TIS                         
Chapter 1
Prenatal Development and Common Genetic Disorders

         Infants and children are frequently evaluated and treated by physical therapists for motor deficits and chronic illnesses. There are multiple reference texts, resources, and documents related to physical therapy intervention for children from the neonatal level up to adulthood. This text is designed to give a broad brush to the developmental sequence of growth and maturity for the normal while discussing the many variances from normal along with potential causes that may create deficits and thus require intervention by therapy. The importance of understanding the abnormal will be in the evaluation and treatment process to design effective treatment regimes to allow the infant/child to progress to the highest practicable level possible. This course may be used as a general reference in the treatment and understanding of physical therapy in the pediatric population.

Chapter 1 will investigate prenatal development and common genetic disorders that may give rise to infants and children who may require rehabilitation intervention. It is important that clinicians understand the role of gestation and influencing factors on the gestational process that may result in abnormal findings or development. The purpose of Chapter 1 is to lay the groundwork from which the next chapters will build on to allow the therapist to understand the developmental processes required in setting treatment parameters and interventions. Chapter 1 will be information based only.

PRENATAL DEVELOPMENT

         Prenatal development begins at the cellular level through a process referred to as fusion when the male germ cell, the sperm, unites with the female germ cell, the ovum to create a new cell referred to as a zygote. The zygote contains the base genetic prototype from which all the remaining cells will develop.

The developing cellular process is divided into two processes:

  1. Meiosis - cellular divisions occurring prior to fertilization. This process provides for random combinations of genetic materials from each parent through production of haploid germ cells. Haploid germ cells contain half the number of chromosomes found in other body cells.


  2. Mitosis - cellular divisions occurring after fertilization that create the ongoing growth and development of the organism. This process creates the diploid cells containing a complete chromosome number. The diploid cells are the result of a combination of one maternal X chromosome with one paternal X or Y chromosome to create either an a) XX female or a B) XY male.
GENETIC DISORDERS

Genetic disorders may occur in association with either cellular process resulting in the most common cause of birth defects. Genetic disorders are responsible for one third of all birth defects and about 85% of those with identified causes.

Genetic disorders may be grouped within three groups:

  1. Chromosome number abnormalities
  2. Chromosome structure abnormalities
  3. Gene mutations
Chromosome Number Abnormalities

Chromosome number abnormalities are the result in nondisjunction errors during cellular divisions in the meiosis process prior to fertilization. Thus, there are unequal distributions of chromosomal material. There are four main types of chromosomal number abnormalities:

  1. Monosomy
      45 XO Turner Syndrome
  2. Trisomy
      47 XYY Syndrome
      47 XXY Syndrome
      Trisomy 21 Down Syndrome
      Trisomy 18 Edward’s Syndrome
      Trisomy 13
  3. Tetrasomies
      48 XXYY
      48 XXXY
  4. Pentasomies
      49 XXXYY
      49 XXXXX
Monosomy is the result of only one of the chromosome pair present at a specific cell site. This is the most common cytogenetic abnormality causing spontaneous abortions. 99% of infants born do not survive. The most common monosomy is Turner Syndrome, 45 XO, is found in 1 in 3000 to 1 in 10,000 newborn females. Characteristics may include transient congenital lymphadema, small stature at birth with web-like appearance of the lateral neck, gonadal underdevelopment, hearing impairment, bone trabecular abnormalities.

Trisomy is the result of 3 of a particular chromosome present at a specific site. Zygotes with 47 chromosomes are produced when a germ cell with 24 chromosomes fuses with one that has 23. Trisomies of sex chromosomes are common and often undetected until later life. Trisomies include:

  1. 47XYY Syndrome characterized by tall stature, shoulder girdle weakness, and generalized incoordination particularly fine motor.
  2. 47 XXY or Klinefelter Syndrome that is present in 1 in 500 males and is the most common cause of infertility in adult males. Characteristics include hypogonadism, long limbs, and slim stature though obesity may be problematic in adults.
  3. Autosomal trisomies with only 21, 18, and 13 recognized as surviving postnatally. These are associated with growth abnormalities and cognitive delays.
Tetrasomies (48 XXYY, 48 XXXY) and pentasomies (49 XXXYY, 49 XXXXX) are characterized by short stature, hypogenitalism, moderate to severe cognitive delay and hypotonia.

