Diagnostic tests

Bedside evaluation

Exam: Upward gaze without diplopia/blurriness >20seconds, neck flexion/extension, counting after a single breath >20seconds.

Diagnosis:

• Ice pack test (80% sensitive) - Apply ice bag to ptosis x2 minutes, should improve.

• Tensilon test - injected IV, should see near immediate improvement

Serologic testing before immune modulating therapy

• AChR Ab: pos in 90% of pts w/ generalized dz, 50-70% pos in ocular, 99% pos in pt w/ thymoma, almost no false positive (rarely in LES, and polymyositis)

• Musk Ab: pos in 38-50% in generalized dz w/ AChR Ab negative; usually neg w/ AChR Ab positive, thymoma, or purely ocular

  • More female, bulbar, prominent resp./prox weakness, no thymoma
  • less responsive to acetylcholinesterase inhibitors or azathioprine
  • good response to plasma exchange and immunosuppression.

• Anti-striated muscle Ab: 30% in pt w/ MG, 80% in pt w/ thymoma, could be marker for thymoma

• 6-12% are seronegative MG: more purely ocular, EMG is identical. Same mgmt.

Electrophysiological testing

• Repetitive nerve study CMAP decremental response in MG, + if >10% decrease. To test must hold AChE inhibitors x12hrs, exercising the muscle x15seconds increases sensitivity.

• Single fiber electromyography: most sensitive dx for MG >95% on pt w/ MG, must differentiate pt w/ Kearns-Sayre syndrome causing Chronic Progressive External Ophthalmoplegia.

• Must image the chest for thymoma, MG could be a paraneoplastic effect of thymoma