Renal Pyramids: Focused Sonography of Normal and Pathologic Processes

Introduction

Sonographic evaluation of the kidneys includes detailed depiction of the renal parenchyma, pelvicaliceal system, and vasculature. There are descriptions of the echogenicity of the normal parenchyma (cortex and medulla) and changes that may occur in pathologic processes (1,2). However, in our experience, we have found that there is a variety of physiologic and pathologic processes that may cause a spectrum of changes in the echogenicity of the renal medullary pyramids, as depicted with high-frequency transducers; these changes have received little attention in the literature.

The purpose of this article is to illustrate the normal appearances and some examples of the various patterns of altered echogenicity that may occur in the renal pyramids in neonates, infants, and older children and to provide pathologic correlation when available. We emphasize the technique and role of focused sonographic evaluation of the pyramids with high-frequency transducers, as this produces the most detailed images of the pyramids; such images are the most helpful in understanding the underlying disease and may help in establishing an accurate diagnosis and facilitate management. By using this focused technique, it is possible to depict alterations in the pyramids in much greater detail than previously documented, and this technique may reveal changes previously not reported.

Sonographic Technique

Detailed anatomic depiction of the renal parenchyma (cortex and pyramids) requires dedication to a meticulous sonographic technique in an attempt to define the normal anatomic appearances and pathologic processes to best advantage.

A routine set of images showing only the whole kidney in each image constitutes an inadequate examination. Although these images may be useful to define the geography of the kidney as a whole and its relationship to adjacent structures, these images will not necessarily provide the detailed imaging required to evaluate the parenchyma adequately.

Limiting the field of view to the half of the kidney closest to the transducer and focused evaluation of only one or two pyramids and the surrounding cortex often helps resolve areas of interest in the pyramids better. This improved resolution is best achieved with linear-array transducers functioning at a high megahertz range (even up to 17 MHz) (3). Focal zone position must be adjusted to be at the level of the region of most interest in the image.

This focused sonographic technique will provide more detailed images in neonates and young infants than in older children simply because of the shorter distance between the transducer and the kidneys. For this reason, much of what is illustrated in this article pertains to the neonatal kidney. Similarly, imaging may be facilitated in many renal transplants if the abdominal wall is not thick because the kidney then often lies closer to the transducer than do native kidneys in older children.

Normal Anatomy

The sonographic appearance of the kidneys in older children is similar to that described in adults. In contrast, however, the sonographic appearance of the kidneys in the neonate differs from that in older children for several reasons. Fetal lobulation is usually quite prominent in the neonate, giving the kidney a more lobulated appearance than in older children (Figs 1, 2). Neonates have much less renal sinus fat than in older children, and the central renal sinus echogenicity is uncommonly appreciated at this age.

Figure 1

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The immature neonatal cortex is very echogenic; in contrast to that in older children, it is hyperechoic relative to the liver—even more so in premature neonates (2). Because of this, sonography depicts corticomedullary differentiation extremely well (Fig 2). Postnatally, the echogenicity of the cortex gradually decreases, and it is usually hypoechoic relative to the liver by 4 months of age, but occasionally this process takes until 6 months. Normal renal pyramids are hypoechoic relative to the renal cortex, independent of patient age, and are more echogenic than normal urine in the collecting system, which is anechoic (Fig 2). Images obtained with high-frequency transducers in neonates (as well as in older children) may depict striations in the cortex owing to the anatomy of the cortical vasculature and in the medulla, where they are due to the collecting ducts (Figs 1, 2).

Compared with the cortex in older children, the immature cortex in the neonate is thinner relative to the size of the pyramids. Therefore, the pyramids appear relatively large (2) (Fig 2).

A small echogenic focus or double, parallel, linear echogenicities at the base of the pyramids represent the arcuate arteries (Figs 1, 2). On either side of the apex of the pyramids, in the region of the papillae, one may see a hyperechoic band, which represents the apposed walls of the calix if it is not distended with urine (Figs 1, 2). If the calix is distended, the papilla will be surrounded by anechoic urine (Fig 2).

