Keywords
pyramids - echogenicity - anuria - renal medulla - echogenic sludge
Key Message
The sonographic findings of increased medullary echogenicity caused by deposits of
urinary sediments, at the tip of the medullary pyramids and observing their gradual
clearance with improving urinary output, highlights the value of focused high-resolution
ultrasound in the management of this transient situation, causing reversible acute
renal failure in the newborn.
Introduction
Almost 95% newborns start urinating within 24hours of life, and nearly always by 48hours.
When urination is delayed, or is scanty, it leads to concern for any urinary abnormality
and further investigations including ultrasound is advised for appropriate management.
The renal medulla and pyramids in neonates normally appear hypoechoic, relative to
the cortex. Various pathophysiologic processes in newborns and children can cause
increased echogenicity of the renal medulla and its pyramids.[1] It has become possible to visualize these appearances using focused ultrasound with
high-frequency transducers.[2] We present the report of a newborn who was anuric till 48hours of life and while
on treatment in the hospital, was monitored serially by using focused high-frequency
ultrasound, till complete resolution of the anuria.
Case History
A 48-hour-old male neonate was brought to the pediatric outpatient department with
complaints of not having passed urine since birth. The baby had a domiciliary delivery
by a trained birth attendant. He cried immediately after birth, was on exclusive breastfeeding
but the mother noticed that he was sleeping quietly most of the day, was not suckling
too well, and hardly demanding any feeds.
On admission (day 1), the baby appeared dehydrated with a temperature of 37.1°C, pulse
rate 142 beats per minute, respiratory rate 45 breaths per minute, and the oxygen
saturation 98% on room air. The blood pressure was 62/33mm Hg. Systemic examination
did not reveal any physical abnormality. No abdominal or pelvic mass was felt. His
nappy pad was dry.
The laboratory parameters showed a blood urea level of 29mg/dL, serum creatinine 3.3mg/dL,
serum sodium 156mEq/L, serum potassium 4.8mEq/L, and C-reactive protein 0.28mg/dL.
The blood gases were within normal limits. Complete blood counts were also in the
normal range for newborns.
An urgent bedside ultrasound was performed by the resident on call, using a Philips
CX 50 portable unit having a convex C5–1 and linear L12–3 probe. It did not reveal
any evidence of urinary tract obstruction or any other gross pathology.
After two fluid boluses (40mL per kg), administered on admission, the baby was started
on maintenance fluids (100mL per kg) without supplemental potassium. Since the urine
output remained low, the maintenance fluids administration was guided by insensible
water loss for a full-term neonate.[3] (Insensible water loss is loss of water through skin, mucous membranes, and respiratory
tract.)[3] Antibiotics were empirically started. Serial renal function tests, serum electrolytes,
and urine output are shown in [Table 1].
Table 1
Serial laboratory parameters, urine output, and weight of the neonate
|
Day 1
|
Day 2
|
Day 3
|
Day 4
|
Day 5
|
Day 6
|
Day 7
|
Day 8
|
|
Urea (mg/dL)
|
29
|
39
|
45
|
44
|
38
|
23
|
11
|
6
|
|
Creatinine (mg/dL)
|
3.3
|
3.25
|
2.82
|
2.27
|
1.93
|
1.11
|
0.7
|
0.49
|
|
Sodium (mEq/L)
|
156
|
141
|
144
|
143
|
141
|
142
|
138
|
140
|
|
Potassium (mEq/L)
|
4.8
|
5.25
|
5.05
|
6.5
|
5.9
|
4.9
|
5.0
|
5.2
|
|
Urine output (mL)
|
10
|
24
|
10
|
30
|
35
|
75
|
135
|
100
|
|
Weight (kg)
|
2.44
|
2.52
|
2.6
|
2.8
|
2.88
|
2.95
|
2.88
|
2.86
|
The following day (day 2) as well as on subsequent days (till day 5), follow-up ultrasonographic
evaluation was performed in the main imaging department using one of the readily available
high-end machines (Samsung RS 80A, Philips Iu 22, or GE Voluson 8). The examination
was performed with a convex pediatric probe followed by high-frequency linear array
probe (11–16MHz), focusing on the renal parenchyma, especially the renal pyramids.
