Credit: E! Online

Get the tissues ready, Teen Mom 2 fans. Tonight's all-new episode focuses on Leah Messer's daughter Aliannah, who was recently diagnosed with a rare form of muscular dystrophy. In an emotional sneak peek clip from E! Online, a neuromuscular specialist explains the severity of Ali's condition to Leah and ex-husband Corey Simms.

"I think, at this time, there's no question. It is a congenital muscular dystrophy," Dr. Tsao says, adding that further tests must be done to determine which portion of the gene is associated, though most signs lead to the Ullrich congential muscular dystrophy.

Although Leah reports that Ali has been improving and walking longer distances, the doctor informs her that over time Ali's existing muscle will be replaced by fat and scar tissue — at which point Ali will likely become wheelchair bound.

Leah does her best to digest this information as Dr. Tsao presents two approaches for further testing: send a skin tissue sample to the National Institute of Health or perform a blood test for gene sequencing. He believes that both tests together will provide the answer and confirm which rare form of muscular dystrophy they're dealing with.

Unfortunately, there's no cure or treatment for any form of muscular dystrophy — and that's the news that hits Leah the hardest.

"Usually we can only do physical therapy, and in her situation she will need a power wheelchair when she goes to school," the doctor explains as Leah looks at him despairingly.

Watch the heart-wrenching sneak peek clip at E! Online, and be sure to read our exclusive interview with Leah Messer for more insight on how she's been handling Ali's diagnosis.

Teen Mom 2 airs on Tuesday nights at 10 p.m. ET on MTV.

Source: E! Online