<unk>  (oculomotor) nerve, innervates eyelid muscles and external ocular muscles (except lateral rectus and superior oblique) 
Nerve also carries parasympathetic fibers on external surface allowing for pupillary constriction
Palsy causes diplopia except in lateral gaze (lateral rectus innervated by CN VI)
Ptosis, headache
Posterior Communicating Artery (PCOM) Aneurysm (compresses nerve)
Ischemia
Diabetes
Trauma
Temporal lobe herniation through tentorium
Myasthenia Gravis
Cavernous Sinus Thrombosis
often associated with other cranial nerve deficits
Neurosyphilis
Autoimmune vasculitis (Lupus)
Aneurysm 
Carotid-cavernous fistula
Mass 
Myasthenia gravis
Thyroid associated orbitopathy
Internuclear ophthalmoplegia
Giant cell arteritis
Diabetes
Temporal lobe herniation through tentorium
often associated with other cranial nerve deficits
Third nerve palsy
Cataract
Fourth nerve palsy
Lens dislocation
Sixth nerve palsy
Macular disruption
Horner syndrome
Basilar Artery Thrombosis