Horner's Syndrome: This condition is characterized by a classic triad of ptosis, miosis, and anhidrosis, resulting from disruption of the sympathetic nerves supplying the eye. It is a common cause of unilateral ptosis and can be due to various etiologies, including stroke, carotid artery dissection, or tumors.

Third Cranial Nerve (Oculomotor Nerve) Palsy: This condition can cause unilateral ptosis due to weakness or paralysis of the levator palpebrae superioris muscle. It can be caused by aneurysms, diabetes, or trauma.
Myasthenia Gravis: An autoimmune disorder that can cause fluctuating unilateral or bilateral ptosis due to weakness of the extraocular muscles, including the levator palpebrae superioris.
Senile Ptosis or Aponeurotic Ptosis: Age-related weakening of the levator aponeurosis, leading to unilateral or bilateral ptosis.

Cavernous Sinus Thrombosis: A potentially life-threatening condition that can cause unilateral ptosis, along with other symptoms like proptosis, chemosis, and cranial nerve palsies. Prompt diagnosis and treatment are crucial.
Giant Cell Arteritis: Although more commonly associated with visual loss, this condition can also cause unilateral ptosis due to ischemia of the extraocular muscles or nerves.
Orbital Malignancy: Tumors in the orbit can cause unilateral ptosis by invading or compressing the levator palpebrae superioris muscle or its nerve supply.

Chronic Progressive External Ophthalmoplegia (CPEO): A mitochondrial disorder that causes progressive weakness of the external eye muscles, including the levator palpebrae superioris, leading to unilateral or bilateral ptosis.
Oculopharyngeal Muscular Dystrophy: A rare genetic disorder characterized by late-onset ptosis and dysphagia, due to progressive muscle weakness.
Congenital Ptosis: Present at birth, this condition can be due to various causes, including poor development of the levator palpebrae superioris muscle or its nerve supply.