Causes of Altered Echogenicity of the Renal Pyramids in Children

In neonates and children, focused sonographic examinations with high-frequency transducers may depict a spectrum of unique changes in echogenicity of the renal pyramids owing to the effects of physiologic or a wide variety of pathologic processes. These are listed in Table 1, which highlights the clinical settings in which we have documented detailed changes in the echogenicity of the pyramids seen in our practice. However, it should not be considered a complete list, as an even wider variety of changes may well be depicted in the future if more investigators adopt this focused technique. As we have expanded our use of this focused sonographic technique, we have been able to depict altered echogenicity in the pyramids in greater numbers of children, in whom an explanation for the changes is not immediately apparent and we are, until now, forced to consider the cause as idiopathic. In this section, we discuss the clinical settings listed in Table 1 and attempt to offer explanations for the changes in echogenicity in the pyramids and provide pathologic proof when available.

Transient Hyperechogenicity of the Pyramids in Neonates and Infants

A transient increase in the echogenicity of the pyramids commonly seen in neonates is the result of physiologic events in the postnatal period (4–8). The prevalence of this finding has been found to vary in normal neonates from 3.9% up to 58% (6).

In this clinical setting, the zone of maximal hyperechogenicity is seen at the apex of the pyramids or papillae, and this may extend (with progressively decreasing echogenicity) up to approximately halfway up the pyramid (Fig 3). The base of the pyramid is usually spared and remains hypoechoic. Usually multiple pyramids are involved, but the finding may not affect all the pyramids and occasionally only one pyramid may be affected. This increased echogenicity is transient and usually resolves in a few days, when the neonate begins to increase the glomerular filtration rate physiologically. However, it has been reported to take up to 10 days to resolve (4), and, in our experience, it may take even longer in premature neonates who are in intensive care units. In rare instances, hyperechoic debris may also be present in the bladder and collecting systems in these neonates, and this debris is thought to represent the same material present in the pyramids (5).

Figure 3

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The same pattern of increased echogenicity may be seen in neonates with renal failure and those with hypernatremic dehydration who may have oliguria at the time of presentation (7,9–12). This finding in such patients is good evidence that the cause of the oliguria is not an underlying primary renal disease. As in the normal neonate, this increased echogenicity is transient and usually disappears after 24–48 hours when the infants have been rehydrated and urine output returns to normal. However, the changes may also sometimes last for up to even 7 days despite good urine output (9,10).

The cause of this transient increased echogenicity was originally thought to be deposition of Tamm-Horsfall protein in the tubules or interstitium of the pyramids (9). Avni et al (9) postulated this based on the observation that this glycoprotein was found in the urine of neonates recovering from acute renal failure, as the degree of hyperechogenicity in the papillae decreased to normal. The same transient findings in normal neonates without renal failure, which were described only subsequently, were presumed to have the same cause (13). However, evidence for this is limited. Some authors have suggested that deposition of urate crystals may account for this transient increased echogenicity, although evidence for this is also lacking (4,7,14).

It may be multifactorial, and indeed different causes may be at play in each of the aforementioned clinical settings.

Obstruction

It has been well documented that severe obstruction of the urinary collecting system or vesicoureteral reflux may lead to changes in the renal parenchyma, which may be recognized sonographically as increased echogenicity of the parenchyma, loss of corticomedullary differentiation, or dysplastic cortical cysts (15). Urinary tract obstruction in neonates and infants may also lead to two patterns of abnormal echogenicity of the renal pyramids; these patterns have only recently been recognized in the literature by Chavhan et al (16). One pattern is a band of increased echogenicity in the pyramidal tissue, which becomes attenuated around markedly dilated calices (Fig 4). The second pattern is fluid-filled tubular structures in the pyramids adjacent to calices that may not necessarily be dilated as a result of the obstruction (Fig 5).

Figure 5

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The band of increased echogenicity is probably due to increased acoustic interfaces as a result of slight dilatation of the collecting ducts in the apex of the pyramid that is attenuated around the markedly dilated calix (Fig 4). This explanation correlates with observations at CT and excretory urography of stasis of contrast material, which accumulates in a crescent shape (Dunbar crescents at urography) in the pyramids draining into markedly dilated calices (17) (Fig 4).

