Do MPS I Treatments Cure Bone & Eye Problems?

While Hematopoietic Stem Cell Transplantation (HSCT) and Enzyme Replacement Therapy (ERT) are life-saving and critical treatments for Mucopolysaccharidosis type I (MPS I), they do not completely cure the disease, particularly when it comes to the bones and the eyes. These treatments work by delivering the missing enzyme to the body through the bloodstream. However, certain areas of the body, like the bones and the corneas, are notoriously difficult for these treatments to reach, meaning you or your child may still experience ongoing problems in these areas.

The Challenge of “Sanctuary Sites”

To understand why HSCT and ERT cannot fix every symptom, it is helpful to understand how these therapies work. Both treatments rely on the bloodstream to deliver the essential missing enzyme to the body’s cells. Tissues that have a rich blood supply—like the liver, spleen, and heart muscle—respond very well to treatment.

However, medical experts refer to the bones and the clear front part of the eyes (the corneas) as “sanctuary sites” ^[1][2]. These are avascular areas, meaning they lack a robust blood supply ^[2]. The cornea naturally has no blood vessels at all, and the growth plates of bones consist of dense cartilage that is very difficult for enzymes to penetrate ^[1]. Because the circulating enzymes cannot easily get inside these tissues, the buildup of complex sugars called glycosaminoglycans (GAGs) continues in these specific areas, leading to persistent symptoms despite an otherwise successful transplant or ongoing ERT ^[3][4].

What This Means for Bone Health

Even with a successful stem cell transplant, bone abnormalities (medically called dysostosis multiplex) often persist and can progress over time ^[4][1]. The cartilage at the ends of growing bones continues to struggle with GAG buildup, which alters how the bones develop and grow ^[1].

As you or your child grows, you will need to continue monitoring skeletal health closely. Many individuals with MPS I eventually require targeted orthopedic surgeries ^[4]. Common issues that may need surgical intervention later in life include:

• Hip problems: Conditions like hip dysplasia (where the hip socket doesn’t fully cover the ball of the thighbone) or hip displacement are very common and often require reconstructive surgery ^[4][5].
• Spinal issues: Curvature of the spine (kyphosis) or spinal cord compression can develop and may necessitate bracing or surgery ^[1].
• Joint stiffness: Ongoing joint restrictions might require physical therapy and sometimes surgical release ^[1].
• Carpal tunnel syndrome: Nerve compression in the wrists is frequent and may require a minor surgery to relieve pressure ^[6].

What This Means for Vision

The cornea is a classic sanctuary site. For this reason, corneal clouding—a signature feature of MPS I—has a minimal response to systemic treatments like ERT or HSCT ^[7]. While these treatments can sometimes stabilize the clouding or slow its progression, they rarely reverse existing cloudiness ^[1][7].

Ongoing care from an ophthalmologist (a specialist in eye diseases) is necessary. If the corneal clouding progresses to a point where it significantly impairs vision, a corneal transplant (penetrating keratoplasty) may be recommended ^[8]. It is important to know that even after a corneal transplant, clouding can sometimes recur years later, though a successful transplant can provide long-lasting visual improvement ^[8]. Other eye complications, such as increased pressure in the eye (glaucoma) or retinal changes, also need regular monitoring ^[9].

Moving Forward: Your Care Team

It is completely normal to hope that a stem cell transplant or starting ERT will be a 100% cure. Celebrating the immense, life-saving benefits of these treatments is important. While HSCT can allow enzyme-producing cells to reach the brain to protect cognitive function, standard intravenous ERT cannot cross the blood-brain barrier ^[10]. However, both treatments are absolutely vital for protecting the heart, liver, and other major organs ^[11].

Managing expectations for the bones and eyes will help you plan for future care. Maintaining a strong, multidisciplinary medical team that includes a pediatric or adult orthopedic surgeon and an ophthalmologist is essential to ensuring the best possible quality of life ^[12].