What is the differential diagnosis for unilateral periorbital pain behind one eye, with associated eyelid swelling, tenderness to palpation, and a dry blistering rash on the forehead, exacerbated by eye movement, and accompanied by blurred vision, in the absence of nasal congestion, neck stiffness, and jaw claudication?

Cluster Headache: This condition is characterized by severe, unilateral orbital or periorbital pain, often accompanied by autonomic symptoms such as eyelid edema, conjunctival injection, and nasal congestion, though congestion is not present in this case. The pain is typically worse with eye movement, and the presence of a dry, blistering rash on the forehead could be related to the intense pain causing localized skin reactions or could be a separate condition. The absence of neck stiffness and jaw claudication, along with the specific location and nature of the pain, supports this diagnosis.

Herpes Zoster (Shingles): The dry, blistering rash on the forehead is highly suggestive of herpes zoster, especially if it follows a dermatomal distribution. The pain behind one eye and the swelling could be part of the zoster involvement, particularly if the trigeminal nerve is affected (herpes zoster ophthalmicus). Blurred vision could occur if the cornea is involved.
Acute Angle-Closure Glaucoma: This condition presents with severe, unilateral eye pain, often accompanied by blurred vision, and can cause the eye to be tender and swollen. However, it typically includes symptoms like nausea, vomiting, and is often associated with a mid-dilated pupil. The absence of these specific symptoms makes it less likely but still a consideration.
Optic Neuritis: Characterized by pain with eye movement and blurred vision, optic neuritis could be a possible diagnosis. However, it typically does not cause the eye to be swollen or tender to touch, and a rash would not be a typical presentation.

Giant Cell Arteritis: Although less likely given the absence of jaw claudication and neck stiffness, giant cell arteritis can present with unilateral headache, scalp tenderness, and visual disturbances. It's crucial to consider this diagnosis due to the risk of permanent vision loss if not promptly treated.
Cavernous Sinus Thrombosis: This is a rare but potentially life-threatening condition that can present with unilateral eye pain, proptosis, and blurred vision. The presence of a rash could be indicative of an underlying infection leading to thrombosis. It's essential to consider this diagnosis due to its high morbidity and mortality.

Tolosa-Hunt Syndrome: A rare condition characterized by unilateral headache, extraocular palsies, and sometimes facial pain, due to nonspecific inflammation of the cavernous sinus or superior orbital fissure. It could explain some of the symptoms but is less likely given the specific combination of symptoms presented.
Syphilis (Secondary): Can cause a wide range of symptoms including rashes and eye problems. However, the specific combination of symptoms described would be unusual for syphilis, making it a rare consideration in this context.