Myasthenic snarl

Bilateral facial weakness can occur in a number of neuromuscular disorders, including neuromuscular transmission disorders, bulbospinal neuronopathy, bilateral facial nerve palsies and some myopathies. Myasthenia gravis may cause marked bilateral facial weakness. The pattern of lower facial and perioral muscle involvement is capricious. In some patients, poor retraction and elevation of the corners of the mouth when attempting to smile or grimace may result in a “vertical smile;” referred to as a myasthenic snarl or sneer.(1-3) There is more activation of the levator labii superioris and levator anguli oris and less activation of the risorius and zygomaticus major causing a smile or grimace lacking the normal retraction of the corners of the mouth.

This patient with severe congenital myasthenia was ventilator dependent and had a PEG. Attempting to show her teeth produced a vertical smile, or myasthenic snarl.(Figure and Video)

Vertical smile in a patient with congenital myasthenia gravis, the "myasthenic snarl."

The diagnosis of congenital myasthenia was made by Dr. Robert T. Leshner after she had carried a diagnosis of muscular dystrophy for many years.

References

1. Campbell WW. DeJong's the neurologic examination, 7th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2013.

2. Campbell WW. Clinical signs in neurology: a compendium. Philadelphia: Wolters Kluwer Health, 2016.

3. Howard JF. The myasthenic facies and snarl. J Clin Neuromuscul Dis. 2000;1:214-5.