Bell's phenomenon

Last reviewed 01/2018

• Bell's phenomenon
  • occurs in the case of a unilateral lower motor neuron palsy
  • is the visible vertical rotation of globe on closing the affected eye (1)
  • it is seen when the patient attempts to shut the eye on the affected side which causes upward movement of the eyeball and incomplete closure of the eyelid.
  • this provides a test of infranuclear competence
    • only a positive response is helpful
    • some normal subjects do not show a Bell's phenomenon.
  • in unconscious patients, a fairly strong corneal stimulus may induce a Bell's phenomenon. If it does, it indicates midbrain-low pons intactness, i.e. cranial nerves III-VII. With structural brainstem lesions above the pons, Bell's phenomenon disappears but the jaw may deviate to the opposite side in the corneal pterygoid reflex.
• bilateral Bell's phenomenon (2)
  • is found in the following conditions
    • myasthenia gravis
    • sarcoidosis
    • bilateral Bell's palsies
    • congenital facial diplegia
    • muscular dystrophy
    • motor neuron disease
    • Guillain-Barré syndrome (rare)
• if an oblique of horizontal ocular deviation is evoked during forcible lid closure, it is said to signify a temporoparietal lesion on the side opposite to the deviation and is called by Cogan 'spasticity of conjugate gaze.'

Reference:

• (1) Facial Paralysis, Medical Content of Bobby R. Alford Department of Otolaryngology-Head and Neck Surgery,Baylor College of Medicine, ©2006 
• (2) J Smith, Bilateral Bell's Phenomenon, BMJ, July 14, 2001, (323:118)