Thailand's 88-year-old King Bhumibol Adulyadej has received treatment for 'water on the brain', the Royal Household Bureau said in a statement on Friday.

The world's longest-reigning monarch has been treated for various ailments during a year-long hospitalisation in the Thai capital. He was last seen in public on Jan. 11, when he spent a few hours visiting his Bangkok palace.

Most Thais have known no other monarch, as the widely revered king ascended the throne 70 years ago. He has spent most of the past six years in hospital, and nervousness over his health and the succession has formed the backdrop to more than a decade of political crisis in Thailand, where the military took power in a coup two years ago.

The palace said examination of the king showed he had 'water on the brain', or hydrocephalus, a build-up of the cerebrospinal fluid that surrounds the brain. He was treated for the same thing last August.

Fluid was drained and the king was being monitored, the statement said.

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

Updates on King Bhumibol's health are typically made public once he is recovering. The palace last issued a statement on his health on May 14, stating he had suffered from a swollen lung and knees.