The second sonographic pattern, depicted as frankly distended tubular structures in the affected pyramids, is due to a greater degree of dilatation of the collecting ducts and is less frequently observed than are the hyperechoic bands (16) (Fig 5). In such affected kidneys, the distended ducts may not be present in all the pyramids; the reason why some of the pyramids are affected in this way and others are not is unclear (Fig 5). It may be due to the anatomy of the pyramid and calix and how the ducts are anatomically aligned. It is also unclear why some kidneys display this pattern of duct dilatation when the degree of caliceal dilatation is usually not as marked as in those kidneys that display the pattern of the hyperechoic band (Fig 4).

Both the band of increased echogenicity and the dilated ducts disappear sonographically after relief of the obstruction (16) (Figs 4, 5).

Ischemia

Diminished renal function due to renal ischemia is not an uncommon occurrence in stressed neonates in intensive care units who have experienced hypoxic or ischemic events (12). Impaired renal perfusion may lead to abnormalities of the pyramids, cortex, or renal vasculature, which include changes such as acute tubular necrosis, medullary or cortical necrosis, and renal vein thrombosis (12). Indeed, in many instances, all of the listed pathologic processes may be present concomitantly in an affected kidney.

There is limited information in the literature about the sonographic appearances of these pathologic processes in neonates. Abnormal increased echogenicity of the cortex or decreased echogenicity of the pyramids is difficult to appreciate in this age group because the cortex is normally relatively hyperechoic, particularly in premature neonates, and the normal pyramids are relatively large and hypoechoic (Figs 1, 2). Despite this, the focused sonographic technique of the pyramids has enabled us to depict significant changes in the echogenicity of the pyramids in some neonates with medullary necrosis (Fig 6) and changes in corticomedullary differentiation in those with more extensive necrosis of both the pyramids and cortex. Correlation with postmortem gross specimens and histologic evaluation of the kidneys has enabled us to appreciate the causes of the various changes in echogenicity seen ante mortem (Fig 6). We have also observed changes in the appearance of the medullary pyramids in renal transplants in patients with acute tubular necrosis; these changes consist of mild, diffuse, heterogeneous increased echogenicity (Fig 7). More studies are required to determine whether the sonographic appearances of these entities are specific in order to convey the most accurate information to the clinicians for appropriate management.

Figure 7

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There is limited information available about the echogenicity of the medullary pyramids in renal venous thrombosis (18–20). However, this information reveals that there is a spectrum of changes in the echogenicity of the pyramids, and the appearance varies depending on the severity of involvement of the kidney, the timing of the examination, and the presence of concomitant disease in the kidney such as acute tubular necrosis and ischemia (18–20). Although in some instances the initial findings may be extremely subtle and even overlooked, the affected kidney usually becomes enlarged and heterogeneous in echogenicity during the acute phase. Corticomedullary differentiation may be completely lost, or in some cases the pyramids may become profoundly hypoechoic or hyperechoic (Fig 8). The thrombi may be directly visualized in the main renal vein as well as in the intrarenal branches, particularly the interlobular branches, along the margins of the pyramids. Serial sonograms may show that these thrombi become hyperechoic as they calcify, and this leads to a lacelike pattern of increased echogenicity along the margins of the pyramids and sometimes more extensively in the parenchyma.

Infection

Candida species are saprophytic fungi in humans and under certain conditions may become pathogenic. In neonatal intensive care units, Candida species are common causes of urinary tract infections, and premature infants with decreased immunity are particularly susceptible to Candida albicans infection (21). Infants with indwelling catheters, immunodeficient states, or congenital urinary tract malformations or undergoing hyperalimentation or prolonged antibiotic or immunosuppressive therapy are also prone to infection with this type of Candida (21,22).

There is little information in the literature about the sequence of sonographic findings in renal candidiasis (21–23). The sonographic manifestations are divided into two groups: parenchymal involvement and fungus balls in the collecting systems. These two groups of findings may coexist in a given patient and in many instances may represent different stages of a sequence of events, with initial involvement of the cortex followed by involvement of the straight tubules and papillae and eventually formation of fungus balls. This has been demonstrated experimentally in mice after hematogenous dissemination of Candida (24).

