Definition and Classification of Multifactorial Diseases
Multifactorial diseases (or diseases with hereditary predisposition) never arise exclusively from defective genetics or solely under the influence of an adverse environment. Their manifestation always requires two conditions: a genetic background and exposure to triggering environmental factors.
Depending on the number of genes forming the predisposition, these disorders are divided into two main groups:
- Monogenic — caused by mutations in a single specific gene.
- Polygenic — caused by a complex combination of multiple genes.
Typical examples of multifactorial pathologies include the most common conditions: ischemic heart disease (IHD), essential hypertension, diabetes mellitus, bronchial asthma, peptic ulcer disease, various rheumatic and psychiatric disorders, as well as congenital malformations.
Etiology of Congenital Malformations
Congenital malformations serve as a clear illustration of multifactorial pathology. They are caused by teratogenic factors — exogenous or endogenous agents that interfere with histogenesis and organogenesis, disrupting normal fetal growth and development throughout pregnancy.
All risk factors for congenital malformations are traditionally divided into two major categories:
- Endogenous factors. Directly related to the parents' physiological state. This includes maternal and paternal age, various abnormalities of the germ cells themselves, and the impact of internal mutagens. In addition, pathological substances generated by metabolic and endocrine disorders in the pregnant woman play a huge role.
- Exogenous (environmental) factors. They account for about 10% of all known risk factors. By nature, they can be:
- Physical (e.g., radiation exposure).
- Chemical (certain medications, drugs, adverse environmental chemicals).
- Biological (viral infections such as rubella, herpes, cytomegalovirus infection, as well as the causative agents of syphilis and toxoplasmosis).
Important note: the unborn child's susceptibility to any teratogen is not a constant value. It is strictly linked to the current stage of embryonic and fetal development.
Chronological Classification of Developmental Defects
Depending on the gestational age at which the pathogenic factor acts, all anomalies are divided into four main groups. The timing of exposure determines the scale and nature of the damage.
- Gametopathies. The target is the germ cells prior to conception. The results are defects caused by mutations in the parental genome.
- Blastopathies. The impact affects the blastocyst in the first 15 days after fertilization, before the formation of the germ layers is complete. Typical consequences include conjoined twins or cyclopia (the presence of a single or fused eyeball in a single median facial orbit).
- Embryopathies. Occur when teratogens affect the embryo from the 16th day to the 8th–9th week. During this time, major organs are laid down. Examples include alcohol, diabetic, thalidomide, and drug-induced embryopathies, as well as severe defects due to the rubella virus.
- Fetopathies. Develop when damage occurs to an already formed fetus (from the 9th week of intrauterine development until birth). These include cryptorchidism, patent ductus arteriosus (Botallo's duct), and prenatal hypoplasia of individual organs or the fetus as a whole.
Pathogenesis
The foundation of any multifactorial anomalies and congenital malformations is a profound disruption at the micro-level. 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 abnormally, become deformed, or completely halt their development.