Condition · Neural

Myasthenia Gravis

Neuromuscular-junction fatigue — a reflex that cannot be sustained

Fatigue at the neuromuscular junction

In myasthenia gravis, transmission across the neuromuscular junction is impaired and fails with sustained activity. The stapedius reflex can be elicited initially but decays abnormally during a continuous tone — a fatigable decay that may partially recover after a period of rest.

Decay without a retrocochlear lesion

Positive decay is usually taught as a retrocochlear sign, but a fatigable, rest-reversible decay should also raise the possibility of a neuromuscular-junction disorder, especially with other myasthenic features.

The reflex signature

Stim. Right

Stim. Left

Probe Right

Present

95 dB HL

Present

95 dB HL

Probe Left

Present

95 dB HL

Present

95 dB HL

The characteristic four-cell grid for this condition.

Thresholds are present and the grid looks normal; the abnormality is in the decay test, where the reflex fatigues and then recovers after rest.

Reflex decay

Positive (abnormal) decay — amplitude reaches 50% at about 6.3 s. Fatigable reflex decay is the characteristic finding — the reflex weakens during the sustained tone as the neuromuscular junction fails, and may recover after rest.

Pure-tone audiogram

○ Right ear✕ Left ear

Hearing is typically normal — myasthenia affects the muscle's neuromuscular junction, not the auditory pathway.

Frequencies plotted: 250, 500, 1k, 2k, 4k, 8k Hz.

References for this page

Warren RL, Gutmann L, Cody RC, Flowers P, Segal AT (1977). Stapedius reflex decay in myasthenia gravis. Archives of Neurology, 34(8), 496–497.
Fowler CG, Wilson RH (1985). Adaptation of the acoustic reflex. Ear and Hearing, 6(5), 263–268.
Katz J (Ed.) (2015). Handbook of Clinical Audiology (7th ed.). Wolters Kluwer, Philadelphia.

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