During the procedure the neonate was kept warm and jelly, heated to room temperature,
applied before scanning. The examination was performed in the supine, decubitus, and
prone positions, the latter allowing adequate visualization of the kidneys without
the obstructive effect of bowel gas shadows. Since the footprint of some of the linear
probes was relatively large, both kidneys could be examined simultaneously in the
transverse plane. This aided in side-by-side comparison of the kidneys in a single
image.
The images were magnified and the focal zone suitably adjusted. Normally, the renal
medulla, composed of several pyramids, appears hypoechoic in a newborn. In this case,
the medulla showed increased echogenicity concentrated more at the tip of the pyramids,
as compared with the base ([Fig. 1]). The bladder was almost empty at the time of this scan (day 2).
Fig. 1 Day 2. Focused scan of the kidney, using a high-frequency linear probe with a trapezoidal
field of view, shows increased medullary echogenicity concentrated at the tip of the
renal medulla (arrows).
This finding of selective concentration of echogenicity at the tip of the renal pyramids,
with relatively clear hypoechoic bases, unlike in a case of medullary nephrocalcinosis,
enabled us to suggest the possibility of transient renal medullary echogenicity as
a cause of acute renal failure. The neonatologist, taking cognizance of the ultrasound
report, continued with the standard treatment protocol of dehydration, which was based
on laboratory parameters and clinical status of the patient.
On day 3, the echogenic sludge was found gravitating slowly into the calyces and pelvis
([Fig. 2]). A small quantity of urine with echogenic debris appeared in the urinary bladder.
The baby had also started passing small quantities of urine in the nappy. Scan done
on day 4, showed the calyces were nearly clear with the echogenic debris now mainly
in the renal pelvis ([Fig. 3]). On day 5 ([Fig. 4]), most of the pyramids showed normal hypoechoic appearance, barring a few residual
echogenic foci within some of them. The pelvis had also cleared significantly. The
urine in the partially filled bladder was now larger in quantity and clearer with
a few echogenic mobile foci still visible within it ([Fig. 5]). The baby was discharged on 8th day in a good condition. He was followed up in
the outpatient department and being in good health, was not subjected to any further
investigations.
Fig. 2 Day 3. The echogenic deposits are seen in the calyces (arrow). Note the clearing of the medulla (black star).
Fig. 3 Day 4. The deposits have shifted from the calyces into the renal pelvis (arrows).
Fig. 4 Day 5. The renal medulla now shows a normal hypoechoic appearance with a few residual
echogenic crystals still visible within them (arrows).
Fig. 5 Day 5. The bladder shows improved filling with echogenic crystals floating within
it (arrows).
Discussion
Ultrasound plays an important role in the evaluation of the urinary tract. The presence
or absence of kidneys, their sizes, uni- or bilateral hydronephrosis, dysplastic lesions,
focal or generalized medullary hyperechogenicity, and evidence of bladder outlet obstruction,
aid in diagnosing the many conditions, including those causing anuria.[1]
With the availability of advanced ultrasound machines, the sonographic appearance
of neonatal kidneys can be studied in great detail, especially by using a high-frequency
probe (11–16MHz) and applying a focused approach. By focused ultrasound approach,
is meant that a small area of the kidney is magnified and studied in detail with the
highest frequency transducer available. Unlike a routine ultrasound scanning, as was
performed at bedside, ultrasound with high-resolution probes, enables macroscopic
evaluation of the structure of the renal parenchyma.[2] A high-frequency convex or better still, a linear array probe focusing on the area
of interest,[2] yields exceptional images.
The kidneys can be evaluated in decubitus or prone positions. The prone position often
works well, as some babies are more comfortable in this position and both kidneys
can be compared side by side.
Neonatal kidneys have an appearance different from adult kidneys.[4] The cortex in neonates is as echogenic as the liver, even more so in preterm babies.[5] The reason is compaction of glomeruli and loops of Henle within a smaller area.
This leads to more interfaces and increased echogenicity.[6] It usually takes 4 to 6 months for the cortical echogenicity to reduce to a normal
lower level when compared with the liver.
