11.3. Lambert Eaton Syndrome (LEMS)
- General Features:
- Presynaptic failure to release acetylcholine packets; there are an adequate number of acetylcholine quanta in each packet from the nerve terminal in response to its depolarization
- Autoimmune etiology suggested by:
- Organs specific antibodies in approximately 25% of patients
- Improvement after plasma exchange and immunosuppressant therapy
- Associated with HLA-B8 and the IgG heavy chain markers
- Antibodies to P/Q VGCC (voltage gated calcium channels; α 1 subunit of the pore)
- Antibodies to synaptic and to the intracellular component of the β subunit of VGCC channels
- Small cell lung cancer is present in approximately 60% of patients
- Autonomic dysfunction putatively related to anti-VGCC on sympathetic and parasympathetic neurons
- Progressive atrophy and Type I fiber loss in muscle biopsy
- Clinical Presentation:
- Peak incidence 50 years; associated with small cell lung cancer
- Leg weakness; proximal > distal
- Dysesthesias and pain occur in a small percent of patients
- Increased muscle strength immediately after voluntary contraction followed by fatigue
- Weakness increased by heat
- Post tetanic increase of reflexes that are initially depressed
- Autonomic symptoms of dry mouth, blurred vision, decreased sweating, and impotence
- Less common cranial nerve dysfunction
- Respiratory failure occurs; difficulty in breathing after surgery in which neuromuscular block agents have been used
- Patients with cancer and those without have similar symptoms
- LEMS may precede lung cancer by greater than two years
- Approximately 3% of SCLC manifest features of LEMS
- Variety of other cancers may demonstrate LEMs
- Laboratory evaluation: (P/Q type)
- VGCC antibodies in 79% of patients
- Positive sera may also reveal N-type VGCC
- EMG:
- Amplitude of the resting CMAP induced by supra maximal stimulation is reduced
- Slow repetitive maximal nerve stimulation less than 10 second causes a further decrement
- Greater than 10 Hz stimulation or 10 seconds of maximal voluntary contraction increases the amplitude of CMAP greater than 100%
- Single fiber EMG demonstrates abnormal firing and blocking
|