Lichen sclerosus: This condition is characterized by thinning of the skin, which can lead to a whitish, patchy appearance, often accompanied by itching and pain. The clinical presentation of lichen sclerosus, especially when it affects the genital or perianal areas, can sometimes be confused with other dermatological conditions. However, its occurrence on the forehead, while less common, still presents with distinctive features such as atrophy and potential for scarring, making it a primary consideration.
Linear morphea: This is a form of localized scleroderma that presents as a linear band of sclerosis, typically on the forehead (also known as "en coup de sabre" when it resembles a saber cut). It can lead to significant cosmetic and sometimes functional impairment. The linear nature and the involvement of the forehead make it a strong candidate for the differential diagnosis.

Psoriasis: Although less likely to present in a linear fashion on the forehead, psoriasis can sometimes mimic the scaly, erythematous aspects of both lichen sclerosus and linear morphea. However, the presence of typical psoriatic lesions elsewhere on the body and the absence of significant atrophy or sclerosis would help differentiate it.
Lichen planus: This condition can present with violaceous, polygonal papules and can sometimes involve the skin in a linear or blaschkoid distribution. However, the presence of Wickham's striae and oral lesions, if present, would help distinguish it from lichen sclerosus and linear morphea.
Scleroderma (systemic): While systemic scleroderma can present with skin thickening similar to linear morphea, the presence of systemic symptoms such as Raynaud's phenomenon, esophageal dysmotility, and pulmonary or cardiac involvement would differentiate it from localized forms.

Malignant transformation in lichen sclerosus: Although rare, there is an increased risk of squamous cell carcinoma in areas affected by lichen sclerosus. Any new or changing lesions within areas of lichen sclerosus should be biopsied to rule out malignancy.
Parry-Romberg syndrome: This rare condition presents with progressive hemifacial atrophy and can sometimes be confused with linear morphea due to its involvement of one side of the face. However, the atrophy in Parry-Romberg syndrome typically involves not just the skin but also the underlying tissues, including muscle and bone.

Nephrogenic systemic fibrosis: This condition, associated with gadolinium exposure in patients with renal failure, can cause skin thickening and sclerosis but is extremely rare and would be considered only in patients with a relevant history.
Eosinophilic fasciitis: Characterized by sudden onset of inflammation and thickening of the skin and fascia, this condition can mimic some features of linear morphea but is much rarer and typically involves the extremities more than the face.