Epidemiology
- Peak Incidence: age 50-70
- Race: occurs in all races and ethnic groups
- Sex: M>F
Etiology
- Sporadic ALS: 90-95% of cases
- Family members are not at increased risk for ALS
- Familial ALS: 5-10% of cases
- 20% of familial cases result from superoxide dismutase 1 (SOD1) enzyme gene mutation
Physiology
- Anterior horn motoneuron disease
- Bilateral diaphragmatic paresis or paralysis (may occur in absence of periph-eral weakness in some ALS cases)
- Pulmonary Hypertension: may occur due to neuromuscular disease with hypoventilation (leads to hypoxia and acidosis, with resultant pulmonary vasoconstriction)
Pathology
- xx
Diagnosis
- ABG: hypoxemia/ hypercapnia (early in course of disease: hypocapnia with tachypnea may be present)
- pCO2 reaches respiratory failure levels when VC falls to <50% of normal (or when respiratory muscle strength falls to <25-30% of normal)
- PFT’s: restrictive
- Decreased FEV1 (with normal or increased FEV1/ FVC ratio): FEV1 curve may be truncated near RV when expiratory muscle weakness is severe
- Decreased VC (due to muscle weakness, decreased lung and chest wall compliance)
- Decreased TLC
- Increased RV (inversely related to MEP)
- Normal-decreased FRC
- Slightly increased Raw (with normal-increased specific airway conductance)
- Normal DLCO (especially if it is corrected for decrease in TLC)
- Decreased MVV (usually): directly related to respiratory muscle strength
- Decreased MIP/MEP (usually decreased to same degree): most reliable tests (spirometry may be normal early, but MIP/ MEP are usually abnormal)
- Decreased lung compliance (due to chronically low lung volumes: microatelectasis/ reduced surface tension/ altered interstitial elastic fibers)
- Transdiaphragmatic pressure: using NG balloon (Pga-Pes)/ normal change >25 cm H2O (referenced to TLC), usually 2-20 cm H2O in bilateral paralysis
- CXR/Chest CT Pattern: may demonstrate low lung volumes
- EMG: dennervation potentials/giant motor units
- NCV: normal
Clinical
- Exertional Dyspnea
- Fatigue
- Tachypnea (typically rapid and shallow)
- Accessory Muscle Use
- Fasciculations
- Spasticity
- Acute/Chronic Hypoventilation (see Acute Hypoventilation, [[Acute Hypoventilation]] and Chronic Hypoventilation, [[Chronic Hypoventilation]])
- Respiratory Failure (due to progressively smaller VT with increased VD/VT ratio): may be preceded by sleep disturbance/morning headache/daytime somnolence
- Aspiration Pneumonia
Treatment
- Oxygen: for hypoxemia (to prevent polycythemia/ cor pulmonale/ cognitive dysfunction/ respiratory muscle fatigue
- Ventilatory stimulants (Progesterone/ Diamox/ Doxapram): not usually useful (drive is intact, but response is impaired)
- Glossopharyngeal breathing: use of tongue/ cheek/ laryngeal/ pharyngeal muscles to inject boluses of air into trachea (permits some speech/ limited cough/ deep breaths to prevent atelectasis)
- Useful only for hours
- Increases VC (by up to 1 L), TLC, peak expiratory flow rates, lung compliance/ relieves dyspnea/ improves V/Q matching/ improves cough
- Inspiratory muscle training
- Cough/secretions: “cough machine mask”/ abdominal compression techniques/ suctioning/ B2 agonists, mucolytics (aid clearance)/ FOB/ Chest PT/postural drainage
- Tracheostomy: allows suctioning but decreases ability to generate pressures to clear small airways
- Mechanical ventilation: may be necessary
Prognosis
- Average life expectancy after onset of symptoms is 3-4 years
References
- xx