Pseudo-pheochromocytoma due to obstructive sleep apnea: a case report

SUMMARY

Obstructive sleep apnea (OSA) is a condition of intermittent nocturnal upper airway obstruction. OSA increases sympathetic drive which may result in clinical and biochemical features suggestive of pheochromocytoma. We present the case of a 65-year-old male with a 2.9-cm left adrenal incidentaloma on CT, hypertension, symptoms of headache, anxiety and diaphoresis, and persistently elevated 24-h urine norepinephrine (initially 818 nmol/day (89-470)) and normetanephrine (initially 11.2 µmol/day (0.6-2.7)). He was started on prazosin and underwent left adrenalectomy. Pathology revealed an adrenal corticoadenoma with no evidence of pheochromocytoma. Over the next 2 years, urine norepinephrine and normetanephrine remained significantly elevated with no MIBG avid disease. Years later, he was diagnosed with severe OSA and treated with continuous positive airway pressure. Urine testing done once OSA was well controlled revealed complete normalization of urine norepinephrine and normetanephrine with substantial symptom improvement. It was concluded that the patient never had a pheochromocytoma but rather an adrenal adenoma with biochemistry and symptoms suggestive of pheochromocytoma due to untreated severe OSA. Pseudo-pheochromocytoma is a rare presentation of OSA and should be considered on the differential of elevated urine catecholamines and metanephrines in the right clinical setting.

LEARNING POINTS

Obstructive sleep apnea (OSA) is a common condition among adults. OSA may rarely present as pseudo-pheochromocytoma with symptoms of pallor, palpitations, perspiration, headache, or anxiety. OSA should be considered on the differential of elevated urine catecholamines and metanephrines, especially in patients with negative metaiodobenzylguanidine (MIBG) scan results.

The hyperpnoea of exercise in health: Respiratory influences on neurovascular control

NEW FINDINGS

What is the topic of this review? Elevated demand is placed on the respiratory muscles during whole-body exercise-induced hyperpnoea. What is the role of elevated demand in neural modulation of cardiovascular control in respiratory and locomotor skeletal muscle, and what are the mechanisms involved? What advances does it highlight? There is a sympathetic restraint of blood flow to locomotor muscles during near-maximal exercise, which might function to maintain blood pressure. During submaximal exercise, respiratory muscle blood flow might be also be reduced if ventilatory load is sufficiently high. Methodological advances (near-infrared spectroscopy with indocyanine green) confirm that blood flow is diverted away from respiratory muscles when the work of breathing is alleviated.

ABSTRACT

It is known that the respiratory muscles have a significant increasing oxygen demand in line with hyperpnoea during whole-body endurance exercise and are susceptible to fatigue, in much the same way as locomotor muscles. The act of ventilation can itself be considered a form of exercise. The manipulation of respiratory load at near-maximal exercise alters leg blood flow significantly, demonstrating a competitive relationship between different skeletal muscle vascular beds to perfuse both sets of muscles adequately with a finite cardiac output. In recent years, the question has moved towards whether this effect exists during submaximal exercise, and the use of more direct measurements of respiratory muscle blood flow itself to confirm assumptions that uphold the concept. Evidence thus far has shown that there is a reciprocal effect on blood flow redistribution during ventilatory load manipulation observed at the respiratory muscles themselves and that the effect is observable during submaximal exercise, where active limb blood flow was reduced in conditions that simulated a high work of breathing. This has clinical applications for populations with respiratory disease and heart failure, where the work of breathing is remarkably high, even during submaximal efforts.

