The MRI spectrum of congenital cytomegalovirus infection

Abstract Human cytomegalovirus (CMV) is an ubiquitous pathogen, with a high worldwide seroprevalence. When acquired in the prenatal period, congenital CMV (cCMV) is a major cause of neurodevelopmental sequelae and hearing loss. cCMV remains an underdiagnosed condition, with no systematic screening implemented in pregnancy or in the postnatal period. Therefore, imaging takes a prominent role in prenatal diagnosis of cCMV. With the prospect of new viable therapies, accurate and timely diagnosis becomes paramount, as well as identification of fetuses at risk for neurodevelopmental sequelae. Fetal magnetic resonance imaging (MRI) provides a complementary method to ultrasound (US) in fetal brain and body imaging. Anterior temporal lobe lesions are the most specific finding, and MRI is superior to US in their detection. Other findings such as ventriculomegaly, cortical malformations and calcifications, as well as hepatosplenomegaly, liver signal changes and abnormal effusions are unspecific. However, when seen in combination these should raise the suspicion of fetal infection, highlighting the need for a full fetal assessment. Still, some fetuses deemed normal on prenatal imaging are symptomatic at birth or develop delayed cCMV‐associated symptoms, leaving room for improvement of diagnostic tools. Advanced MR sequences may help in this field and in determining prognosis, but further studies are needed.

and asymptomatic newborns remain at risk of sequelae. 7 Long-term sequelae are expected in 40%-60% of symptomatic survivors, and 10%-20% of asymptomatic children. 7,[11][12][13] This number may be much higher if longer follow up would be routinely available and accounted for lesser symptoms such as delayed cognitive, motor or language development, or behavioral issues, in some cases only detected at the start of school age. 14 Despite extensive literature available, cCMV remains an underdiagnosed condition particularly in cases not associated with characteristic prenatal/neonatal findings or with late-onset symptoms, delaying initiation of treatment. This is partially related to the lack of universal CMV screening of pregnant women, which is not recommended given the lack of proven effective therapy during gestation. 15 Until recently there were no viable options to prevent maternalfetal CMV transmission or improve outcome of infected fetuses, leading prenatal imaging to focus on detecting existing central nervous system (CNS) anomalies associated with dismal developmental outcomes. With the introduction of in utero treatment options, identifying fetuses at risk for neurodevelopmental sequelae becomes more relevant, although one should acknowledge the fact that this treatment options are still of experimental nature and have potential side effects. 16,17 A significant percentage of fetuses deemed normal on prenatal imaging become symptomatic at birth or develop delayed cCMV-associated symptoms, leaving room for improvement. This, together with increased awareness and the shifts in paradigms of transmission, screening and treatment a review on this topic was warrant, focusing on MRI and advances in imaging, and how these may contribute to counseling of cCMV patients.

| METHODS
We identified published studies and review papers related to congenital CMV infection by searching the MEDLINE database for Englishlanguage articles published from 1962 (first available article) through July 2019 using the keywords "cytomegalovirus" or "CMV" and "congenital." References of the selected articles were further checked for papers that may have escaped the primary search. Priority was given to information published in higher impact journals and studies with clear methodology and high number of subjects over case reports, case series or convenience samples. All relevant literature used can be found in the reference section of this paper.