In those cases with involvement of the pyramids, the sonographic changes are more frequently seen in the region of the papillae, which become hyperechoic (Fig 9). This finding may be difficult to differentiate from transient hyperechogenicity of the pyramids in neonates and from nephrocalcinosis, particularly in premature neonates, who are at risk for both conditions (see pertinent discussions). Correlation with positive blood or urine cultures for Candida species is extremely helpful in this situation. The papillae may eventually slough and mycetomas, often referred to as fungus balls, may develop in the collecting system. These fungus balls appear as nonshadowing hyperechoic foci within the collecting system, although sterile aggregates of debris may mimic this appearance (23).

Figure 9

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Polycystic Renal Diseases

Polycystic renal diseases (eg, autosomal recessive polycystic disease, Beckwith-Wiedemann syndrome) may give rise to a spectrum of altered echogenicity, which is often more obvious in the pyramids than in the cortex. These changes include (a) increased echogenicity of the pyramids due to mild tubular distention; (b) macroscopic tubular distention, which eventually leads to the development of small macroscopic cysts; and (c) hyperechoic foci due to crystal deposition. These changes vary in severity from patient to patient and may be diffuse or focal.

Autosomal Recessive Polycystic Kidney Disease.—Autosomal recessive polycystic kidney disease (ARPKD) is the most common inherited renal cystic disease that manifests in childhood, with an estimated prevalence of one in 20,000 live births (25). It is characterized by cylindrical and saccular dilatation of the collecting tubules, which is always accompanied by periportal hepatic fibrosis.

The sonographic depiction of dilated tubules, often radially distributed in relation to the kidney as a whole and arranged in parallel columns, shows good correlation with the histologic findings described in ARPKD and has been found to be useful in differentiation of this condition from other entities that may also show cyst formation in the kidneys (3,25). This tubular dilatation is often bilateral and affects the whole kidney, although focal and unilateral renal involvement has also been described (25).

Figure 11

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The sonographic depiction of cysts in ARPKD is common (Figs 10, 11). These cysts share the same pathogenesis of the tubular dilatation and usually appear as small cysts randomly scattered throughout the kidneys; they predominantly affect the medullary pyramids but may also be found in the cortex. Another common sonographic feature seen in ARPKD is the presence of punctate hyperechoic foci, 1–3 mm in size and often casting ring-down artifact, found in the areas of tubular dilatation (Figs 10, 11). These hyperechoic foci are thought to represent crystalline material, likely calcium compounds, and seem to be associated with the development of renal failure (25–27).

Autosomal Dominant Polycystic Kidney Disease.—Autosomal dominant polycystic kidney disease (ADPKD) comprises at least three phenotypically similar but genetically distinct entities. In this group of conditions, multiple scattered discrete cysts are usually noted in the cortical and medullary regions of the kidneys; the involvement is generally bilateral but may be asymmetric (3). The intervening parenchyma not involved by the cysts, including spared pyramids, has a normal sonographic appearance. Although the changes are probably already present at birth in all cases, they may not become evident at sonography until later in life.

In rare instances, particularly when it manifests in the perinatal period, ADPKD may appear as diffuse increased echogenicity of the renal parenchyma with loss of corticomedullary differentiation, resembling the appearances of ARPKD (27). We have found that with use of a focused sonographic technique, some of these perinatal cases of ADPKD show changes limited to the medullary pyramids, with macrocysts and tubular ectasia resembling the autosomal recessive form very closely (Fig 12).

Beckwith-Wiedemann Syndrome.—Beckwith-Wiedemann syndrome has heterogeneous causes and clinical presentations and can be briefly defined as an overgrowth disorder. The diagnosis is usually made in the presence of macroglossia, anterior abdominal wall defects, and macrosomia. However, the list of clinical features that can be seen in Beckwith-Wiedemann syndrome is quite extensive (28,29). The main indication for these patients to undergo abdominal sonography is screening for embryonal tumors, particularly Wilms tumor in the kidneys. However, there are other nonneoplastic abnormalities in the kidneys in Beckwith-Wiedemann syndrome that can be recognized at sonography (Fig 13). These reflect histologic and metabolic abnormalities that have been described in these patients, including dilatation of the distal tubules, often referred to as renal medullary dysplasia, and nephrocalcinosis that appears to be related to hypercalciuria, which has been demonstrated in 22% of patients with Beckwith-Wiedemann syndrome (28,29).

