College Answer

(a)   Patency of the oropharyngeal airway is due to activity of paired sets of upper airway muscles.

The presence of respiratory activity in the muscles of the soft palate, pharyngeal walls and tongue prevents otherwise floppy strictures from being sucked into the airway. Obstruction during sleep may be due to a combination of factors:

1)         reduced  airways  size  -enlarged  tonsils/adenoids,  macroglossia  myxoedema,  acromegaly, malignancy. A large percentage of OSA patients have a structurally small airway.

2)         neuromuscular tone- reduced tone occurs in REM sleep, particularly in postural muscles of the pharynx, palate etc.

3)         neuromuscular coordination – the normal coordination of increased upper airway tone withinspiration is lost.

(b) Potential long-term complications include:

Cardiac- hypertension, nocturnal angina/arrhythmias

Pulmonary- respiratory failure, cor pulmonale

Neurological- headache, somnolence, dementia

Psychiatric – depression, personality changes

Other - impotence, polycythaemia, glaucoma

Discussion

Pathophysiology of sleep apnoea:

• The oropharynx is a muscular tube which depends on muscle tone for patency.  
• Three muscle groups are involved:
  • muscles influencing hyoid bone position (geniohyoid, sternohyoid)
  • the muscle of the tongue (genioglossus)
  • the muscles of the palate (tensor palatini, levator palatini)
• During sleep there is a loss of background muscle tone.
  • This occurs mostly during REM sleep
  • Neuromuscular coordination is lost: the normal increase in upper airway tone with inspiration does not occur.
• This is exacerbated by drugs, alcohol, bulbar stroke, myopathy, etc.
• Several anatomical defects may coexist, which narrow the upper airway:
  • Acromegaly
  • Retrognathia
  • Macroglossia, eg. in Down syndrome
  • Fat infiltration of oropharyngeal tissues
  • Oedema (as a part of generalised oedema)
  • Upper airway infection, eg. tonsillitis
• The combination of narrowed airway, reduced airway muscle tone and lost inspiratory coordination results in complete upper airway obstruction with inspiration.
• The resulting apnoeic episodes have several pathological features:
  • Hypoxia
  • Hypercapnea
  • Extremely negative intrathoracic pressure
• This has pathological consequences.
  • Hypertension
  • Pulmonary hypertension (due to chronic hypoxic vasoconstriction)
  • Right ventricular hypertrophy and right heart failure
  • Increased risk of myocardial infarction
  • Atrial fibrillation (3-4 fold higher odds)
  • Increased risk of stroke
  • Decreased seizure threshold (independently associated with epilepsy)
  • Diabetes (somehow, it is an independent risk factor)
  • Increased risk of post-operative reintubation

Consequences of sleep apnoea

• Due to chronic REM sleep deprivation:
  • Daytime somnolence
  • Mood disturbances
  • Cognitive impairment
• Due to chronic hypoxia:
  • Pulmonary hypertension (due to chronic hypoxic vasoconstriction)
  • Right ventricular hypertrophy and right heart failure`
  • Polycythaemia
• Due to chronic hypercapnea:
  • Reset respiratory drive centre
• Due to the influence of the above on cardiovascular function:
  • Hypertension
  • Increased risk of myocardial infarction
  • Atrial fibrillation (3-4 fold higher odds)
  • Increased risk of stroke (likely due to polycythaemia and hyperviscosity)
• Associated with OSA, but not necessarily caused by it:
  • Decreased seizure threshold (independently associated with epilepsy)
  • Diabetes (somehow, it is an independent risk factor)
  • Increased risk of post-operative reintubation
  • Impotence