In his recent Muscle & Nerve article, 4 the author comprehensively reviews autonomic nervous system testing, describing in detail indication, protocol, normal values, and clinical usefulness of different tests of cardiovascular autonomic regulation. The tests mentioned in this article undoubtedly measure autonomic hypofunction, and although they carry important prognostic information, as in diabetic neuropathy, 7 autonomic dysfunction may also present as excessive sympathetic or parasympathetic outflow, i.e., hyperfunction. 5 This holds particularly true for patients with Guillain-Barre Β΄syndrome (GBS), where vagally mediated arrhythmias, such as profound bradycardia or heart arrest, may occur in up to 23% of patients. 1,6 With the relative ease of cardiac pacing, the recognition of these potentially lethal dysrhythmias is of utmost clinical importance.
Although we have found an impairment of standardized autonomic tests in nearly all of our patients at the height of the disease, GBS patients with serious bradyarrhythmias did not differ from those without bradycardia regarding their responses to autonomic function tests. 2 Eyeball pressure (EP) is the strongest known stimulus for producing asystole. 3 We found that abnormal sensitivity to EP correctly identified 2 out of 3 patients necessitating cardiac pacing or cardiopulmonary resuscitation. 1 The parasympathetic dysfunction score derived from the heart rate responses to Valsalva maneuver, deep breathing, and active change of posture in 13 patients and 130 examinations was similar in patients with (EP+) and without (EP-) abnormal sensitivity to EP (Fig. 1). Similarly, there was no statistically significant difference between the two groups for the single tests (Valsalva ratio: 1.58, 1.52-1.64, median and interquartile ranges, vs. 1.45, 1.24-1.76; heart rate response to deep breathing: 10.9, 8.8-12.6 beats per minute vs. 7.3, 5.1-11.7 beats per minute; heart rate response to active change of posture: 1.22, 1.15-1.35 vs. 1.19, 1.08-1.27, Mann-Whitney rank sum test), although the median values of all tests tended to be higher in patients with abnormal EP. Thus, EP may provide additional information about the state of the autonomic nervous system, and especially about vagal overreactivity. Like the standardized autonomic function tests mentioned in the article, 4 it is simple, fast, and readily obtained, and contrary to them, it may also be performed in patients unable to cooperate with these tests, namely in mechanically ventilated patients.
We suggest that eyeball pressure testing should be incorporated into the battery of autonomic function tests, particularly in patients in whom vagal overreactivity is suspected.