Essence and classification of multifactorial diseases
Multifactorial diseases (or diseases with hereditary predisposition) never arise solely due to defective genetics or exclusively under the influence of an unfavorable environment. Their manifestation always requires the coincidence of two conditions: the presence of a genetic basis and the action of provoking external triggers.
Depending on how many genes form the predisposition to the disorder, two main groups of such diseases are distinguished:
- Monogenic — depend on mutations in one specific gene.
- Polygenic — caused by a complex combination of multiple genes.
Typical examples of multifactorial pathologies include the most common problems: coronary artery disease (CAD), hypertension, diabetes mellitus (DM), bronchial asthma, peptic ulcer disease, various rheumatic and psychiatric disorders, as well as congenital malformations (CM).
Etiology of congenital malformations (CM)
Congenital malformations serve as a vivid illustration of multifactorial pathology. They are caused by teratogenic factors — exogenous or endogenous agents that interfere with histogenesis and organogenesis processes, disrupting normal growth and development of the fetus throughout pregnancy.
All CM risk factors are traditionally divided into two broad categories:
- Endogenous factors. Directly related to the state of the parents' organism. These include the age of the mother and father, various anomalies of the germ cells themselves, and the exposure to internal mutagens. In addition, a huge role is played by pathological substances formed during metabolic and endocrine disorders in a pregnant woman.
- Exogenous (environmental) factors. They account for about 10% of all known risk factors. By their nature, they can be:
- Physical (e.g., radiation exposure).
- Chemical (certain medications, narcotic substances, adverse environmental chemical factors).
- Biological (viral infections such as rubella, herpes, cytomegalovirus infection, as well as the causative agents of syphilis and toxoplasmosis).
Important nuance: the unborn child's susceptibility to any teratogens is not a constant value. It is strictly tied to the current stage of embryonic and fetal development.
Chronological classification of malformations
Depending on the gestational age at which the pathogenic factor acted, all anomalies are divided into four main groups. It is the time of exposure that determines the scale and nature of the damage.
- Gametopathies. The targets are the germ cells themselves even before conception. The result is malformations caused by mutations in the parental genome.
- Blastopathies. The blow hits the blastocyst in the first 15 days after fertilization, when the formation of germ layers is not yet complete. Typical consequences: the appearance of conjoined twins or cyclopia (the presence of one or two fused eyeballs in a single orbit along the midline of the face).
- Embryopathies. Occur when teratogens affect the embryo in the period from the 16th day to the 8th–9th week. At this time, the main organs are laid down. Examples: alcoholic, diabetic, thalidomide, and drug-induced embryopathies, as well as severe malformations due to the rubella virus.
- Fetopathies. Develop when the already formed fetus is damaged (from the 9th week of intrauterine development until birth). These include cryptorchidism, patent ductus arteriosus (Botallo's duct), as well as prenatal hypoplasia of individual organs or the fetus as a whole.
Pathogenesis
The basis of the development of any multifactorial anomalies and congenital malformations is a profound failure at the microlevel. Pathogenesis consists of a gross distortion of normal intercellular and intermolecular interactions. Because of this, the delicate mechanisms of morphogenetic processes break down — tissues and organs are laid down incorrectly, grow with anomalies, become deformed, or completely stop in their development.
Briefly: answers to topical questions
What developmental anomalies are characteristic of blastopathies?
Blastopathies are characterized by pathologies arising in the first 15 days from the moment of zygote formation (disorders of implantation, orientation of the embryoblast, and development of extraembryonic organs). The main anomalies include:
- Conjoined twins — incompletely separated symmetrical or asymmetrical twins (twin malformations).
- Cyclopia — the presence of one or two fused eyeballs in a single orbit along the midline of the face.
- Empty gestational sacs — occur as a consequence of aplasia or early death of the embryo with its subsequent resorption.
- Ectopic pregnancy — the result of impaired implantation.
What specific anomalies and diseases belong to fetopathies?
Fetopathies include pathological conditions arising from fetal damage in the period from the 9th week of intrauterine development until birth. Specific examples include:
- Cryptorchidism.
- Patent ductus arteriosus (Botallo's duct).
- Prenatal hypoplasia of any organ or the fetus as a whole.
- Diabetic fetopathy: ultrasound signs are indicated in the source, including fetal dimensions >75th percentile and disproportionate fetal sizes.
- Infectious fetopathies: infection after the 3rd month of pregnancy leads to their development. When the fetus is infected with cytomegalovirus infection at late stages, true malformations do not form; the disease manifests from the first days of life as a severe acute infectious process with multiorgan lesions.
What is the mechanism of the teratogenic action of chemical factors (drugs) on the fetus?
The mechanism of the teratogenic action of chemical factors and drugs includes:
- Indirect action of chemical agents: they cause pathological changes in the mother's body.
- Direct action: chemical agents directly penetrate the placental barrier.
- Genotoxic action: the damaging effect of chemicals on DNA; it can manifest as a teratogenic effect.
- Impairment of embryonic and/or fetal development: teratogenic factors disrupt development throughout pregnancy, affecting histo- and organogenesis, fetal growth and development.
- Pathogenetically, this is associated with the distortion of intermolecular and intercellular interactions, which leads to the disruption of morphogenetic processes.
For drugs from the source affecting hemostasis and thrombosis, it is indicated: they easily cross the placental barrier, exert a teratogenic effect, and inhibit the formation of osteocalcin — a vitamin-K-dependent bone tissue protein, which leads to impaired skeletal formation in the fetus.
To remember the chronology of malformations, use the phrase "Where Was This Factor?" (in Russian «Где Был Этот Фактор?»): Gametopathies (before conception), Blastopathies (up to 15 days), Embryopathies (up to 9 weeks), Fetopathies (until birth).
Class and exam questions
What determines the severity and nature of a congenital defect?
First of all, on the gestational age at which the teratogenic factor acted, as well as on the stage of embryonic or fetal development at that moment.
How does embryopathy differ from fetopathy?
Embryopathies occur when the embryo is damaged from the 16th day to the 8th–9th week of pregnancy, and fetopathies occur when the fetus is affected after the 9th week and up to the moment of birth.
Which risk factors for congenital malformations are endogenous?
These include mutagens, germ cell anomalies, parental age, and substances formed in the mother's body due to endocrine or metabolic disorders.
What else to study on this topic
- Mechanisms of formation of polygenic and monogenic predisposition
- The impact of specific viral infections (rubella, cytomegalovirus) on organogenesis
- Pathogenesis of thalidomide and alcohol embryopathies
- Features of the development of diabetic fetopathies
- The role of intercellular interactions in normal morphogenesis
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