Vitiligo: This is the most likely diagnosis due to the characteristic depigmentation of skin in a segmental pattern, which is a hallmark of segmental vitiligo. The condition often presents unilaterally on the face, following a dermatomal distribution.

Pityriasis Alba: Although more common in children, pityriasis alba can present with hypo-pigmented patches on the face, which might be confused with vitiligo. However, the presence of fine scales and a history of atopic dermatitis can help differentiate it.
Post-Inflammatory Hypopigmentation: Certain inflammatory conditions like eczema, psoriasis, or contact dermatitis can lead to hypopigmentation after the inflammation has resolved. The history of a preceding inflammatory condition can aid in diagnosis.
Idiopathic Guttate Hypomelanosis: This condition presents with small, discrete, hypopigmented macules, often on sun-exposed areas, including the face. It's more common in older adults and those with significant sun exposure.

Tuberous Sclerosis: Although rare, tuberous sclerosis can present with hypopigmented macules (ash leaf spots), which might be mistaken for vitiligo. The presence of other systemic features like seizures, intellectual disability, or skin lesions such as angiofibromas is crucial for diagnosis.
Piebaldism: A rare genetic condition characterized by congenital depigmentation, often on the forehead or scalp, which can be mistaken for segmental vitiligo. The presence of white hair and the congenital nature of the condition help differentiate it.

Waardenburg Syndrome: A genetic disorder that can present with depigmentation of the skin and hair, along with deafness and other ocular abnormalities. The combination of hearing loss and pigmentary abnormalities is key to diagnosing this condition.
Nevus Depigmentosus: A congenital condition presenting as a well-defined area of hypopigmentation, which can be mistaken for segmental vitiligo. The congenital nature and the absence of progression are distinguishing features.