valsalva manoeuvre
   Start ECG machine (limb leads only, use lead II)
   Patient blows into sphygmomanometer and maintains pressure at 40mmHg for 15 seconds, continue recording for 30 seconds after release of pressure.
   Measure shortest R-R interval during manoeuvre and longest after.
   Valsalva ratio = longest after/shortest during.
   Take mean of three readings.
2. Heart rate variation during deep breathing
   Start ECG machine
   Ask patient to breathe quietly at a rate of six breaths over one minute (5 seconds in and 5 seconds out).
   Mark ECG at start of each inspiration and expiration.
   Measure maximum and minimum R-R interval for each cycle and convert to beats/min.
   Result is mean difference (max - min) for heart rate during deep breathing.
3. Heart rate response to standing
   Start ECG recording with patient lying.
   Ask the patient to stand, continue recording ECG for 1 minute.
   Measure shortest R-R interval around the 15th beat after standing and the longest around the 30th beat.
   Calculate longest/shortest = 30:15 ratio.
   Tests of sympathetic damage
   Blood pressure response to standing
   Measure blood pressure lying and then 2 minutes after standing
   Record postural difference

INTERPRETATION

TESTS Normal Borderline Abnormal
Valsalva ratio =1.21 1.11-1.20 =1.10
(max-min) HR >15 11-14 <10
(30:15 ratio) >1.04 1.01-1.03 =1.00
fall in BP =10 11-29 =30
These tests can be used to determine the degree of abnormality present: if two or more of the parasympathetic tests plus the sympathetic tests are clearly abnormal then this indicates significant autonomic damage, earlier damage is signified by abnormalities in at least two parasympathetic tests.

SENSITIVITY AND SPECIFICITY

Caution should be taken in interpreting these tests in patients who are poorly co-operative and in the elderly.

REFERENCE

Clarke B.F. and Ewing D.J., BMJ 285, 918-920 (1982).