Chromosome Structure Abnormalities

Chromosome structure abnormalities may result from mosaicism, translocation, chromosome deletion, or microdeletions.

Mosaicism typically results from a nondisjunction error during mitotic cell division and may occur in sec chromosomes or as autosomes.

Translocation results from a breakage and transfer of chromosomal material to unrelated, intact chromosome pairs. May be caused by environmental influences such as drugs, radiation, viruses, or chemical teratogens rather than inheritance. This condition may or may not result in abnormal development.

Chromosomal deletion is induced by chromosome breakage and the broken potion is lost rather than attached. Similar to translocation, may be caused by environmental influences. Most noted of chromosomal deletions is Cri-du-chat syndrome with results in abnormal laryngeal development that produces a weak and high-pitched cry. Cognitive deficits, mircrocephaly are always present. Also, hypotonia, hypertelorism, scoliosis, and congenital heart abnormalities are often present.

Microdeletions may span several contiguous genes and produce Prader-Willi syndrome and Angelman syndrome. Gene Mutations

Specific gene mutations account for 7-8% of all congenital anomalies. Mutations can occur randomly and besides an inheritance factor may be cause by environmental agents such as radiation and carcinogenic chemicals.

  1. Autosomal dominant inheritance - indicates that the mutant gene was contributed by one parent.
      Achondroplacia - the most frequent cause of short stature caused by disturbance to the endochondral ossification resulting in bilateral shortness of the humerus, femur, and macrocephaly associated with hydrocephalus.

      Ectrodactly - cleft hand or cleft foot in which two or more digits are fused

      Polydactyly - Extra digits present on the hand or foot

      Osteogenesis Imperfecta - error in collagen development resulting in multiple bone fractures generally categorized in four types with varying degrees of mortality and fragile bony structure.

  2. Autosomal recessive inheritance - the gene mutation is transmitted by both parents.
      Cystic fibrosis - progressive disorder of the exocrine glands with progressive digestive and pulmonary problems most often fatal though life spans have been lengthening with appropriate treatments.

      Childhood spinal muscular atrophy resulting in progressive flaccid paralysis. Defined in three types with decreasing severity of the disease and improved life span with weaknesses noted occurring in later childhood for Types II and III. Type 1, is usually fatal before 3 years of age.

  3. X-linked inheritance - transmitted on the X-chromosome primarily affecting males while females may be minimally affects or may be unaffected carriers.
      Rett Syndrome - lethal in males before birth, females with syndrome have a normal development up to the age of 6 months followed by gradual cognitive and motor losses, deceleration of head growth, onset of hypotonia, ataxia and may be associated with autism.

      Hemophilia - clotting disorder of the blood resulting in excessive bleeding or hemorrhage.

      Duchenne’s muscular dystrophy - muscle wasting disease

      Fragile X Syndrome - most common cause of mental retardation in males after Down syndrome. It is associated with older paternal age with characteristics of increased head circumference, prominent forehead, generalized hypotonia, torticollis, and scoliosis.

      Lesch-Nyan syndrome - x-linked recessive causing excessive production of uric acid with deleterious effects on the brain and liver. Associated with spasticity beginning at 6-8 months of age, choreoathetosis, growth deficiency, autism, and tendency to self mutilate.
ENVIRONMENTAL INFLUENCE ON PRENATAL DEVELOPMENT

Environmental Agents - environmental agents such as radiation, heavy metals, infectious or chemical teratogens may cause chromosomal abnormalities, especially deletion and translocation, and increase the rate of gene mutations. When in the pregnancy the mother came into contact with a teratogenic agent also determines the outcome of the fetus.
MATERNAL SUBSTANCE ABUSE

Alcohol Ingestion and Fetal Alcohol Syndrome (FAS)
         First described in the 1960s, however the full impact of maternal alcohol ingestion on fetal development and frequency of occurrence was not recognized until the 1990s. Children with FAS may have multiple, sometime subtle, developmental problems including hypotonia, gross motor delays and fine motor limitations. In addition, they may present with flattened nasal and bridge appearance, with microcephaly, learning disorders, hyperactivity, and joint and cardiac abnormalities.

