Epidemiology
- Rare disorder
- Predisposing factors:
- Sex: M>F
- Peak age: aged 20-50 (but can occur at any age)
- Familial cases: have been reported
- Definition: idiopathic hypoventilation in the absence of any neuromuscular disease or mechanical ventilatory defect
Physiology
- Central hypoventilation due to abnormal chemoreceptor function, some lesion of medullary reticular formation, or abnormal carotid body chemoreceptor function
- Resultant central hypoventilation leads to hypoxia/ acidosis (with resultant pulmonary vasoconstriction)
- Nocturnal hypoventilation: disorder is often worse during sleep due to greater physiologic reliance on metabolic control during sleep
- Ventilatory response to hypoxemia/ hypercapnia: decreased (supplemental oxygen may increase or decrease ventilation)
Diagnosis
- ABG: hypoxemia (usually normal A-a gradient, occasion-ally elevated due to V/Q mismatch or microatelectasis)/ hypercapnia (pCO2 may be normal during wakefulness in some cases/ elevated serum bicarb may be only clue to disorder)
- Patients can voluntarily decrease pCO2 on demand
- PFT’s
- FEV1: normal
- VC: normal
- TLC: normal
- MVV: normal
- MIP/MEP: normal
- Exercise test: increase in pCO2 and decrease in pO2
- CBC: polycythemia
- EMG/NCV: normal
Clinical
(asymptomatic or insidious onset)
- Dyspnea: uncommon
- Fatigue/ lethargy
- Disturbed sleep
- Excessive daytime somnolence
- Morning headache
- Severe respiratory depression after sedation/anesthesia
- Cyanosis
- Prolonged breathold (without dyspnea)
- Sleep-disordered breathing (usually CSA-CSR)
- Pulmonary Hypertension/Cor Pulmonale (see Pulmonary Hypertension)
- Acute/Chronic Hypoventilation (see Acute Hypoventilation and Chronic Hypoventilation)
Treatment
- Avoid respiratory depressants
- Progesterone (40 mg/day): useful over long-term in patients with obesity-hypoventilation/hypercapnic COPD
- Benefit is predicted by ability to decrease pCO2 by hyperventilation
- Acetazolamide (Diamox): for patients with primary metabolic alkalosis with CO2 retention, but not pure respiratory acidosis
- Nocturnal oxygen: exacerbates nocturnal hypercapnia in most cases
- Phrenic nerve pacing: test phrenic nerve for viability (using diaphragmatic electrodes)
- Implant cervical or thoracic (preferred, better pacing) electrodes
- Begin pacing gradually and assess function (rib cage moves paradoxically inward during paced breaths) with fluoro/ PFT’s: best to pace for 6-12 hours each side alternating
- Complications: nerve damage/ infection/ upper airway occlusion (due to negative pressures/solved by trach or pacing from SCM EMG)
- Mechanical ventilation (Rocking bed, positive pressure, etc.): usually required/nocturnal use alleviates daytime symptoms
- Negative pressure may predispose upper airway collapse during sleep
References
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