US doctor arrives in London to assess 11-month-old Charlie Gard

— -- An American doctor specializing in treating rare genetic conditions has arrived in London to examine Charlie Gard, an 11-month-old boy suffering from a critical illness that has damaged his brain and rendered him unable to breathe without assistance.

Dr. Michio Hirano, chief of the division of Neuromuscular disorders and a professor of neurology at Columbia University in New York City, arrived at Great Ormond Street Hospital today to help assess Charlie and weigh in on the next, or final, chapter of the baby's life.

A U.K. judge extended invitations to Hirano, as well as a doctor from the Bambino Gesu hospital in Rome who has not been named, after evidence on a new experimental treatment was presented by the New York-based doctor by video link last Thursday in court. The hospital said each doctor will be given an "honorary contract," which provides access to all the baby's medical records and the hospital facilities over the next days. They will be allowed to examine Charlie and speak with his doctors and parents.

Charlie suffers from mitochondrial depletion syndrome, a rare genetic disease that causes progressively increasing muscle weakness that leads to organ failure and becomes life-threatening within a few years. Though he is less than a year old, the baby has been on life support for several months.

His situation has drawn international attention in part because of the conflict between his parents, Connie Yates and Chris Gard, and doctors at Great Ormond Street Hospital. The baby's parents want him to receive an experimental treatment to extend his life, while doctors at the hospital believe that such an intervention will not ultimately help him and only prolong his pain and suffering. The hospital, which wants to Charlie off life support, has prevailed in several rounds of court cases. The case is now in the hands of the High Court.

Their fight to try experimental treatments in the effort to save Charlie's life has received some high profile backers, including Pope Francis and President Trump, who said in a tweet issued on July 3 that "If we can help little #CharlieGard, as per our friends in the U.K. and the Pope, we would be delighted to do so."

The treatment options offered by Hirano and his Italian counterpart involve drugs that they claim will increase muscle function and may be able to repair the damage done to Charlie's brain by his illness. The judge in the case said he was allowing the U.S. and Italian doctors access to determine whether Charlie's brain damage is structural, which they said cannot be repaired, or functional, which they said could potentially be reversed.

Their assessment of Charlie will be completed by Thursday and the case is expected to return to court early next week.

Following a court review of their assessment on Tuesday, the judge plans to rule on whether Charlie will be allowed to leave the U.K for either the U.S. or Rome to receive the experimental treatment or be taken off of life support at Great Ormond Street Hospital.

Connie Yates, Charlie's mother, will be present while the doctors assess Charlie's current condition.

ABC News' Mike Trew, Joseph Simonetti and Bianca Seidman contributed to this report.