The renal medulla of neonates is larger as compared with the older age group and hypoechoic
([Fig. 6]). The renal sinuses are devoid of fat in early neonatal period and do not appear
echogenic. The tip of a pyramid of the renal medulla is enclosed by a calyceal cup,
the walls (fornices) of which appear echogenic when opposed to each other. They appear
isoechoic, when spread apart by urine. Neonates with a normal ultrasound appearance
as described have a normal urine output within the first 24hours.[1]
Fig. 6 Normal kidney of a neonate showing echogenic cortex and hypoechoic medulla (arrows).
The increased echogenicity, as was seen in the reported case, appeared to be due to
deposition of endogenously produced substances at the tip of the renal papilla and
was slow to clear, hence causing transient outflow obstruction. As the glomerular
filtration rate began improving, the echogenic sludge cleared from the base of renal
pyramid, shifting toward the tip. The base of the pyramids therefore showed a normal
hypoechoic appearance. This process of clearance took several days.[7] The echogenic debris gradually emptied into the calyces, then gravitated into the
pelvis and flowed out into the bladder, being ultimately washed off by urination.
The entire process of clearance of the echogenic debris from the urinary tract, could
be witnessed during real-time scanning. It coincided with improvement in renal status
of the patient.
Neonatal renal failure is a serious condition and has been attributed to severe perinatal
asphyxia, sepsis, disorders of the urinary tract, and various nephrotoxic drugs.[8] The outcome is often poor and may lead to death or long-term complications in survivors.
However, a condition called “neonatal transient renal failure associated with medullary
hyperechogenicity” has been described in literature and has a favorable outcome.[7] Makhoul et al described a case series of 9 neonates of this benign entity with excellent
outcome.[7]
Transient medullary hyperechogenicity is seen in 3.9 to 58% neonates.[9] While the clearance of the echogenic debris, observed in real-time ultrasound, with
rapid improvement in the clinicopathological status in a matter of few days, precludes
any obstructive pathology, the etiology of transient renal medullary echogenicity,
is still unclear.[10]
It is probably a physiologic event but several alternatives have been suggested.[9] Based on the reviewed literature and our own experience, we compiled [Table 2], which is a summary of the likely etiologies of increased renal medullary echogenicity.[11]
[12]
Table 2
Differential diagnosis of increased neonatal renal medullary echogenicity
|
Conditions
|
Clinical factors
|
Ultrasound features
|
|
Autosomal recessive polycystic kidney disease (ARPKD)
(ADPKD may also mimic ARPKD in some cases)
|
May be detected in prenatal ultrasound. May be syndromic
(Beckwith–Wiedemann syndrome)
|
Enlarged kidneys with diffusely hyperechoic pyramids with dilated ducts forming cystic
space. Presence of numerous discrete echogenic foci casting a comet tail shadow
|
|
Lesch-Nyhan syndrome
|
Increased urate crystal deposits
The drug allopurinol used for treatment may also, as a side effect, be a causative
factor, by depositing xanthine crystals in the pyramids
|
Diffuse echogenicity of the pyramids with central sparing
|
|
Glycogen storage disease
|
Type 1 is commonly associated with renal involvement
|
Nephromegaly with increased corticomedullary echogenicity secondary to glycogen, urate,
and calcium deposition
|
|
Sickle cell anemia
|
Older children in the first decade, usually affected. Unlikely in newborns
|
Increased echogenicity of the entire kidney/throughout medulla/central medulla or
tips of pyramid[14]
|
|
Obstructive uropathy
|
Longer course of illness. May accompany pain, UTI, hematuria
|
Features of obstructive uropathy with curvilinear medullary echogenicity around the
dilated calyces
|
|
Acute renal failure
|
H/o sepsis, prematurity, birth anoxia, drug intake, etc.
|
Findings may mimic the ultrasound findings or transient increase in echogenicity
|
|
Hyponatremic state
|
Low serum sodium levels
|
May show identical US findings
|
|
Renal medullary nephrocalcinosis of the neonate
|
Longer course of the disease
Patients may be normocalcemic or hypercalcemic
|
Diffuse/peripheral increase in echogenicity of the renal medulla without / with slight
shadowing. The central area and extreme periphery of the medulla may be spared even
in the peripheral type of increased echogenicity.
