Myasthenia Gravis: This is a common cause of isolated ptosis, especially if the ptosis worsens throughout the day. Myasthenia gravis is an autoimmune disorder that affects the neuromuscular junction, leading to fluctuating muscle weakness.

Third Cranial Nerve (Oculomotor Nerve) Palsy: This can cause ptosis due to the nerve's control over the levator palpebrae superioris muscle. The ptosis may be accompanied by other symptoms like diplopia or pupillary dilation, but isolated ptosis can occur.
Horner's Syndrome: This syndrome, resulting from disruption of the sympathetic nerves to the eye, can cause mild ptosis, often accompanied by miosis and anhidrosis.
Senile Ptosis: Age-related weakening of the levator palpebrae superioris muscle or its aponeurosis can lead to isolated ptosis without other significant symptoms.

Aneurysm of the Posterior Communicating Artery: Although less common, an aneurysm compressing the third cranial nerve can cause isolated ptosis. Missing this diagnosis could lead to severe consequences, including subarachnoid hemorrhage.
Pituitary Apoplexy: Sudden hemorrhage or infarction of a pituitary adenoma can compress surrounding structures, including the third cranial nerve, leading to acute ptosis. This condition requires immediate medical attention.

Chronic Progressive External Ophthalmoplegia (CPEO): A mitochondrial disorder that leads to progressive weakness of the external eye muscles, including the levator palpebrae superioris, causing 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 underdevelopment or malformation of the levator palpebrae superioris muscle. It may not always be recognized as "isolated" since it's often identified early in life.