Dyspnea in the supine position due to neuromuscular causes

Specialty: Respiratory.

• neuromuscular orthopnea
• intolerance to recumbency due to muscle weakness

Why it occurs

• Amyotrophic lateral sclerosis (ALS) with selective and progressive involvement of the diaphragm and intercostal muscles (when adopting the supine position, the patient experiences almost immediate asphyxiation because the flaccid abdominal muscles cannot compensate for the lack of diaphragmatic inspiratory force)
• Myasthenia gravis in the phase of myasthenic crisis or severe bulbar muscle weakness
• Chronic inflammatory demyelinating polyneuropathy (CIDP) with involvement of the cervical motor roots
• Limb girdle muscular dystrophy with late involvement of the axial respiratory pump
• Spinal muscular atrophy type III or IV with progressive diaphragmatic weakness.

Initial workup

Comparative spirometry sitting and lying down; Measurement of maximum respiratory pressures (MIP and PEM); Diaphragmatic ultrasound in recumbency; Nocturnal polysomnography with transcutaneous CO2 monitoring; Blood analysis with determination of antibodies against the acetylcholine receptor (AChR) or anti-MuSK if myasthenia is suspected.

Standard management

• Non-invasive mechanical ventilation (BiPAP) using a nasal or oronasal mask scheduled during sleep and periods of recumbency as mandatory ventilatory support
• Pyridostigmine — 60 mg orally every 6 hours in myasthenia gravis
• Intravenous immunoglobulin (2 g/kg administered in 2-5 days if there is a myasthenic crisis or Guillain-Barré decompensation).