Trigeminal Neuralgia: This condition is characterized by sudden, severe, and stabbing pain or numbness in parts of the face. It can occur without any visible abnormalities on an MRI, making it a likely diagnosis for facial numbness with a negative MRI.

Multiple Sclerosis (MS): Although an MRI is often used to diagnose MS, early or mild cases might not show significant lesions. Facial numbness can be a presenting symptom of MS.
Vitamin Deficiency (e.g., Vitamin B12): Deficiencies in certain vitamins, particularly B12, can cause neurological symptoms including numbness in the face.
Hypothyroidism: This condition can cause a variety of neurological symptoms, including numbness or tingling in the face.

Lyme Disease: Neurological manifestations of Lyme disease can include facial numbness. It's crucial to consider this diagnosis, especially in endemic areas, as untreated Lyme disease can lead to serious complications.
Sarcoidosis: This autoimmune disease can affect any part of the body, including the nerves, leading to facial numbness. It's essential to consider sarcoidosis due to its potential for serious complications if left untreated.
Cancer (e.g., Leptomeningeal Metastases): Although less common, cancer can cause facial numbness. A high index of suspicion is necessary, especially in patients with a history of cancer.

Sjögren's Syndrome: An autoimmune disorder that can cause neurological symptoms, including facial numbness, due to inflammation of the nerves.
Guillain-Barré Syndrome: A rare autoimmune disorder where the body's immune system mistakenly attacks part of its peripheral nervous system, which can lead to numbness and tingling in the face among other symptoms.
Mitochondrial Myopathies: A group of disorders caused by dysfunctional mitochondria, which can lead to a variety of neurological symptoms, including facial numbness.