Figure 13

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At sonography, the tubular dilatation manifests as increased echogenicity of the pyramids due to an increase in acoustic interfaces (Fig 13). This tubular dilatation may progress to the formation of scattered cysts, which are usually simple in appearance but occasionally may be complex in echogenicity and therefore may mimic a neoplastic mass (28). Another cause of increased echogenicity of the pyramids in these patients is nephrocalcinosis.

Metabolic Causes

Nephrocalcinosis.—The most common metabolic abnormality responsible for histologic changes in the pyramids that may be depicted as increased echogenicity at sonography is nephrocalcinosis (30–32). Although nephrocalcinosis primarily affects the renal pyramids, it may occasionally be appreciated in the cortex as well (33,34). There are many causes of nephrocalcinosis in children, the most common of which are listed in Table 2 and have been categorized according to the presence or absence of elevated levels of calcium in the blood and urine (30–32,34–37). However, the discussion of each individual cause is beyond the scope of this article.

In 1979, Bruwer (38) used the term Anderson-Carr-Randall progression to describe a pattern of nephrocalcinosis leading to renal calculi formation; the pattern is based on the radiologic and histologic findings of these authors. According to this pattern, the nephrocalcinosis occurs most prominently in the papillae and along the margins of the papillae adjacent to the fornices of the calices (38). It is in this area that the highest levels of calcium are found. As the nephrocalcinosis progresses, the calcium deposits erode into the calices, leading to early calculus formation. In children, the sonographic equivalent of this pattern is seen primarily in premature neonates and much less commonly in older children (39).

In 1986, Patriquin and Robitaille (39) described four sonographic patterns of medullary nephrocalcinosis. The first pattern is a band of increased echogenicity occurring in the periphery of the pyramid with sparing of the central part of the pyramid. The second pattern is a more hyperechoic periphery with faint increased echogenicity of the center of the pyramid. The third pattern consists of marked increased echogenicity throughout the whole pyramid. The fourth pattern is characterized by a solitary focus of increased echogenicity at the tip of the pyramid along the margins of the fornix. These authors also used the term Anderson-Carr-Randall progression to refer to all four of these patterns of nephrocalcinosis. However, it is only the fourth pattern that conforms strictly to the Anderson-Carr-Randall progression as described by Bruwer (38). The first three patterns are different and probably represent a progressive sequence of increasing calcium deposition and are not usually seen as a progression from the fourth pattern.

The first three patterns described by Patriquin and Robitaille (39) are what is usually depicted if one uses older sonographic equipment or if one performs the examination with lower-megahertz transducers. However, we have shown with focused sonography that it may not be the most peripheral part of the pyramid that initially becomes hyperechoic (Fig 14). The most peripheral part of the pyramid often remains hypoechoic, and the zone of increased echogenicity is just internal to this. Furthermore, focused sonography with high-frequency transducers shows that the zone of increased echogenicity is not simply a solid band but is made up of multiple punctate hyperechoic foci, which represent the intratubular crystal deposition of calcium (Fig 15). If greater amounts of calcium are deposited, the echogenic foci may even adopt a linear configuration as they follow the course of the tubules (Fig 16).

Although nephrocalcinosis may eventually progress to involve most of the pyramid, acoustic shadowing is rarely seen even in this situation (Fig 14). The lack of shadowing may reflect the way in which the calcium is laid down within the pyramid (40). Acoustic shadowing may be appreciated only in rare cases of extreme involvement of the pyramids (Fig 17) or if there is development of associated calculi in the adjacent calices. In the latter instance, it may be difficult to determine how much of the increased echogenicity is related to the nephrocalcinosis in the pyramid and how much is due to the stone in the adjacent calix.

