Potter syndrome pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Pathophysiology
Terminology: Syndrome vs. Sequence
Potter Syndrome is not technically a syndrome as it does not collectively present with the same telltale characteristics and symptoms in each and every case. It is more accurately described as a "sequence" or chain of events that may have different beginnings (absent kidneys, cystic kidneys, obstructed ureters or other causes), but which all end with the same conclusion (absent or reduced volume of amniotic fluid) . This is why Potter Syndrome is often called Potter Sequence or Oligohydramnios Sequence by some clinicians and researchers. The term Potter Syndrome is most frequently associated with the condition of oligohydramnios sequence regardless of the root cause of the absence or reduced volume of amniotic fluid. However, as noted in this article, the term Potter Syndrome was initially coined in order to refer to fetuses and infants with BRA. It was not until later that the term became more encompassing as it was noted that other causes of failed fetal urine production also resulted in similar physical characteristics and prognoses of the fetuses and infants with BRA (that which Potter originally described in 1946). Since then, the term Potter Syndrome has become a misnomer and experts have attempted to not eliminate the terminology, but to modify it in a way so as to be able to determine the different root causes by creating a nomenclature system. However, this classification system has not caught on in the clinical and research fields.
Classic form
Classic Potter Syndrome occurs when the developing fetus has bilateral renal agenesis, which also presents with agenesis of the ureters. BRA has been estimated to occur at a frequency of approximately 1:4000 to 1:8000 fetuses and neonates. However, recent analysis has estimated that the condition may occur at a much greater frequency. The condition has been reported to occur twice as common in males as in females, suggesting that certain genes of the Y chromosome may act as modifiers. However, no candidate genes on the Y chromosome have yet been identified.
BRA appears to have a predominantly genetic etiology and many cases represent the most severe manifestation of an autosomal dominant condition with incomplete penetrance and variable expressivity. There are several genetic pathways that could result in this condition. To date, few of these pathways or candidate genes have been considered or analyzed regarding BRA. The majority of possible candidate genetic pathways are autosomal recessive in nature and do not coincide with the frequency or penetrance at which BRA occurs in the human population. Additionally, candidate genetic pathways would be expected to involve genes expressed in the developing urogenital system (UGS). Often, these same genes and/or pathways of interacting genes are also expressed in the developing UGS as well as the Central Nervous System (CNS), gut, lung, limbs, and eyes.
Normal kidney development
See kidney development.
Importance of fetal urine
Development of the mature kidney begins between weeks 5 and 7 of gestation. Fetal urine production begins in early gestation and comprises the majority of the amniotic fluid in the second and third trimesters of pregnancy. The fetus continuously swallows amniotic fluid, which is reabsorbed by the gastrointestinal tract and then reintroduced into the amniotic cavity by the kidneys via urination. Oligohydramnios occurs if the volume of amniotic fluid is less than normal for the corresponding period of gestation. The fetal urine is critical to the proper development of the lungs by aiding in the expansion of the airways - alveoli, by means of hydrodynamic pressure and by also supplying proline which is a critical amino acid for lung development. Alveoli are the small sacs in the lungs that exchange oxygen with the blood. If the alveoli, and thereby the lungs, are underdeveloped at the time of birth the infant will not be able to breathe air properly and will go into respiratory distress shortly after birth due to pulmonary hypoplasia (underdeveloped lungs). This is the primary cause of death to Potter syndrome infants secondary to renal failure. The fetal urine also serves to cushion the fetus from being compressed by the mother's uterus as it grows.