The first child in the world to receive a double hand transplant is able to write, feed and dress himself on his own, just 18 months after the surgery, doctors say in an update published in a medical journal.

Zion Harvey lost his hands and legs below the knees after contracting staphylococcal sepsis as a toddler. The potentially fatal medical condition triggered by the body's response to an infection resulted in multiple organ failure.

By age four, his mother had given him a kidney.  As an organ transplant recipient, Zion was already on anti-rejection therapy, making him a candidate for hand transplants that surgeons would otherwise shy away from in a child, Dr. Marco Lanzetta of the Italian Institute of Hand Surgery, said in a journal commentary.

The hand transplants took place in July 2015 when Zion was eight years old.

In Tuesday's issue of the journal The Lancet, Dr. Scott Levin and Dr. Sandra Amaral of Children's Hospital of Philadelphia and their co-authors say Zion has exceeded his previous adapted abilities to dress, feed and wash himself without hand transplants.

Zion's goals? To climb monkey bars and grip a baseball bat.

He's done that and more, and has even thrown the first pitch at a Major League Baseball game.

"Hand transplantation in a child can be surgically, medically and functionally successful under carefully considered circumstances," the study's authors wrote.

First, Zion demonstrated that the tendons, joints and muscles had attached to the hands donated from a deceased child. Doctors also saw that his brain was communicating with the limbs.

Double Hand Transplant Child

Zion Harvey, centre who received a double hand transplant in July 2015, shakes hands with a health care worker in Aug. 2016. Doctors giving an 18-month update call it a surgical, medical and functional success. (Dake Kang/Associated Press)

Doctors documented his advances and setbacks, including eight rejections of the hands. The medical team managed the rejections with strong medications. X-rays and other scans showed how bones healed and remodelled.

After the transplants, the six to eight months of rehabilitation were intense for Zion. The medical team had to consider the unique challenges of an eight-year-old, giving lots of encouragement, rest breaks and using biofeedback video games and puppets to keep him engaged and exercising.

He often expressed pride in his accomplishments during therapy, doctors wrote.

"From an emotional standpoint, he remains a remarkable young man," Levin said in a hospital video. 

'Never give up on what you're doing'

 Zion's spirit shone in the hospital video. It shows his progress at printing a thank you note to the donor parents, cutting with scissors, grabbing a slice of pizza, hugging his mom Pattie Ray and playing with an action figure.

"If any kid is watching this and you're going through a rough time, never give up on what you're doing. You'll get there eventually," Zion said.

He's already using the fine muscles of his hand, said Dr. Gregory Borshel, a plastic surgeon at Toronto's Sick Kids Hospital who was not involved in Zion's case. 

"Giving a child like that hands is truly life transforming," said Borshel, who treats nerve injuries. 

Zion's nerves have had to regrow all the way into the hand, which doctors say happens much faster in kids.

Children his age tend to rebuff prosthetic hands, Borshel said.

With transplant patients, "they can manipulate them, they look like hands and so they will accept them as self, whereas a prosthetic is not regarded as self."

Borshel said Zion's case has caused Sick Kids to consider expanding their transplant service to try to offer it to families.