Abnormal renal function may not be present even with diffuse involvement of the entire pyramid (41). Follow-up sonographic examinations of kidneys affected with diffuse nephrocalcinosis may show no change in the sonographic appearances of the pyramids despite resolution of the cause of the nephrocalcinosis (42). This lack of change has been shown experimentally in rabbits with phosphate-induced nephrocalcinosis and has also been our clinical experience. One illustrative example is nephrocalcinosis related to Williams syndrome, which is associated with hypercalcemia in the first months of life. In these patients, the calcium levels become normal in the second half of the 1st year of life, although the nephrocalcinosis, once established, is permanent throughout childhood (43). Our experience in clinical practice as well as previous animal studies have also shown that the changes in the pyramids due to nephrocalcinosis are more easily appreciated at sonography than at CT, which is less sensitive even when well-developed changes are seen at sonography (44).

Several cases of medullary nephrocalcinosis associated with renal cysts have been reported, mainly in the nephrology literature (45–48). Most of these patients had distal renal tubular acidosis (45), but medullary nephrocalcinosis associated with renal cysts has also been seen in children with apparent mineralocorticoid excess syndrome (46), Bartter syndrome type III (47), and cardiofaciocutaneous syndrome (48). The exact mechanism of the formation of these cysts remains controversial and may not be the same in each patient. The cyst formation is usually attributed to chronic hypokalemia secondary to the preexisting condition in most of these children or to nephron obstruction caused by the nephrocalcinosis itself (45,47). Only one case has been reported in the radiology literature (48), to our knowledge. However, we have personally seen three such cases in children, all of whom had well-established bilateral nephrocalcinosis and in whom sonography showed the development of medullary cysts. At follow-up, the medullary cysts showed an increase in number and size with eventual involvement of the cortex (Fig 17).

In the very early stages of calcium deposition, sonography may reveal uneven and asymmetric involvement of the pyramids. However, these patterns of increased echogenicity due to nephrocalcinosis eventually involve all of the pyramids in both kidneys quite evenly and symmetrically. Navarro et al (49,50) have shown that persistent, true unilateral or marked asymmetric involvement of the kidneys may occur if there is unilateral obstruction of the main renal artery or vein or the ureter. In these clinical settings, it is the kidney affected by the vascular or ureteric obstruction that is unaffected or markedly less affected by the nephrocalcinosis (49,50). These obstructions may protect the kidney by causing a decrease in the blood perfusion and therefore a reduction of the high levels of calcium to which the contralateral kidney is exposed (49,50).

Specific mention should be made of a particular clinical setting associated with uneven and asymmetric increased echogenicity in the pyramids; this finding occurs in preterm infants and is also due to nephrocalcinosis (51). It causes a pattern of increased echogenicity that differs from the others, as it is initially seen most markedly in the apical or papillary region of the pyramid and at the interface between the papilla and calix (Fig 18) (40,52). This finding appears to correspond to the pattern of nephrocalcinosis originally described as the Anderson-Carr-Randall progression by Bruwer (38) and appears to be the type 4 pattern described by Patriquin and Robitaille (39). As it progresses, the process may involve the pyramid more diffusely and may also cause formation of small calculi in the calices adjacent to the affected papillae (40). Follow-up sonographic examinations in affected neonates often show complete resolution of these changes as the neonates mature. However, the time for resolution varies considerably (53–55).

The exact cause of the nephrocalcinosis in this group of neonates appears to be multifactorial (52,54–56). The diuretic drug furosemide, a known hypercalciuric agent, was originally thought to be the main etiologic factor, but its dose and length of therapy may not be related to the severity of the abnormality seen in the kidneys at sonography; in fact, there are reports of nephrocalcinosis occurring in preterm infants who have never received furosemide (54,56). More recently, it has been appreciated that other factors in the neonatal period appear to play a role in the development of this type of nephrocalcinosis, including elevated urinary excretion of oxalate and uric acid, decreased urinary excretion of citrate, use of dexamethasone, parenteral nutrition, long-term ventilation, slightly impaired acid-base homeostasis, disturbed mineralization, and immature or impaired renal function (52,54).

However, the patterns are not specific for the cause of the increased echogenicity (30).

