Somnolence syndrome is defined by a group of symptoms including extreme drowsiness, clumsiness, lethargy and slow mental processing. It may also present alongside signs of increased intracranial pressure, with the presentation of symptoms such as headache, nausea, vomiting, anorexia and irritability. Occasionally the syndrome may also be accompanied by a transient recurrence of a previously resolved neurological deficit.rr
It is a common side effect for patients undergoing cranial irradiation and total body irradiation. Its symptoms can have a marked effect on a patient’s quality of life, and can be distressing for both patients and their carers.
Somnolence syndrome has been reported to occur in 13-79% of patients undergoing cranial irradiation.r It has also been reported to occur in patients who have undergone total body irradiation.
The duration of symptoms has been reported from 2 days to 6 weeks, with symptoms slowly resolving completely and spontaneously.rrr
For cranial irradiation, the syndrome has been reported from doses between 12 Gy and 54 Gy.r The incidence is thought to be related to the volume of brain irradiated. For total body irradiation, doses as low as 12 Gy have been reported to trigger the syndrome.