Echogenicity may involve the cortex also. Ultrasonographic and clinic state lasts
longer
|
|
Ischemia
|
H/o perinatal asphyxia
|
Mild diffuse heterogeneously increased echogenicity. Exclude renal vein thrombosis
|
|
Candida infection
|
Urinary tract infection, catheterization, immunodeficiency, congenital urinary tract
abnormalities
|
Hyperechoic pyramid, resembling transient increased echogenicity of the renal pyramids.
Echogenic non shadowing ball-like sloughed off papilla/mycetoma in the renal outflow
tract
|
|
Transient increased echogenicity of uncertain etiology[15]
|
May not be associated with any biochemical abnormality
|
Diffusely echogenic renal medulla[11]
|
|
Transient increased echogenicity related to maturation of tubules
|
Diminished urinary output
|
Hyperechoic tips of renal papilla only.[14] May be the same or a similar entity
|
Abbreviations: H/o, history of; US, ultrasound; UTI, urinary tract infection.
Note: Summary of the postulated etiologies of increased renal medullary echogenicity
in children (11, 13, and Authors' experience).
The main considerations were urinary tract obstruction and renal medullary nephrocalcinosis[2]:
-
In urinary tract obstruction, increased medullary echogenicity may be observed as
a curvilinear rim encircling the dilated calyces. This could be due to the stacking
up of obstructed dilated pericalyceal tubules, creating many reflective interfaces
as in multiple small renal cysts, thereby resulting in increased medullary echogenicity.[13]
-
In renal medullary nephrocalcinosis, there is either diffuse ([Fig. 7]) or peripheral increase in renal medullary echogenicity with central sparing ([Fig. 8]), unlike in this reported case which showed increased echogenicity predominantly
localized at the tips of the renal pyramids with relatively clear bases that were
abutting the renal cortex. It is a disease which lasts longer and may be associated
with hypercalcemia. The ultrasound findings are, therefore, different in this disorder.
-
The neonate had hypernatremia, hence hyponatremic dehydration causing increased medullary
echogenicity could be discounted.
-
The possibility of deposition of endogenous substances (likely uric acid) has been
postulated to be one of the causes, leading to tubular stasis and transient acute
renal failure.[6] Hyperuricemia has been documented in many of these neonates.[6] Uric acid crystalluria has been observed in up to 70% of these neonates. The possibility
of urate crystals deposited in the papilla could not be excluded since we could not
obtain urine samples.
-
Deposition of Tamm–Horsfall protein produced by tubular cells has also been postulated
by some authors, but could not be substantiated in this patient.
-
Perinatal asphyxia resulting in renal hypoperfusion may also be a factor by causing
hyperuricemia and tubular cell damage. As the newborn was delivered at home, perinatal
asphyxia might have contributed to the renal failure. Inappropriate initiation of
feeding may also have aggravated the renal failure in the patient.
Fig. 7 A case of nephrocalcinosis, showing diffusely echogenic renal medulla (arrows).
Fig. 8 Another case of nephrocalcinosis, showing echogenicity in periphery of the renal
medulla with central sparing (arrows).
In conclusion, we present this case report because it highlights the utility of focused
high-resolution ultrasound in the diagnosis and management of patients of transient
acute renal failure in neonates. The ultrasonographic findings of increased renal
medullary hyperechogenicity, due to stasis of echogenic sludge at the tip of the renal
papilla, followed by gradual clearance with correction of dehydration and improvement
in the urinary output, were highly suggestive of this unusual entity causing transient
renal failure in the newborn. That the appearance of the kidneys normalized in a few
days, underlines the importance of recognizing this condition causing transient stasis
nephropathy.[13] The ultrasonologist is in a good position to recognize this condition thereby contributing
substantially in the management of these patients.
Learning Points
-
Ultrasound, especially focused high-resolution ultrasound, is an invaluable tool in
the investigation of renal anomalies, in neonates and infants.
-
The sonographic features of transient oliguria/anuria due to a benign self-limiting
cause can be different from other more serious conditions such as renal medullary
nephrocalcinosis.
-
Presence of echogenic sludge mainly concentrated at the tip of the renal pyramids
with gradual clearance, as the glomerular filtration rate improves, enables suggesting
this transient cause of acute renal failure in the neonate.
-
Follow-up ultrasound may be required only if clinical or laboratory findings raise
a red flag.
