Hemihyperplasia is a rare disorder where an excessive production of cells causes one side of the body, or part of one side, to grow larger than the other. This condition can be present at birth or become apparent as a child grows, with the degree of asymmetry varying widely. For some, the difference is barely visible, while in others it is quite pronounced. This overgrowth can affect the limbs, face, and internal organs.
Underlying Causes and Associated Syndromes
Hemihyperplasia can be an isolated condition or a feature of a broader genetic overgrowth syndrome. The most common of these is Beckwith-Wiedemann syndrome (BWS), a condition involving excessive growth during childhood. Researchers have linked many cases of hemihyperplasia and BWS to issues on a specific region of chromosome 11, which contains genes that regulate growth.
Normally, genes from one parent are expressed while those from the other are silenced, a process called genomic imprinting. In BWS and isolated hemihyperplasia, this balance is disrupted by genetic mutations or epigenetic changes, which alter gene activity without changing the DNA. This leads to an overactivation of growth-promoting genes or inactivation of growth-suppressing genes, causing asymmetrical overgrowth.
Other genetic conditions, including Proteus syndrome, Klippel-Trenaunay syndrome, and Neurofibromatosis Type 1, can also feature hemihyperplasia. Genetic analysis helps determine if the condition is isolated or part of a larger syndrome, which informs the approach to care.
Identifying the Signs
Visible asymmetry is the primary sign of hemihyperplasia, where one arm or leg may be noticeably longer or thicker than its counterpart. This difference can affect a child’s gait or posture. The overgrowth involves not just bone but also soft tissues like muscle and skin, making the affected area appear larger.
Facial asymmetry is another common sign. This can present as one cheek being fuller, a difference in the size of the eyes or nostrils, or an uneven jawline. The teeth on one side of the jaw may also be larger than on the other.
Hemihyperplasia can also affect internal organs, such as the kidneys or liver, causing one to be larger than the other. This internal overgrowth is identified during medical evaluations. The pattern of overgrowth is unpredictable, sometimes progressing rapidly and at other times showing only modest changes.
Diagnosis and Evaluation
Diagnosis begins with a physical examination by a clinical geneticist or a specialist in growth disorders. The physician documents physical asymmetries and takes precise measurements of limb length and circumference. These measurements quantify the overgrowth and establish a baseline for monitoring.
Imaging studies are used to assess internal signs. An abdominal ultrasound examines the size of the kidneys and liver for enlargement. X-rays provide clear images to measure differences in the long bones of the arms and legs.
Genetic testing is often advised to determine if the hemihyperplasia is isolated or part of a syndrome like Beckwith-Wiedemann. This testing involves analyzing a blood sample for the specific genetic or epigenetic changes on chromosome 11 known to cause the condition. A definitive diagnosis helps formulate a long-term management and surveillance plan.
Health Risks and Necessary Monitoring
A primary concern with hemihyperplasia is an elevated risk of developing certain childhood cancers, known as embryonal tumors. The two most common are Wilms tumor (kidney cancer) and hepatoblastoma (liver cancer). While the risk is higher than in the general population, the vast majority of children with hemihyperplasia do not develop cancer.
Due to this increased risk, a screening protocol is a standard part of care for early detection when tumors are most treatable. Monitoring involves regular abdominal ultrasounds, performed every three months, to inspect the kidneys and liver. This screening continues until the child is about eight years old, covering the period of highest risk.
The surveillance protocol also includes a blood test to measure alpha-fetoprotein (AFP) levels, as elevated AFP can indicate a hepatoblastoma. This blood test is done every three months until the child is four years old. These screening guidelines allow medical teams to monitor the child’s health and intervene quickly if concerns arise.
Managing Physical Asymmetries
Management focuses on the functional and cosmetic challenges of physical asymmetry. A significant issue for many children is a leg length discrepancy, which can affect walking, running, and posture. A common non-surgical solution is a custom shoe lift fitted to the shorter leg’s shoe to level the pelvis and improve gait.
For a more substantial leg length difference, surgery may be considered as a child nears skeletal maturity. A common procedure is epiphysiodesis, which halts growth at the growth plate of the longer leg, allowing the shorter leg to catch up. The surgery is timed with the adolescent growth spurt for the best outcome.
For asymmetries affecting other areas like the face, plastic and reconstructive surgery may be used. These procedures, such as soft tissue reduction or bone contouring, aim to improve symmetry. They can also address functional issues like difficulty with chewing.