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A 59-year-old nasopharyngeal carcinoma survivor developed double vision and eyelid drooping seven years after chemoradiation — ultimately diagnosed as radiation-induced third cranial nerve palsy. The case highlights a rare but serious late complication of head and neck radiotherapy, distinct from tumor recurrence or microvascular disease. Management remains largely supportive, with surgical options reserved for stable, refractory cases.
A 59-year-old man treated seven years earlier for nasopharyngeal carcinoma with chemoradiation (70 Gy to the tumor bed and left cavernous sinus) presented with progressive vertical double vision and left upper eyelid drooping. Workup — including MRI, CT angiography, PET scan, and labs for myasthenia gravis and thyroid disease — ruled out tumor recurrence, aneurysm, and neuromuscular causes. MRI revealed new enhancement along the left third cranial nerve's cisternal segment, pointing toward either microvascular ischemia or radiation-induced neuropathy.
The key differentiator was time: microvascular third nerve palsies typically recover within 8–12 weeks, but this patient showed no improvement at 12 weeks — and continued to worsen. That trajectory, combined with his irradiated cavernous sinus and coexisting radiation necrosis of the temporal lobe, led to a diagnosis of radiation-induced third cranial nerve neuropathy. He is currently managed with a Bangerter filter for diplopia relief, with strabismus surgery deferred given the absence of a fusible position and his medical comorbidities.
Key Takeaways:
Why it matters: As survival rates for head and neck cancers improve, late radiation toxicities — including rare cranial neuropathies — will become more common. Clinicians must maintain a high index of suspicion for radiation-induced injury in long-term survivors presenting with new neurological deficits.
