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Osteogenesis Imperfecta

Osteogenesis imperfecta or OI, also known as brittle bone disease, is a genetic disorder that is characterized by weak and fragile bones that break easily. Osteogenesis imperfecta is caused by a genetic defect that disables the body to make strong bones. A person may experience occasional fractures or may have multiple fractures throughout life depending upon the severity of Osteogenesis imperfecta present. Apart from fragile bones, people with OI suffer from teeth problems - Dentinogenesis Imperfecta, hearing loss, muscle weakness, loose joints (joint laxity) and skeletal malformations.


Causes of OI

Osteogenesis imperfecta is either inherited from a parent who has the defective gene or could be a result of new mutations. Due to the defective gene, an important protein substance called type I collagen is not produced in the body. This protein plays an important role in forming connective tissues in bones and also helps in forming ligaments, teeth and the white outer tissue of the eyeballs (sclera). Due to poor production of the protein, bones become brittle and fragile and tend to break easily. Most of the mutations in OI exist in the two type I collagen genes, COL1A1 and COL1A2 and account for almost all the forms of OI.


Classification of Osteogenesis imperfecta

Depending upon the severity of the Osteogenesis imperfecta, the condition is divided into type 1, type 2, type 3, type 4. These types are classified mostly by fracture frequency and by characteristic features. Recently, research has identified three more additional variations to Osteogenesis imperfecta known as type 5, 6 and 7.


Type 1

Type 1 Osteogenesis imperfecta is the mildest and the most common form of OI. More than 50% patients suffer from Type 1 Osteogenesis imperfecta. In this type, though body produces normal type I collagen but only half the normal quantity. People with Type 1 OI may experience fewer fractures, and most often the condition may go unnoticed for several years after their birth.


Type 2

Type 2 Is the most severe form of Osteogenesis imperfecta often resulting in bone deformities in the child. Type 2 OI normally turns out to be fatal with the production of very little or poor quality collagen being produced in the body. Infants with type 2 OI are born with fragile rib cage and underdeveloped lungs. They usually die either in the womb or soon after birth.


Type 3

Osteogenesis imperfecta type III is severely progressive type associated with symptoms like short stature, a triangular face, severe scoliosis, grayish sclera, and Dentinogenesis imperfecta (impaired and irregular teeth with yellow-blue tinge). Infants with type 3 OI have fractures at the time of birth itself, and few infants reveal a fractured and eventually healed bones in the womb itself.


Type 4

Type 4 Osteogenesis imperfecta can range from very mild to severe form often resulting in growth retardation in children. A child with type 4 OI is short with bow shaped legs. Symptoms like tinted sclerae (white of the eye) and dental deformities may also be present at the time of the birth. Child normally suffers from Long bone fractures, vertebral compression, scoliosis, and ligament laxity with type 4 OI.

Type 4 also has two sub types called type 5 and type 6 OI. Though clinically they resemble type 4, types 5 and 6 have unique patterns to the bones. Type 5 exhibits features like ossification of interosseous membrane of the forearm with radial head dislocation, large callus formation and an abnormal histopathological pattern. Type 6 will have elevated alkaline phosphatase and blue-white sclerae.


Diagnosis

Most severe forms are diagnosed before birth itself. Ultrasound scanning during second trimester may reveal deformity of limbs, abnormally short fetus, irregular skull shape, lack of mineralizations and narrow chest cavity. Few cases are diagnosed soon after the birth and mild type of OI is found out much later in life when such individuals suffer from repeated fractures. However, the following diagnostic methods are used to assess the condition.


  • Clinical examination, wherein history of frequent fractures with minimal trauma is noted

  • X-Ray

  • Genetic testing of a blood sample (DNA )

  • Skin biopsy to assess collagen production.

Treatment

Since Osteogenesis imperfecta is a genetic condition, it does not have a cure. Patient will be treated symptomatically and will be aided with external tools to provide maximum possible mobility. Efforts are also taken to improve muscle strength and boost the bone mass in the patient through physical therapy. Professionally designed exercise programmes are highly beneficial and play an important role in treating the patients suffering from OI. Patient may also be prescribed required nutritional supplements like calcium, and vitamin D along with physical therapy. Few suitable candidates are also treated with surgical procedure called intramedullary rod surgery wherein metal rods are inserted through the length of the long bones to support and strengthen them.


Of late, Bisphosphonates drugs are being used in treating Osteogenesis imperfecta. Bisphosphonates are used to decrease the amount of bone resorption. It also helps in preventing fractures and improve person's functional mobility. There is also research being done to understand the role of gene therapy in treating Osteogenesis imperfecta.


Prognosis

Prognosis of Osteogenesis imperfecta depends upon the severity of the conditions. Despite bone deformity, restricted activity, and short stature, often patients with OI lead productive and near to normal lives.


