BANGKOK (Reuters) - Thailand's 88-year-old king, the world's longest reigning monarch, is recovering after being treated last month for "water on the brain" and narrowing of the arteries, the Royal Household Bureau said in a statement on Sunday.

King Bhumibol Adulyadej has been treated for various ailments over the past year in hospital in Bangkok. He was last seen in public on Jan. 11, when he spent several hours visiting his palace in the Thai capital.Anxiety over the king's health and the succession has formed the backdrop to over a decade of political upheaval that has included two coups and sometimes violent street protests. 

An x-ray showed the king had "a lower amount" of cerebrospinal fluid surrounding his brain after a build-up was drained last month, said the palace statement.

Echocardiograms showed a "satisfactory function of the King's heart muscles," it said. The king occasionally had a mild fever and was recovering from it, said the statement. 

News about the royal family is tightly controlled in Thailand. Laws protecting the royals from insult make it a crime to defame, insult or threaten the king, queen, heir to the throne or regent.

(Reporting by Patpicha Tanakasempipat; Editing by Andrew R.C. Marshall and Keith Weir)