Thailand's King Bhumibol Adulyadej is to undergo a lumbar puncture procedure, the Royal Household has said in a rare statement on the monarch's health.Doctors said the procedure, also known as a spinal tap, would drain excess fluid on the king's spine which was causing unsteadiness when he walked.
King Bhumibol, who is 83, is hugely respected in Thailand and has been the country's unifying figure for decades.
He has mostly been in hospital since 2009, making occasional short trips.
The Thai Royal Household said the king had recovered from the lung infection for which he was originally taken to hospital, but had remained in hospital for rehabilitation and nourishment.
In a statement, the household said the lumbar puncture was a common procedure for elderly people and would take place on Monday night.
King Bhumibol is the world's longest reigning monarch.
He has no official political role but has been credited with intervening at moments of acute political tension to find a non-violent resolution.
There is near universal affection for him among the Thai public but the country also has harsh "lese-majeste" laws which punish any criticism of the monarchy with up to 15 years in prison.