Turner Syndrome is a genetic condition affecting females, stemming from the complete or partial absence of one of the two X chromosomes. While it presents with a range of features, one of its most common characteristics is short stature.
The Genetic Basis of Short Stature in Turner Syndrome
Short stature in Turner Syndrome is linked to the absence or alteration of an X chromosome. About half of individuals with Turner Syndrome have monosomy X (45,X), meaning each cell has only one X chromosome instead of the usual two. Other instances involve a partial deletion or rearrangement of an X chromosome, or mosaicism, where some cells have the typical two X chromosomes and others have only one.
The primary cause of short stature is haploinsufficiency of the SHOX gene (Short Stature Homeobox-containing gene). This gene is located on the X chromosome. Having only one functional copy instead of two reduces the amount of SHOX protein produced, which directly impacts normal bone development and growth, leading to the reduced height observed in individuals with Turner Syndrome.
Characteristic Growth Patterns and Predicted Height
Individuals with Turner Syndrome exhibit a distinct growth trajectory. Growth is often normal during infancy, but a noticeable slowdown in growth velocity begins around two to four years of age and continues throughout childhood. This reduced growth rate makes short stature nearly universal in girls with this condition.
The absence of a pubertal growth spurt, which normally accounts for a substantial increase in height during adolescence, significantly contributes to reduced adult height. Without growth-promoting treatments, the average adult height for individuals with Turner Syndrome is approximately 4 feet 8 inches (142 cm to 147 cm), about 8 inches (20 cm) shorter than the average for females without the condition. Healthcare providers use specialized growth charts developed specifically for girls with Turner Syndrome, which differ from standard growth charts. These charts help track a child’s height velocity and identify when intervention may be beneficial.
Growth Hormone Treatment for Height
Growth hormone therapy is the primary medical intervention used to increase adult height in individuals with Turner Syndrome. This treatment, often involving daily subcutaneous injections of recombinant human growth hormone (somatropin), works by stimulating linear growth. It aims to augment the natural growth process, which is impaired in these individuals.
Treatment typically begins in early childhood, often between 4 to 6 years of age, and preferably before 12 to 13 years, to achieve the best results. Early initiation allows for a longer period of growth hormone exposure before natural growth plates begin to close. Growth hormone therapy can increase final adult height, with average gains ranging from 2 to 4 inches or more (5 to 10 cm). The duration of treatment is extended until growth velocity decreases to less than 2 cm per year and bone age reaches at least 13-14 years, indicating that the growth plates have largely fused. Monitoring includes regular height measurements and assessment of insulin-like growth factor-I levels.
Long-Term Considerations for Height and Bone Health
Beyond directly influencing height, managing Turner Syndrome involves long-term considerations for the skeletal system. Individuals with Turner Syndrome have an increased risk of developing osteopenia and osteoporosis, conditions characterized by reduced bone mineral density. This heightened risk is primarily linked to the chronic estrogen deficiency that often occurs due to premature ovarian failure in Turner Syndrome.
To help maintain bone health, estrogen replacement therapy (ERT) is a common and important component of long-term management. ERT, typically initiated around 11-13 years of age, helps promote bone mineralization and reduces the risk of fractures. Along with ERT, ensuring adequate intake of calcium and vitamin D, and encouraging regular physical activity, are also important strategies to support bone health and overall well-being in individuals with Turner Syndrome.