Myasthenia Gravis: This autoimmune disorder is characterized by fluctuating muscle weakness and fatigability, which can include ptosis (drooping of the eyelid) and proximal muscle weakness. The symptoms can be unilateral or bilateral and often worsen with activity and improve with rest.

Stroke or Transient Ischemic Attack (TIA): Although less common for isolated ptosis and proximal muscle weakness without other focal neurological deficits, a stroke or TIA in specific brain areas (like the midbrain for ptosis) could potentially cause these symptoms.
Horner's Syndrome: This syndrome can cause ptosis, but it is typically accompanied by miosis (pupil constriction) and anhidrosis (lack of sweating). Muscle weakness is not a primary feature, but it could be considered if the patient has a lesion affecting both sympathetic pathways and motor nerves.
Lambert-Eaton Myasthenic Syndrome (LEMS): An autoimmune disorder that can cause proximal muscle weakness, but ptosis is less common compared to Myasthenia Gravis. LEMS often involves autonomic dysfunction and can be associated with small cell lung cancer.

Pituitary Apoplexy: A medical emergency where a pituitary tumor suddenly hemorrhages or infarcts, potentially causing acute ptosis due to compression of nearby nerves, along with other symptoms like severe headache, visual disturbances, and acute hormonal deficiencies.
Cavernous Sinus Thrombosis: A rare but potentially life-threatening condition that can cause ptosis among other symptoms due to the involvement of cranial nerves passing through the cavernous sinus. It often presents with proptosis, chemosis, and decreased ocular motility.
Third Cranial Nerve (Oculomotor Nerve) Palsy: While typically causing diplopia, ptosis, and pupillary dilation, a third cranial nerve palsy due to an aneurysm (especially a posterior communicating artery aneurysm) is a "do not miss" diagnosis because of its potential for catastrophic consequences if not promptly treated.

Mitochondrial Myopathies: A group of disorders affecting the mitochondria, which can lead to muscle weakness, including proximal muscle weakness, and sometimes ptosis. These conditions are genetic and often have a gradual onset.
Congenital Myasthenic Syndromes: Rare genetic disorders that affect the neuromuscular junction, similar to Myasthenia Gravis but present at birth or in early childhood. They can cause ptosis and muscle weakness.
Botulism: A rare but serious illness caused by a toxin produced by the bacteria Clostridium botulinum, which can lead to muscle weakness, including ptosis, among other symptoms like difficulty swallowing and speaking.