To present a case of idiopathic intracranial hypertension (IIH) manifesting with third cranial nerve palsy and to review previously reported cases of IIH with similar involvement.

IIH typically presents with chronic headaches, pulsatile tinnitus, transient visual obscurations, and papilledema. The fulminant form accounts for approximately 2–3% of cases and is characterized by acute onset of symptoms and rapid visual decline within four weeks. Sixth cranial nerve palsy occurs in about 10% of IIH cases, while involvement of other cranial nerves, such as the third cranial nerve, is rare.

A 16-year-old girl presented with one week of occipital headache, nausea, and progressive blurry vision accompanied by right eyelid ptosis. She reported recent significant weight gain in the setting of risperidone use. Neurological examination revealed partial right third cranial nerve palsy with ptosis and a dilated pupil without relative afferent pupillary defect, along with bilateral severe papilledema. MRI and MRV excluded mass lesions, aneurysm, and inflammatory causes but demonstrated findings consistent with IIH. Lumbar puncture revealed a markedly elevated opening pressure of 86 cm H2O with normal CSF contents. The patient underwent ventriculoperitoneal shunting, and at two-week follow-up, her third cranial nerve palsy had nearly resolved, leaving only mild residual ptosis.

A literature review (including this patient) identified seven reported cases of IIH with third cranial nerve involvement (six female), all presenting with new-onset IIH.  Three cases had markedly elevated CSF opening pressures (> 50 cm H2O), met criteria for fulminant IIH, and underwent CSF shunting.

Third cranial nerve palsy is a very rare manifestation of IIH. Fulminant IIH should be considered in the differential diagnosis of acute third cranial nerve palsy, particularly when associated with markedly elevated opening pressures. Prompt recognition and early CSF diversion are important to prevent irreversible visual and cranial nerve deficits in fulminant IIH.