Clinical Utilities
Myasthenia gravis (MG) and Lambert-Eaton syndrome (LES) are acquired disorders of neuromuscular transmission. MG is caused by pathogenic autoantibodies (autoAbs) binding to muscle's nicotinic acetylcholine receptor (AChR) or, in a small minority of patients, muscle-specific receptor tyrosine kinase (MuSK); LES is caused by autoAbs binding to motor nerve terminal's voltage-gated P/Q-type calcium channel. Synaptic transmission fails when autoAbs cause a critical loss of junctional cation channel proteins that activate the muscle action potential. Both MG and LES can affect children (see #83371 "Myasthenia Gravis [MG] Evaluation, Pediatric") as well as adults, although LES is very rare in children. MG is 10 times more frequent than LES, but it is sometimes difficult to distinguish the 2 disorders, clinically and electromyographically. In adults with MG there is at least a 15% occurrence of thymoma or other neoplasm. These neoplasms are an endogenous source of the antigens driving production of the autoAbs characteristic of each disorder. LES is frequently associated with small-cell lung carcinoma. Thus far, MuSK antibody has not been associated with any neoplasm. Autoimmune serology is indispensable for both the initial evaluation and monitoring of patients with acquired disorders of neuromuscular transmission. The neurological diagnosis depends on the clinical context and electromyographic findings, and is confirmed more readily by a serological profile than by any single test. Not all of the antibodies in this profile impair neuromuscular transmission (e.g., N-type calcium channel antibodies, antibodies directed at cytoplasmic epitopes accessible in detergent solubilized P/Q-type calcium channels and muscle AChRs, or antibodies against sarcomeric proteins that constitute the striational antigens). Useful For: Confirming the autoimmune basis of a defect in neuromuscular transmission (e.g., MG, LES) Distinguishing LES from 2 recognized autoimmune forms of MG Raising the index of suspicion for cancer, particularly primary lung carcinoma (N-type calcium channel antibody) Providing a quantitative autoAb baseline for future comparisons in monitoring a patient's clinical course and response to immunomodulatory treatment. Note: single antibody tests may be requested in the follow-up of patients with positive results previously documented in this laboratory.

CPT Codes
83519-59/ACh receptor (muscle) binding antibody 83519-59/ACh receptor (muscle) modulating antibody 83519-59/Calcium channel binding antibody, P/Q-type 83519-59/Calcium channel binding antibody, N-type 83520/Striational (striated muscle) antibodies 83519-59/ACh receptor (muscle) blocking antibody (if appropriate) 84182/CRMP-IgG Western blot (if appropriate)

Reference Range
ACh RECEPTOR (MUSCLE) BINDING ANTIBODY
      < or = 0.02 nmol/L
ACh RECEPTOR (MUSCLE) MODULATING ANTIBODY
      0-20%  (reported as __% loss of AChR)
CALCIUM CHANNEL BINDING ANTIBODY N-TYPE
      <20 pmol/L
CALCIUM CHANNEL BINDING ANTIBODY P/Q-TYPE
      <20 pmol/L
STRIATIONAL (STRIATED MUSCLE) ANTIBODIES
      <1:60
ACh RECEPTOR (MUSCLE) BLOCKING ANTIBODY
      0-25% (reported as __% blockade of AChR)
      Curare-like drugs used during general anesthesia
      can cause a false-positive result.
CRMP-5-IGG WESTERN BLOT
      Negative (reported as positive or negative)

Component Information