Fossorial Damaraland mole rats do not exhibit a blunted hypercapnic ventilatory response

Damaraland mole rats (DMRs, Fukomys damarensis) are a eusocial fossorial species that spend the majority of their life in densely populated underground burrows, in which they likely experience intermittent periods of elevated CO₂ (i.e. hypercapnia). The primary physiological response to hypercapnia in most mammals is to increase depth and rate of breathing (i.e. hyperpnoea), but this response is often blunted in species that inhabit hypercapnic environments. In their natural habitat, DMRs putatively experience a gaseous environment ranging from normocapnic (0.1% CO₂) to hypercapnic (6.0% CO₂) conditions (Roper et al. 2001 J. Zool. 254, 101-107). As such, we hypothesized that DMRs would exhibit blunted hypercapnic ventilatory and metabolic responses, relative to those of non-fossorial rodent species. To test this hypothesis, we exposed awake, freely behaving DMRs to normoxic normocapnia (21% O₂, 0% CO₂, balance N₂) or graded normoxic hypercapnia (21% O₂, 0, 2, 5, 7 and 10% CO₂, balance N₂), and measured ventilation and metabolism using whole-body plethysmography and indirect calorimetry, respectively. We found that ventilation and metabolism were unchanged during prolonged normocapnia, whereas during graded hypercapnia, ventilation was elevated at 2% CO₂ and above. As a result, O₂ extraction efficiency at the lungs decreased with increasing hyperpnoea. Conversely, metabolic rate did not increase until 10% CO₂, presumably due to the metabolic cost of hyperpnoea. Taken together, our results suggest that despite their fossorial lifestyle, DMRs do not exhibit adaptations in their ventilatory or metabolic responses to environmental hypercapnia.

Sensitivity of the human ventilatory response to muscle metaboreflex activation during concurrent mild hypercapnia

NEW FINDINGS

What is the central question of this study? What is the relationship between the level of systemic hypercapnia and the magnitude of the additional hyperpnoea produced in response to a standardized level of muscle metaboreflex activation? What is the main finding and its importance? When a standardized activation of the muscle metaboreflex was combined with exposure to increasing levels of hypercapnia, the hyperpnoea this caused increased linearly. The concept of a synergistic interaction between the muscle metaboreflex and the central chemoreflex in humans is supported by this finding.

ABSTRACT

Ventilation increases during muscle metaboreflex activation when postexercise circulatory occlusion (PECO) traps metabolites in resting human muscle, but only in conditions of concurrent systemic hypercapnia. We hypothesize that a linear relationship exists between the level of hypercapnia and the magnitude of the additional hyperpnoea produced in response to a standardized level of muscle metaboreflex activation. Fifteen male subjects performed four trials, in which the end-tidal partial pressure of carbon dioxide ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>P</mml:mi> <mml:mrow> <mml:mrow><mml:mi>ET</mml:mi> <mml:mo>,</mml:mo> <mml:mi>C</mml:mi></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> ) was elevated by 1, 3, 7 or 10 mmHg above resting values using a dynamic end-tidal forcing system. In each trial, subjects were seated in an isometric dynamometer designed to measure ankle plantar flexor force. Rest for 2 min in room air was followed by 15 min of exposure to one of the four levels of hypercapnia, at which 5 min further rest was followed by 2 min of sustained isometric calf muscle contraction at 50% of predetermined maximal voluntary strength. Immediately before cessation of exercise, a cuff around the upper leg was inflated to a suprasystolic pressure to cause PECO for 3 min, before its deflation and a further 5 min of rest, concluding exposure to hypercapnia. The PECO consistently elevated mean arterial blood pressure by ∼10 mmHg in all trials, indicating similar levels of metaboreflex activation. Increased ventilation during PECO was related to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>P</mml:mi> <mml:mrow> <mml:mrow><mml:mi>ET</mml:mi> <mml:mo>,</mml:mo> <mml:mi>C</mml:mi></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> as described by the following linear regression equation: Change in minute ventilation (l min^-1 ) = 0.85 ×  <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>P</mml:mi> <mml:mrow> <mml:mrow><mml:mi>ET</mml:mi> <mml:mo>,</mml:mo> <mml:mi>C</mml:mi></mml:mrow> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> (mmHg) + 0.80 (l min^-1 ). This finding supports our hypothesis and furthers the idea of a synergistic interaction between muscle metaboreflex activation and central chemoreflex stimulation.