Introduction

Whilst navigating the waters of medical jargon, you may be wondering about the differences between two seemingly similar conditions. Pyelectasis and hydronephrosis have been used interchangeably in many instances, and that’s indicative of the overlapping nature of the two terms. However, there are differences in diagnostic test findings, the underlying causes, and incidences of both conditions. In this article, we will explore how pyelectasis and hydronephrosis differ from each other.

Understanding renal anatomy and function

The kidneys, each lying on either side of the posterior abdomen (retroperitoneal), typically extend from the T12 to L3 horizontal planes of the vertebral column. Due to the presence of the liver on the right side of the abdomen, the right kidney is positioned slightly lower than the left. Above each kidney are the adrenal glands, which are also enveloped by the renal fascia.

As expected of human anatomy, the internal structure of the kidneys is, indeed, intricate. The renal parenchyma encompasses two essential areas: the cortex (outer layer) and the medulla (inner layer). The renal cortex mainly contains the units of filtration, known as nephrons. Extensions of the cortex continue along the medulla, which gives the medulla its distinct pyramidal/triangular sections. These are known as renal pyramids.

The products/urine leaving the renal pyramids enter a small region known as the minor calyx and then the major calyx. As minor calyces merge to create a major calyx, the two structures are not physically separate, but a continuation of each other. The products within the major calyces are facilitated into a central point within the kidney, known as the renal pelvis; this is essentially where all the fluid (urine) finally collects, before being funnelled out through the renal pelvis and down the ureters. The ureters are a conduit leading to the urinary bladder, at which urine is stored, until there is a signal or need for urination.

The main functions of the kidneys involve filtration of the blood and excretion of waste products. The kidneys are also responsible for water and electrolyte balance. As a result, the kidneys can have an impact on several aspects, such as blood pressure regulation, water output/loss, arrhythmias (due to electrolyte imbalances), and metabolic changes.

What is pyelectasis?

Pyelectasis, also known as pelviectasis, refers to the dilatation/ enlargement of the renal pelvis, specifically.^1,2 This is the part of the kidney where urine collects before passing into the ureter and then to the bladder. Reports of pyelectasis, where the term is used accurately and in relation to renal pelvis findings only, are commonly found in foetuses during antenatal ultrasounds/ scanning. It typically resolves on its own before birth or even shortly afterwards, so pyelectasis is considered a relatively benign condition. Overall, pyelectasis only happens in around 1% of all pregnancies, with a higher incidence in male foetuses.³

As most cases of pyelectasis are seen in the prenatal period, much of the current scientific literature focuses on foetal pyelectasis.¹ Given the spontaneous nature of this condition, the cause for foetal pyelectasis (in most cases) is still unknown. However, there is usually no associated pathology of concern. Interestingly, some reports showed that pyelectasis can appear and disappear multiple times throughout pregnancy.^2,4 

What is hydronephrosis?

Hydronephrosis refers to the dilatation of the renal pelvis and the calyces (i.e., minor calyces and the major calyx), due to an accumulation of urine.^1,5 Like pyelectasis it can affect one or both kidneys and is often detected by ultrasound scans. However, hydronephrosis can affect people of any age and can be an acute or chronic presentation. The typical clinical presentation of hydronephrosis includes flank pain, or abdominal pain, urinary symptoms and signs of kidney dysfunction. Hydronephrosis in adults tends to have an underlying cause:

• Kidney stones
• Maternal hydronephrosis (i.e., pregnancy)
• Ureter obstruction or narrowing: due to infection, injury, other masses or surgery.
• Benign prostatic hyperplasia/ enlarged prostate.
• Congenital abnormalities
• Some cancers or tumours e.g., renal, bladder, prostate, and ovarian cancers

Another word that may be used in place of hydronephrosis is pelvicaliectasis (not to be confused with pelviectasis), which also refers to a dilatation of the renal pelvis and calyces, but the two words are, in fact, not synonymous.⁶ Hydronephrosis describes such dilation but only in the context of an obstructive cause. If no underlying obstruction can be identified, pelvicaliectasis may be a more suitable term to use.

Differentiating pyelectasis and hydronephrosis

Diagnostic approaches

Imaging is essential for the identification of pyelectasis and hydronephrosis. This will allow for visualisation of the kidneys and other parts of the renal tract, which can reveal or hint towards any underlying causes or pathologies. Options for imaging studies include ultrasound scans, computerised tomography (CT) scans, and magnetic resonance imaging (MRI). An ultrasound scan is a quick, effective, and relatively cheap method of identifying fluid accumulation in the body; thus, it can be used to identify the presence of hydronephrosis and pyelectasis. Given that ultrasound scans are also routinely performed during pregnancy, antenatal scans can detect such abnormalities.⁷ As mentioned earlier, involvement of the renal pelvis alone would be pyelectasis and involvement of both the renal pelvis and the calyces would indicate hydronephrosis. CT and MRI scans may not be necessary imaging modalities; however, they can be useful in visualising the renal tract and may show obstructions or complications of hydronephrosis, which would not be picked up by ultrasound scanning.

In some literature, there are references to classifications of renal pelvic dilations. For pyelectasis, as it’s mainly seen in foetuses, more recent publications seem to be in support of gestation-dependent thresholds, rather than set values applied across different ages.^4,7 Whilst it is unclear as to what degrees of pyelectasis would be considered mild, moderate or severe, most authorities have adopted the threshold for severe pyelectasis to be 6mm.² For hydronephrosis, the thresholds are also different. The Society of Foetal Urology (SFU) developed a grading system from 0 to 4 to assess the extent of hydronephrosis⁽⁸⁾. However, this was originally devised for neonatal and infant hydronephrosis. According to Radiopaedia, the validity and reliability of all hydronephrosis grading systems are not warranted, so classifying the degree of hydronephrosis is a challenge.⁸

Treatment

As mentioned earlier, most cases of foetal pyelectasis are self-resolving; this could be during or shortly after pregnancy. Thus, treatment is not usually required. If there are concerns or the diagnosis is not very clear, there may be close monitoring with repeat ultrasounds. Interventions (surgical) will be required if there are severe blockages, or obstructions which are unlikely to self-resolve or reflux.³ In some cases, pyelectasis can eventually become hydronephrosis once it involves the renal calyces. Babies might be monitored for potential hydronephrosis.

The treatment of hydronephrosis depends on the underlying cause and its severity. In adults, placing a urinary catheter into the bladder or the kidneys (severe or complicated cases) is often the first line of treatment. This drains urine out, which should relieve symptoms of retention.⁹ Further treatment may be needed to target the cause of this retention. For example, medications or surgery may be indicated for an enlarged prostate or surgery may be indicated for removal of large kidney stones. In the case of antenatal hydronephrosis, this is likely to resolve over time. Under specialist advice, a child may be prescribed antibiotics to reduce the risk of urinary tract infections, until the scans show that there are no more findings of hydronephrosis.

Untreated hydronephrosis, whether it presents antenatally, in children or adults, can result in abdominal pain, discomfort, urinary retention, kidney damage, and infection. An infection could eventually lead to sepsis and therefore urgent medical attention and monitoring are required.

Summary

Whilst pyelectasis and hydronephrosis are used interchangeably in some instances, the two are actually different. Pyelectasis refers to the dilatation of the renal pelvis, whilst hydronephrosis involves both the renal pelvis and renal calyces. In a way, pyelectasis and hydronephrosis could be regarded as findings on different parts of a continuous spectrum, indicating some kind of renal dilatation and fluid/urine accumulation within the kidneys.