Urate Crystal Deposition.—Increased echogenicity of the pyramids can also occur as a result of deposition of urate crystals in the collecting tubules in association with interstitial nephritis (30,57,58) (Fig 19). This finding has been described in adult patients with gout and in children with Lesch-Nyhan syndrome, who have in common high blood levels of uric acid. It has also been speculated that allopurinol, a drug used to treat hyperuricemia, may also be involved in causing increased echogenicity of the pyramids by producing xanthine crystal deposits in the kidneys as a side effect (57,58).

Figure 19

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Glycogen Storage Disease Type 1.—Glycogen storage diseases are inherited disorders of the metabolism of glycogen. Although there are over 12 forms of this condition, type 1 glycogen storage disease is among the most common types associated with renal involvement (59). The enzyme defects present in this entity lead to excessive accumulation of glycogen in the liver, kidney, and intestinal mucosa (60). Renal findings described in this condition include nephromegaly, high renal blood flow, proximal or distal tubular dysfunction, focal segmental glomerulosclerosis, amyloidosis, Fanconi-like syndrome, nephrocalcinosis, nephrolithiasis, and hyperuricosuria (59,60).

Sonographic appearances of the kidneys include bilateral enlargement, increased cortical echogenicity, and increased medullary echogenicity (Fig 20). The increased echogenicity is secondary to nephrocalcinosis and urate crystal deposition as well as glycogen deposition, which at biopsy has been demonstrated to occur in the tubular epithelial cells (60,61).

Figure 20

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Sickle Cell Disease

Increased echogenicity of the renal cortex and medullary pyramids is well documented in children with sickle cell disease (30,62–65). In the cortex, the increased echogenicity is the result of glomerular hypertrophy and later glomerular and interstitial fibrosis (62). The increased echogenicity of the pyramids in children with sickle cell disease is the result of vascular congestion in the pyramids, as has been proved at histologic analysis in one patient (30). However, papillary necrosis has also been implicated as a pathogenic factor, since the papilla is the very site in the renal medulla where the countercurrent mechanism creates an environment favorable for sludging of erythrocytes in the vasa recta (66).

Whatever the pathophysiology involved in the increased echogenicity of the medullary pyramids in sickle cell disease, the sonographic changes are not evident in infants and first develop only in older children during the first decade of life (62). The literature describes several patterns of abnormal renal echogenicity in sickle cell disease that can be found by using low-frequency sector-array transducers: (a) diffuse increased echogenicity throughout the kidney with loss of corticomedullary differentiation; (b) increased echogenicity involving the central region of the renal parenchyma, including the pyramids and the cortex between the pyramids; (c) increased echogenicity confined to the periphery of the medullary pyramids; and (d) increased echogenicity confined to the medullary pyramids but occurring uniformly throughout the pyramids (62).

Focused sonography of the pyramids confirms that there is indeed a spectrum of patterns of increased echogenicity in children with sickle cell disease (Fig 21). The increased echogenicity may be evenly diffuse throughout the pyramid or may be present only centrally in the pyramid or only peripherally (Fig 21). However, we are still uncertain of the reason for these differences in sonographic patterns and how they correlate with differing histologic changes in the pyramid.

Conclusions

This article has shown that in neonates and children, focused sonographic examinations of the pyramids may depict a spectrum of unique changes in echogenicity due to the effects of physiologic processes or a wide variety of pathologic processes that may affect the collecting ducts or the interstitium of the pyramids. The article illustrates that the patterns of altered echogenicity are optimally depicted with high-frequency transducers, particularly linear-array transducers. We have highlighted the clinical settings in which we have documented detailed changes in the echogenicity of the pyramids seen in our practice. However, this should not be considered a complete list, as an even wider variety of changes may well be depicted in the future if more investigators adopt this focused technique.

The patterns of altered echogenicity alone may reflect a specific cause but in many instances are nonspecific, with clinical and biochemical correlation required to establish a more precise diagnosis. However, there is a lack of histologic data to completely explain the mechanism of many of these changes in echogenicity in all of the processes. As we have expanded our use of this focused sonographic technique, we have been able to depict altered echogenicity in the pyramids in greater numbers of children in whom an explanation for the changes is not immediately apparent, and we are forced to consider the cause as idiopathic. More work is required to expand the use of this focused technique together with clinical, biochemical, and histologic correlation in an attempt to offer more complete explanations for the changes in echogenicity in the pyramids.