Postural dysautonomia in response to head-up tilt in a military pilot aspirant: Aeromedical considerations

INTRODUCTION

Dysautonomia is a broad terminology used to describe a number of physiological disturbances of the autonomic nervous system (ANS). Under non-stressful conditions, the two components of ANS - Sympathetic and Parasympathetic nervous systems are in balance. In the event of stress, the sympathetic response predominates and this leads to physical signs and symptoms. One of the stressful situations that the human body faces in day to day life is orthostasis. On assumption of upright posture, there is immediate shift of blood of 300–800 mL from upper body to lower body.^[1] This pooling of blood is sensed as a decrease in blood volume and blood pressure at the cephalic end and it initiates an autonomic response mediated by cardiopulmonary (senses low blood volume), carotid and aortic (sense low blood pressure) baroreceptors. This triggers a compensatory mechanism by which the parasympathetic tone decreases and there is a surge of sympathetic discharge, thereby restoring blood circulation at the head level.^[2] This response is considered physiological to the postural change and is evident by an immediate increase in heart rate (HR), slight reduction of systolic blood pressure (SBP) and slight increase or maintenance of diastolic blood pressure.^[3] However, some susceptible individuals show abnormal responses to orthostasis. The abnormal responses to orthostatic challenge can vary from development of persistent tachycardia, anxiety, and hypotension and can ultimately progress to pre-syncope or frank syncope.

This paper highlights a case of postural dysautonomia that was detected during evaluation of a female flying aspirant at the Institute of Aerospace Medicine (IAM), India.

CASE REPORT

A 23-year-old female military pilot aspirant reported to IAM for evaluation of Air Sickness and Syncope. She was undergoing her flying training at the Air Force Academy, Indian Air Force. One day, during her pre-flight preparation for an early morning sortie, the cadet was putting on her shoes and sitting on a chair. On getting up, she felt giddy and lost consciousness. It was a solitary episode of loss of consciousness without signs of convulsion. She was evaluated at her local unit hospital and was diagnosed as a case of neurocardiogenic syncope. Since confirmed diagnosis of neurocardiogenic syncope with a known precipitating event was established, she was not restricted from flying duties as per standard regulatory guidelines and she continued with her flying training.

Two months later, in a span of 4 days, she reported with nausea and vomiting on three occasions during her flying training. On all three occasions, the symptoms of nausea and vomiting appeared after she was subjected to aerobatic manoeuvres during her training sorties. She reported that she never developed any symptoms of motion sickness while traveling in vehicles on road or as a passenger in civil airlines. Before that she had undertaken about 20 h of flying on the same jet trainer aircraft (Pilatus PC7) flying routine flight handling sorties (with exposures up to 2.5 Gz) without any symptoms. She was referred to the local aeromedical training centre where she underwent Air Sickness desensitisation therapy. However, she was unable to complete the protocol due to persistent vomiting. Thereafter, she was referred to IAM for further evaluation and aeromedical disposal.

At IAM, the pilot aspirant was extensively evaluated for any cardiovascular or neurological abnormality. All routine investigations including electrocardiographic and electroencephalographic studies, magnetic resonance imaging (Brain), treadmill test and 24-h Holter study were within normal limits. Head-up tilt (HUT) test was carried out in view of her previous episode of syncope.

Immediately on exposure to 70° HUT, she developed tachycardia which persistently remained high even till 20 min of exposure. She also developed symptoms of tingling, numbness, shortness of breath, warmth and pain in the neck. The test was terminated at that point. The results of HUT were consistent with symptoms of postural dysautonomia as per the American College of Cardiology/American Heart Association/Heart Rhythm Society guidelines.^[4] She was evaluated by Aviation Psychologist and was found to have high performance anxiety. Due to her exhibiting normal clinical condition on ground, she was deemed fit on ground by cardiologist and neurologist.

Before commencing her motion sickness desensitisation at IAM, the individual was fluid loaded with increased salt and water intake and a test retest assessment was carried out with repeat HUT and passive standing test. The results of both the tests were similar to the HUT response earlier in that she presented with persistent tachycardia without hypotension accompanied by xerostomia and pre-syncopal symptoms. Both the HUT responses are presented in [Table 1].

DISCUSSION

One way of evaluating the cardiovascular reflexes on orthostatic stress is by tilt table testing.^[3] Postural orthostatic tachycardia syndrome (POTS) is characterised by persistent tachycardia of ≥30 beats/min (or absolute HR ≥120 beats/ min) on assumption of upright posture from supine position (≥40 beats/min for those between 12 and 19 years age) with subjective symptoms of dysautonomias such as diaphoresis, tremulousness, palpitation, fatigue and xerostomia in absence of orthostatic hypotension [Table 2]. These signs and symptoms have to be persistent for at least 6 months if a person were to be labelled as a case of POTS.^[6] However, some guidelines do not mention the time duration of symptoms for diagnosis of POTS. The symptoms are provoked by orthostatic challenge and relieved by lying down. The symptoms of POTS can be absent on active standing due to ‘skeletal muscle pump’ brought about by the contraction of lower limb muscles when a person stands, thereby increasing the venous return and arterial blood pressure. During passive standing such as in HUT, however, the skeletal muscle pump does not function and blood tends to pool in lower limbs, thus activating the reflex baroreceptor mechanism of cardiovascular system.^[7] This case report talks about a case in which the symptoms of dysautonomia were established during evaluation in HUT.

CONCLUSION

The case report describes an aircrew who was evaluated for air sickness and syncope and developed signs of postural dysautonomia while being subjected to orthostatic challenge in the form of HUT. However, the challenge remains whether one should consider the dysautonomic response as aeromedically significant, especially in view of no clinical compromise being evident during daily routine physical activities.