(WGN) — It’s an invisible disease and a rare condition that starts in childhood. It’s a devastating diagnosis, but there is some new progress toward treating neurofibromatosis — or NF.

The first sign is a distinctive marking on the skin, something that looks like a birthmark. But what lies beneath can cause debilitating symptoms and damage to vital structures in the body.

Three-year-old Sam Mohideen has NF, and looking at him alongside his siblings, Max and Maria, you’d never know his health challenges.

“Sam is a ball of energy but he is also the happiest, kindest third child that I know,” said his father, Shahan Mohideen. “He loves his family. He loves being in the thick of it. He is fearless, and he is just fun.”

Dr. Robert Listernick, an academic general pediatrician with Lurie Children’s Hospital, said the first sign of the condition is typically a birthmark.

“In young children, usually the presenting sign is birthmarks that we call ‘café au lait’ spots, coffee with milk,” Listernick said. “We call them birthmarks, but they are often not present at birth but develop in the first several years of life.”

Erin Mohideen, Sam’s mother, said that’s what she thought the marks on her son were.

“I thought they were birthmarks, honestly,” Sam’s mother Erin Mohideen said. “He had a few freckles. He had a few ‘café au lait spots,’ and so I thought, ‘Look at this special kid with some different skin, that’s OK, right?’”

What looks like a birthmark is an indication of a deeper problem — a tumor that runs all the way down Sam’s arm to the base of his thumb. The benign lesion, called a plexiform neurofibroma, arises from the nerve covering. It’s too complex to be removed.

“Unfortunately, it’s involving the brachial plexis, which is a big bundle of nerves here which might affect function,” Listernick said, pointing to a location near the armpit.

But there’s now hope for children with the condition.

“2020 was a horrible year for the world. For the NF community, it was a great year because we now have the first FDA-approved drug,” Listernick said.

Patients with neurofibromatosis lack a gene that suppresses tumor growth. A type of drug known as a biologic, Selumetinib, can prevent growth and actually shrink lesions while taking away the pain.

Erin and Shahan Mohideen recently learned Sam has more lesions.

“He has additional tumors near his spine, in his chest cavity, on his neck and then one in the back of his brain,” Erin Mohideen said. “I don’t know what the future holds. We don’t know what that looks like.”

For patients with the inherited disease, tumors range in size and location and can cause damage to bones, organs and nerves. Some patients are born with scoliosis or develop it over time.

“About 15% of children with NF1 develop a tumor on the optic nerve and that can cause vision loss,” Listernick added.

Listernick the new treatment will be “a life changer for many patients with NF1.”

“Knowing that that is an option and knowing that is in our back pocket in case something goes wrong with Sam is great,” Shahan Mohideen said.

Right now, the soon-to-be preschooler is carefree and not hindered by his condition.

“Knowing and showing his resilience and zest for life in battling through this I think is important to show,” Shahan Mohideen said.

The Children’s Tumor Foundation has resources for patients and families. Sam’s family also has a team for the Shine A Light NF Walk.

For more information from the Neurofibromatosis Network, go to its website.