Maternal Drug Use
         It is difficult to determine effects of specific drugs on the fetus because maternal substance abuse often includes multiple drugs. Research studies report a range of outcomes from no specific developmental sequelae to CNS impairment.

GROWTH: INFLEUNCES AND PROGRESSION

         The regulation of growth is highly complex and is dependent upon the interaction of nutrition, genetic, hormonal, mechanical, and environmental factors. There are many normative charts available that give growth parameters considered to be normal. Fetal growth depends upon the physical size of the mother’s uterus under normal conditions.

Average birth weight is between 5.5 (2500g) and 9 lbs.
  1. AGA - appropriate/average for gestational age - birth weight between 10 and 90th percentiles
  2. LGA- large for gestational age - birth weight above the 90th percentile
  3. SGA - small for gestational age - birth weight below the 10th percentile
  4. LBW - low birth weight - less than 2500 g
    1. full-term infant weight less than 5.5 lb. is consider SGA and LBW
    2. preterm infant weighing less than 5.5 lb. is considered LW not necessarily SGA
  5. VLBW - very low birth weight - birth weight is less than 1500 g
  6. ELBW - extremely low birth weight - birth weight is less than 1000 g
  7. Micropreemie - birth weight is less than 750 g
Failure to Thrive (FTT) - is a growth deficiency occurring during infancy and early childhood. It is attributable to psychosocial and/or biological factors. Generally, length, weight, and height are below the 3rd percentile for age. There may be environmental factors contributing to the disorder including inadequate nutrition, inconsistent parenting, and environmental neglect. Organic disease is present in less than 50% of children typically diagnosed with FTT.

BIOMECHANICAL ASPECTS OF GROWTH

Mechanical forces, mediated by genetic and chemical factors, influence the child’s growth from the earliest weeks of development.

  1. Plasticity of adjacent tissues leads to complex series of motions that either constrain or allow growth.
  2. Movement is an essential factor to complete joint development during the second and third prenatal months.
      Fetal akinesia sequence - results in multiple congenital contractures
      Spina bifida infant may demonstrate less movement
  3. Muscle cells align developmentally according to the direction of pull placed upon them. Muscles appear more compliant than bone to force and thus respond to stretch by lengthening.
  4. Bone appears to be noncompliant as compared to muscle. Wolff’s law describes the association between bone structure and mechanical demands placed on it.
  5. Deforming forces appear to have much greater impact on the lower extremity than the upper extremity in development.
Examples of joint and bone malalignment:
  1. Scoliosis and kyphoscoliosis - may develop prenatally due to the development of wedge-shaped vertebral bodies or fused vertebrae. The most common reasons appear to be related to asymmetrical muscle pull or lack of support of the trunk related to paralysis. Cerebral palsy, spina bifida, spinal muscle atrophy, poliomyelitis, osteogenesis imperfecta, and muscular dystrophies are all contributing conditions to joint or bone malignment.
  2. Coxa valga- increased angle of inclination of the femoral neck relative to the femoral shaft when viewed in the frontal plane. Abnormal stresses may negatively influence acetabulum develop and contribute to an increased risk of hip dislocation.
  3. Coxa vara - decreased angle of inclination of the femoral neck relative to the femoral shaft when viewed in the frontal plane. Decreased pressure on the acetabulum and increased bending stress on the femoral neck leads to predisposition for femoral neck fractures and slipped capital femoral epiphysis.
  4. Femoral anteversion - anterior rotation (version or torsion) in the transverse plane of the upper femur. Infantile anteversion is present in cerebral palsy children with in toeing gait patterns.
  5. Femoral retroversion - posterior rotation in the transverse plane of the upper femur. Generally not seen as a drawback to the normal person, and is an advantage for ballet dancers desiring hip external rotation.
  6. Genu valgus, varus - increased lateral (valgus), or medial (varus) knee angulation in the frontal plane. It may be due to hamstring, quadriceps, hip adductor, or hip abductor weakness or spasticity.
  7. Medial, lateral tibial rotation - tibia rotates in relation to femur at the knee joint. It is very common in neonates and is often due to ligamentous laxity.
  8. Medial, lateral tibial torsion - rotation within the tibial shaft leading to spiral development of the bone associated with muscle imbalance of the knee-ankle and ankle-foot muscles especially in children with spastic cerebral palsy.

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