Leonardo's family has set up a GoFundMe account to help pay for his medical expenses. [Representational image.]Creative Commons.

Gorham-Stout disease (GSD) – also known as vanishing bone disease or the phantom bone disease – is a rare bone disorder that causes progressive bone loss. While what exactly causes the disease is still unknown, affected patients go through steady bone destruction and resorption, with even multiple bones getting involved at times.

GSD, a rare decease recorded in only 300 people in the world, mostly affects the ribs, spine, pelvis, skull, collarbone (clavicle), and jaw, and could also cause pain and swell in the affected area. Its severity can differ from person to person, and can even lead to disfigurement and functional disability in extreme cases.

Earlier this month, a two-year-old boy called Leonardo Aguillon from Zeeland, Michigan, was admitted to a hospital with pneumonia symptoms, but chest x-rays and CT scans revealed that what he's suffering from GSD. In his case, the symptoms were just coughing fits, as his father Antonio shared with Daily Mail Online.

The condition had reached such extreme levels in the boy's case that his sternum and half of his left rib cage were missing. His prognosis is yet to be figured by doctors, and at the same time, they are worried that spread of the disease to his spine could even paralyze or kill him.

"[We had] a reaction of fear and uncertainty," said his father, an IT specialist at a local college, who is determined to spread awareness and find a cure for the disease. "We had never heard of this disease so we had no idea what were dealing with... it was extremely alarming."

CT scan and chest X-rays were able to reveal that Leonardo's condition was actually GSD.Creative Commons.

Luckily in Leonardo's case, GSD has been regional – meaning, it is currently affecting his rib cage only – in which case, symptoms include difficulty in breathing, chest pain, and weight loss. It can also include chylothorax – a condition where lymphatic fluid leaks and accumulates in the chest, causing infections or respiratory diseases, Daily Mail reported.

But in Leonardo's case "there were no symptoms," said Antonio. "Except whenever he got sick he would breathe a little heavier, but that's normal for any kid."

Around March 9, Leonardo began showing signs of pneumonia, and was diagnosed with infection initially. But later on, it was discovered that his sternum and half of his left rib cage had dissolved. His pneumonia started getting worse too and it was only after sedation that doctors were able to stabilize him, three days later.

Doctors are still unsure about how to treat the extremely rare condition because of how little research has been done on it, but as of now, they have put Leonardo on medication to stop his bones from dissolving and to prevent the disease from spreading further.

Dr Caleb Bupp, a geneticist at Spectrum Health who is not treating Leonardo, told WZZM-TV: "What seems like happens is those lymphatic ducts that are in certain bones, overactivated and sort of degrade the bone," addressing why it could absorb the bones.

"One of the thoughts and theories is that if you can stop the body from absorbing bone with medications that sort of addresses that," he added.

Leonardo's family have to wait for three weeks before knowing if the medication is slowing the dissolving of bones or not. "I'm definitely hopeful that he's going to stabilize and be able to cope with the disease," his father said. "But it's not easy to keep positive without more information."