What is Bell's (Bell's phenomenon) phenomenon?

Bell's Phenomenon

Bell's phenomenon is a protective reflex in which the eyeball rolls upward and slightly outward during eyelid closure, which helps protect the cornea from exposure and injury. 1, 2

Definition and Clinical Significance

• Bell's phenomenon is a physiological protective reflex that occurs during eyelid closure, where the globe turns upwards and slightly outwards to avoid corneal exposure 3, 2
• This reflex is named after the Scottish anatomist Sir Charles Bell, who first described this phenomenon 4, 5
• Bell's phenomenon serves as a critical protective mechanism for the cornea, particularly important in patients with facial nerve palsy who may have impaired eyelid closure 1, 6
• The phenomenon can be observed during normal blinking but is more pronounced when attempting to forcefully close the eyelids against resistance 2

Variations and Grading

• Bell's phenomenon shows significant variability in both type and magnitude of response among normal individuals 2
• A grading system has been developed to classify Bell's phenomenon into five grades 6:
  • Grade +2: Strong positive (pronounced upward movement)
  • Grade +1: Weak positive (mild upward movement)
  • Grade 0: No Bell's phenomenon (no eye movement)
  • Grade -1: Weak inverse (mild downward movement)
  • Grade -2: Strong inverse (pronounced downward movement) 6
• Age-related changes occur in Bell's phenomenon, with stronger responses (grade +2) more common in younger individuals (4-40 years) and weaker or absent responses more common in older adults (>60 years) 6

Clinical Relevance in Bell's Palsy

• In patients with Bell's palsy, the inability to close the eyelid completely (lagophthalmos) combined with an intact Bell's phenomenon helps protect the cornea from exposure 4, 1
• Assessment of Bell's phenomenon is an important part of the clinical evaluation of patients with facial nerve disorders, as it helps determine the risk of corneal exposure and need for protective measures 1, 5
• Eye protection is strongly recommended for Bell's palsy patients with impaired eye closure to prevent corneal damage, with measures including lubricating drops, ointments, moisture chambers, and mechanical protection 1

Inverse Bell's Phenomenon

• Inverse Bell's phenomenon is a rare variation where the eye moves downward instead of upward during eyelid closure 3, 7
• This can occur in normal individuals, patients with Bell's palsy, or following surgical procedures such as ptosis correction 3, 6
• Inverse Bell's phenomenon may still provide some protective effect for the cornea, though not to the same degree as the typical upward movement 6
• Transient inverse Bell's phenomenon has been reported following ptosis surgeries, including levator resection and frontalis sling-suspension procedures 3, 7

Neurological Basis

• The neural mechanism of Bell's phenomenon involves a reflexive pathway distinct from volitional eye movements 8
• While volitional vertical eye movements activate the frontal eye field (FEF), Bell's phenomenon as a reflexive movement appears to involve different neural pathways 8
• The supplementary motor area (SMA) proper is active during both volitional eye movements and eyelid opening/closing tasks 8

Clinical Assessment

• Bell's phenomenon can be assessed by asking the patient to close their eyes and then observing the movement of the globe as the examiner attempts to open the eyelids 2
• The presence and strength of Bell's phenomenon should be documented as part of the examination in patients with facial weakness or paralysis 1, 5
• Absence of a normal Bell's phenomenon in patients with lagophthalmos increases the risk of corneal exposure and damage, necessitating more aggressive protective measures 1, 6

Importance in Patient Management

• Understanding a patient's Bell's phenomenon is crucial for determining appropriate eye protection strategies in facial nerve disorders 1
• Patients with impaired eyelid closure but intact Bell's phenomenon may require less aggressive interventions than those with both impaired closure and absent Bell's phenomenon 1, 6
• The presence of ocular surface lesions is significantly higher in individuals with grade 0 (absent) Bell's phenomenon compared to those with other grades 6