Sensations from the face are carried by the trigeminal nerve, which is the fifth cranial nerve, and ends up in a nucleus extending from the midbrain to the upper reaches of the spinal cord. The two sides of the face are each controlled by its own trigeminal nerve. The trigeminal has three main branches, and these have many smaller branches. The pattern of facial numbness follows the distribution of the nerves.
Opthalmic Sensory Loss
The ophthalmic division of the trigeminal, also known as V1, transmits sensation from the upper eyelid, the forehead and the cornea. The corneal reflex, in which the eyelids blink in response to the cornea being touched, uses a branch of the ophthalmic nerve to transmit the sensation, while the facial nerve is responsible for closing the eyelid. An acoustic neuroma, which is a growth around the nerve responsible for hearing, can press on the trigeminal nerve, causing loss of sensation in the cornea, and can lead to loss of the corneal reflex.
Mandibular Nerve
The mandibular division of the trigeminal, or V3, gathers sensations from the lower part of the face, including the jaw and lower teeth. Loss of sensation, often accompanied by tingling, can occur after dental extractions. According to "Adams and Victor's Principles of Neurology," numbness can be permanent. Numbness in the center of the jaw, an area innervated by a branch of the mandibular nerve called the mental nerve, can be the first sign of serious diseases, including cancerous infiltration, multiple sclerosis and autoimmune diseases, such as scleroderma.
Onion Pattern Sensory Loss
Sometimes the pattern of loss of sensation in the face doesn't follow the course of the nerve, but the pattern in which the signals gather together as they enter the central nervous system. According to "Sobotta--Atlas of Human Anatomy," signals from the more central areas of the face reach higher levels in the brain stem. If the problem is in the brain stem rather than in the peripheral nerve, sensation can be lost in a circle containing the lips and nostril, while sparing the wider circle containing the cheeks and nose. This complicated pattern can be uncovered with a thorough neurological exam.
References
- "Sobotta - Atlas of Human Anatomy"; Reinhard Putz: 2008
- Darthmouth.edu: Facial Sensations and Movements
- "Harrison's Principles of Internal Medicine, 17th Edition"; Antoni Fauci (ed); 2008
- "Adams and Victor's Principles of Neurology"; Maurice Victor and Allan Ropper; 2001
