A 75-year-old man has experienced headaches for the past 2 months. On physical examination, his vital signs are temperature, 37deg C; pulse, 68/min; respirations, 15/min; and blood pressure, 130/85 mm Hg. His right temporal artery is prominent, palpable, and painful to the touch. His heart rate is regular, and there are no murmurs. His erythrocyte sedimentation rate is 100 mm/hr. A temporal artery biopsy is performed, and the segment of the temporal artery excised is grossly thickened and shows focal microscopic granulomatous inflammation. He responds well to corticosteroid therapy. Which of the following complications of this disease is most likely to occur in untreated patients?