How Marfan Syndrome Affects the Skeletal System
Marfan syndrome is a genetic disorder that impacts the body's connective tissue, the material that provides structure and support to cells, organs, and tissues. The condition arises from a mutation in the FBN1 gene, which contains the instructions for making a protein called fibrillin-1. Fibrillin-1 is a key component of the microfibrils that give connective tissue its strength and elasticity. When this protein is faulty, the connective tissue becomes weak, leading to a wide range of health issues. While Marfan syndrome can affect multiple body systems, its impact on the skeletal system is often the most visible, causing a unique set of physical characteristics and deformities.
Tall Stature and Disproportionate Limbs
One of the most recognizable signs of Marfan syndrome is its effect on an individual's height and body proportions. The weakened connective tissue is unable to properly regulate the growth of the body's long bones, resulting in a distinctively tall and slender physique. These skeletal features are frequently the first clues that lead to a diagnosis.
The limbs—arms and legs—are often disproportionately long in comparison to the trunk, a condition known as dolichostenomelia. A common way to assess this is to measure a person's arm span, which in individuals with Marfan syndrome is typically greater than their height. This is not simply a matter of being tall; it is a specific imbalance in body proportions that results from the unregulated growth plates in the long bones.
Another hallmark feature is arachnodactyly, or "spider fingers." This term describes fingers and toes that are exceptionally long and slender. This occurs due to the overgrowth of the small bones in the hands and feet. The combination of long fingers and lax joints gives rise to two simple clinical indicators: the wrist sign and the thumb sign. A positive wrist sign occurs when an individual can wrap the thumb and little finger of one hand around the opposite wrist with the digits overlapping. A positive thumb sign is present when the thumb, held across the palm in a fist, extends beyond the edge of the hand. While not definitive on their own, these signs are strong indicators of the underlying skeletal changes.
Deformities of the Torso
The structural weakness of connective tissue also leads to distinct deformities in the chest and spine, as the bones and cartilage lack the support needed to maintain their proper shape and alignment.
Chest Wall Deformities
The chest wall is particularly susceptible to change, often resulting in a condition known as a pectus deformity. This happens because the cartilage connecting the ribs to the sternum (breastbone) is overly flexible. The most common form is pectus excavatum, where the sternum sinks inward, creating a noticeable hollow in the chest. Less commonly, the sternum may be pushed outward in a "pigeon chest" deformity called pectus carinatum. While often a cosmetic concern, a severe case of pectus excavatum can compress the heart and lungs, potentially leading to shortness of breath or reduced exercise tolerance.
Spinal Curvature
The spine often develops abnormal curves because the ligaments that hold the vertebrae in alignment are too lax. Scoliosis, a sideways curvature of the spine, is the most common issue. In people with Marfan syndrome, scoliosis can develop earlier in life and progress more rapidly than in the general population. Another potential issue is kyphosis, an exaggerated forward rounding of the upper back. Both conditions can cause significant back pain and, in severe cases, may require treatment with bracing or surgery to prevent further progression.
Craniofacial and Dental Abnormalities
The effects of weakened connective tissue extend to the structure of the head and face, resulting in a distinct set of characteristics that can present functional challenges.
The shape of the head and face is often affected. Many individuals develop a long, narrow face, a condition known as dolichocephaly. This is frequently accompanied by other features, such as underdeveloped cheekbones (malar hypoplasia), which can make the mid-face appear flat. The eyes may also seem deep-set, a feature called enophthalmos.
The lower jaw is often small and appears set back, a condition called mandibular retrognathia. This combination of features contributes to the elongated facial profile characteristic of the syndrome. Inside the mouth, the palate is typically high and narrowly arched. This constriction of the upper jaw often leads to dental crowding and a poor bite, or malocclusion, which can interfere with chewing and speaking.
The temporomandibular joint (TMJ), which connects the jaw to the skull, is also prone to problems. Joint hypermobility can cause frequent clicking, popping, and pain during movement. In some cases, the joint may even partially dislocate, making TMJ dysfunction a common complaint.
Joint, Foot, and Hand Abnormalities
Beyond the major deformities of the torso and limbs, the laxity of ligaments and tendons in Marfan syndrome affects smaller joints throughout the body, leading to hypermobility and instability.
Joint hypermobility, or "loose joints," is a common feature. This excessive flexibility means joints can move beyond their normal range of motion, increasing the risk of sprains, dislocations, and chronic pain. While it contributes to the positive wrist and thumb signs, this laxity affects joints throughout the body, including the knees, shoulders, and hips.
This ligamentous laxity also has a significant impact on the feet. Many individuals develop pes planus, a severe flatfoot deformity. This occurs because the ligaments supporting the arch of the foot are too weak to bear the body's weight, causing the arch to collapse. This can lead to foot pain, ankle instability, and difficulty with walking and standing for long periods.
