Trigeminal Neuralgia: This condition often presents with sudden, severe, shock-like or stabbing pain in parts of the face, but it can also cause numbness or tingling. The involvement of the trigeminal nerve, which supplies sensation to the face and parts of the scalp, makes it a plausible cause for the symptoms described.

Multiple Sclerosis (MS): MS can cause a wide range of neurological symptoms, including numbness or tingling in the face and scalp due to demyelination of nerve fibers. The unilateral presentation could be part of a relapsing-remitting pattern.
Stroke or Transient Ischemic Attack (TIA): Although less common, a stroke or TIA can cause facial numbness, especially if the brainstem or areas supplying the face are involved. The absence of a rash does not rule out vascular causes.
Peripheral Neuropathy: Conditions affecting peripheral nerves can lead to numbness in specific areas, including the face and scalp. Diabetes, vitamin deficiencies, and autoimmune diseases are potential underlying causes.

Lyme Disease: Although typically associated with a rash (erythema migrans), Lyme disease can cause neurological symptoms, including facial numbness or paresthesia, without an obvious rash in some cases. Early recognition is crucial for effective treatment.
Sarcoidosis: This autoimmune disease can affect any part of the body, including the nervous system, leading to facial numbness among other symptoms. It's a diagnosis that requires consideration due to its potential for serious complications if left untreated.
Cancer (e.g., Brain Tumors, Leptomeningeal Metastases): Tumors affecting the brain, cranial nerves, or meninges can cause localized numbness. These conditions are less common but critical to identify early due to their significant impact on prognosis and treatment.

Sjögren's Syndrome: An autoimmune disorder that primarily affects glands, especially salivary and tear glands, but can also have neurological manifestations, including neuropathies that might cause facial or scalp numbness.
Vasculitis (e.g., Giant Cell Arteritis): Inflammation of blood vessels can lead to a variety of symptoms, including neurological ones, depending on the vessels affected. While less common, it's an important consideration in the differential diagnosis due to its potential for serious complications.
Neurosyphilis: A complication of untreated syphilis infection, which can cause a wide range of neurological symptoms, including numbness or tingling in the face and scalp. Its consideration is important due to the availability of effective treatment.