A 19 year old man presents with weakness of both upper and lower limbs for 10 minutes, in association with severe pain on both sides of the neck. There is no history of recent trauma.

His medical history is significant for homozygous sickle cell disease, with no other associated hemoglobinopathies. Treatment with hydroxyurea has been initiated on several occasions, but his compliance has been poor.

He has had 5 episodes of acute chest syndrome during the last 3 years, which were treated with exchange transfusions. The last such episode was 5 months ago.

He does not smoke or consume alcohol and denies drug abuse. A complete blood count shows a Hb of 8.7 g/dL, WBC of 12,000/mm3, platelet count of 449,000/mm3, and hematocrit of 26%.