A lawyer for imprisoned Russian opposition leader Alexei Navalny, who has complained of serious back and leg pain in custody, says doctors have found him to be suffering from a double hernia.
Olga Mikhailova told the independent TV channel Dozhd that Mr Navalny also is beginning to lose sensation in his hands.
Mr Navalny went on a hunger strike last week to protest against what he called poor medical care.
On Tuesday, the leader of the Navalny-backed Alliance of Doctors union was detained by police after trying to get into the prison to talk to doctors.
Mr Navalny, 44, is Russian President Vladimir Putin’s fiercest domestic opponent.
He was arrested in January upon returning to Moscow from Germany, where he spent five months recovering from a nerve-agent poisoning that he blames on the Kremlin.
Russian authorities have rejected the accusation.
Still, labs in Germany and elsewhere in Europe confirmed that Mr Navalny was poisoned with the Soviet-era Novichok nerve agent.
Mr Navalny has rejected the conviction as fabricated, and the European Сourt of Human Rights found it “arbitrary and manifestly unreasonable”.
Authorities transferred Mr Navalny last month from a Moscow jail to the IK-2 penal colony in the Vladimir region, 53 mileseast of the Russian capital.
The facility in the town of Pokrov stands out among Russian penitentiaries for its especially strict inmate routines, which include standing at attention for hours.
Within weeks of being imprisoned, Mr Navalny said he developed severe back and leg pains and was effectively deprived of sleep because a guard checks on him hourly at night.
He went on a hunger strike on March 31, demanding access to proper medication and a visit from his doctor.
Russia’s state penitentiary service has said that Mr Navalny is receiving all the medical help he needs.
Ms Mikhailova did not specify where Mr Navalny’s hernias are located, but said one of them is difficult to treat and that a neurologist consulted by Mr Navalny’s organisation said the treatment prescribed in the prison was ineffective.
In an Instagram post on Monday, Mr Navalny said three of the 15 people he is housed with have been diagnosed with tuberculosis, a contagious disease that spreads through the air. He said he had a strong cough and a fever with a temperature of 38.1 degrees Celsius (100.6 F).
On Monday, the state penitentiary service said Mr Navalny had been the prison’s sanitary unit after a checkup found him having “signs of a respiratory illness, including a high fever”.
Ms Mikhailova said that Mr Navalny’s fever had lowered, but he is still coughing and is weak from the hunger strike.