Terminologies :

  • Osteogenesis : Growing new bone
  • Osteocytes : Bone cells
  • Osteoblasts : Bone cells building new bone structure
  • Osteoclasts : Bone cells which scavange bone tissue

Monosomy

Monosomy is a rare chromosome anomaly. Human cells normally contain 23 pairs of chromosomes, with a total of 46 chromosomes in each cell. Monosomy refers to the loss of one chromosome in cells. Any such change of chromosomes shall cause problems pertaining to growth, development and function of the body's systems. Monosomy is a genetic defect caused by an incomplete set of chromosomes. The changes in chromosomes occur during the formation of reproductive cells in early fetal development.


Monosomy can be identified during prenatal testing, especially in women who are at high risk. Prenatal testing such as an amniocentesis can reveal monosomy. As the test results could be very complicated, it is important to receive genetic counseling before undertaking this test. While a negative result indicates that no abnormalities were detected, a positive result suggests that a problem may be present. Since false positives and negatives can also happen, follow up additional testing is also recommended.


Aneuploidy is the term used to refer to chromosomal defects, a gain or loss of chromosomes from the normal 46. In monosomy, which is a kind of anueploid, there is the loss of one chromosome in cells. Another common form of aneuploidy is trisomy where people have three copies of a particular chromosome 21 in each cell instead of the two copies. One common example of the condition caused by trisomy is Down Syndrome.


Turner syndrome is a known example of the condition caused by monosomy. In this syndrome, women typically have only one X chromosome instead of the usual two. Significantly, Turner syndrome is the only full monosomy that is found in human beings. In other full monosomy, the individual will not survive development.


Cri du chat syndrome and 1p36 Deletion Syndrome are instances of partial monosomy caused by deletion of the short p arm of chromosome 5 and chromosome 1 respectively.


Chi du chat syndrome is characterized by a number of symptoms and in particular a malformed larynx which causes the voice to sound strangely high pitched. Chromosome 1p36 deletion syndrome is considered one of the commonest chromosome deletion syndromes. It is characterized by features such as developmental delay, feeding difficulties, low muscle tone, distinctive facial features, hearing loss, heart problems, seizures, vision defects and a large fontanelle that is slow to close. The incidence of monosomy 1p36 has been estimated to be 1 in 5000 to 1 in 10000 live born children. Interestingly, more females than males have been reported.


Myopathy

Myopathy or muscular disease that includes muscle inflammation and muscle weakness. Myopathies affecting the skeletal muscle can have many origins - inherited, drug induced or endocrine issues. Mostly a Myopathy is transitory in nature and rarely results in complete loss of function. Muscular Dystrophy is possibly an exception in that it can be severe and sometimes even fatal if it occurs early in life.


Genetic Myopathies

These inherited Myopathies occur due to a genetic defect in the synthesis of a protein. There are many kinds in genetic Myopathies:


  • Central Core Disease : Weakness in the muscles affecting hips and legs resulting in problems in running, jumping and climbing stairs

  • Centronuclear Myopathy or Myotubular Myopathy : Weakness in the muscles affecting face, legs, arms and the trunk resulting in drooping upper eyelids, facial weakness or foot drop

  • Myotonia Congenita : Muscles in the face, arms or legs are affected and result in muscular stiffness (myotonia) after contracting of muscles - usually after a trigger in the form of stress, fatigue, cold or a long period of no motion

  • Nemaline Myopathy : Weakness in the muscles of arms, legs and the trunk resulting in poor or absent reflexes, long or narrow face, abnormal facial features

  • Paramyotonia Congenita : Stiffness of muscles in the face, forearms and hands

  • Periodic Paralysis : Temporary muscle weakness episodes in hypokalemic form (low calcium) as a result of vigorous exercises, intake of food high in carbohydrates, stress, alcohol, insulin, pregnancy or infection. In the hyperkalemic form (high calcium) it can occur as a result of vigorous exercises, stress, pregnancy, skipping food, high potassium levels or steroids

  • Mitochondrial Myopathies : Progressive weakness of muscles in the eye (ocular myopathy) or arms and the legs or multisystem issues


Endocrine related Myopathies : Hormone deficiency can cause Myopathies. Hyperthyroid Myopathy is the result of excess secretion of thyroxine from the thyroid gland affecting muscles in the shoulders, hips or eyes. Hypothyroid Myopathy occurs when too little hormone is secreted and results in stiffness, cramps and weakness of legs and arms muscles.


Inflammatory Myopathies : Some Myopathies result in inflamed, weakened or wasted muscles. Dermatomyositis affects the connective tissue and the severity of the affected muscle loss can result in crippling movement.

Chronic muscle inflammation is called as Myositis. It is usually caused due to allergic reaction, infectious disease or rheumatism. Sometimes Myopathies are hereditary. Symptoms of Myopathy can also include cramps, spasms and stiffness. There is progressive deterioration in muscle strength resulting in pain and fatigue on walking and tripping and falling. This is not due to nerve dysfunction. Some patients might notice facial weakness, foot drop, droopy eyelids and poor reflexes in affected muscles.


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Bibliography / Reference

Collection of Pages - Last revised Date: April 23, 2019