| Infection
CMV is part of the Herpesviridae family. It is transmitted via smear infection, through direct contact of mucous surfaces with infectious body fluids, 17,18 establishing a lifelong latent infection in the host, with periodic reactivations which can be a source of disease. 19 The prevalence of cCMV ranges from 2 to 20 per 1000 live births, depending on factors such as geography and socioeconomical status. 6,[20][21][22][23][24] Several factors contribute to the burden of cCMV in childhood, both in terms of morbidity and mortality. These include the limited awareness of parents and healthcare professionals, [25][26][27] underrecognition of infection (often asymptomatic in pregnant women and newborns), lack of established screening programs, delayed-onset of sequelae (making retrospective diagnosis challenging), and the absence of vaccines and limited proven efficacy of current treatments. 16,22 Maternal infections are described as primary (no preconceptional immunity) vs nonprimary (preconceptional immunity; including reactivations and infections by a different strain). This division has been used to stratify the risk of fetal transmission as well as the severity of sequelae. 10 However, the dogma that prior maternal immunity can protect against symptomatic cCMV infection has been challenged 10,22 by former 28 as well as recent studies, 20,21,[29][30][31][32][33][34][35][36][37][38] and the impact of quantitative preconceptional immunity in intrauterine CMV transmission rates remain undefined. 10,22 In some prospective studies the risk of long-term neurodevelopmental sequelae has been shown to be comparable in primary and nonprimary infections. 30,33,34,[39][40][41] A unique feature of cCMV is that its prevalence increases with an increase in CMV prevalence in the maternal population, without reaching a level at which cCMV incidence falls. 10,42,43 As such, the high seropositivity in the antenatal maternal population means that a high percentage of congenitally affected babies are born to immune mothers. 44 What is already known about this topic?
• Congenital cytomegalovirus (CMV) infection is a leading cause of congenital sensorial-neural hearing loss and developmental delay.
• Congenital CMV remains underdiagnosed, partially due to lack of screening and awareness, but also due to unrecognized prenatal findings suspicious for CMV infection in prenatal imaging.
• Despite unspecific, ultrasound and magnetic resonance imaging (MRI) findings are essential in diagnosis and prognostication.
What does this study add?
• This study provides an overview of fetal brain and body MRI findings.
• It presents illustrative images and tables to facilitate performance and interpretations of fetal MRI in the context of congenital CMV.
• Application of advanced MR sequences in overcoming limitations of structural brain imaging.
Timing of seroconversion/fetal infection is a strong predictor of postnatal sequelae 9,12,40,[45][46][47] with periconceptional and early gestational associated with a higher risk of symptomatic disease at birth. 39,48 Populations at higher risk of fetal transmission throughout pregnancy include women with underlying immunity deficits primary or acquired. 10,49 cCMV is the most common nongenetic cause of SNHL, accounting for 25%-30% of sensorineural hearing loss in childhood. 50 Longterm follow up studies have determined that 8%-10% of patients will present neurodevelopmental abnormalities, independent of the presence of symptoms at birth. 51,52 Maternal or fetal screening is currently not recommended or performed, although a recent American study established the costeffectiveness of universal screening in pregnancy, granted CMV incidence is at least 0.82%, or even lower if behavioral interventions to prevent transmission and further pharmacological interventions are being offered. 15 The current consensus is that serology tests (IgG, IgM, IgG avidity) should be offered to pregnant women who develop flu-like symptoms not attributable to another specific infection, or when imaging findings (ultrasound [US] or magnetic resonance imaging [MRI]) are suggestive of cCMV. 16 Intrauterine infection can be confirmed by polymerase chain reaction detection of CMV DNA in the amniotic fluid (AF) after 21 gestational weeks (GW) and at least 6 weeks after infection, as a sufficient time is required for placental crossing and for fetal renal function to become sufficient for viruria to be detectable. 9 Patient education and institution of hygiene measures have an effectiveness of up to 85% in prevention of primary CMV. 53,54 If maternal infection cannot be prevented, the most widely spread treatment options are hyperimmune globulin 55,56 and antiviral drugs (valacyclovir, valganciclovir) to reduce the likelihood of fetal transmission or ameliorate fetal sequelae, 55-60 although no randomized controlled trials have proven a definitive benefit.
As such, treatment options should be discussed on an individual basis.

| The role of MRI in congenital CMV
Imaging in cCMV has two main objectives: detection of fetal structural anomalies for correct diagnosis and provision of prognostic information.
In the absence of universal screening, detecting and raising the possibility of a fetal CMV infection is paramount. This raises the importance of AF or newborn testing, allowing fetal treatment in selected cases, and avoiding a lengthy and costly diagnostic journey for children with cCMV born with nonspecific symptoms (mean specificity of neonatal symptoms: 12%) or developed delayed symptoms. 5 Further, an accurate diagnosis helps parents in future reproductive decision making, and reduce the stress and anxiety from an uncertain diagnosis.
While US is the method of choice for fetal imaging, MRI has an established added value in the detection of fetal brain anomalies in several conditions, 61 and has a growing potential for the evaluation of structural but also functional as well as metabolic imaging methods.
Detection of cCMV-related CNS anomalies is a predictor of poor outcome among infected fetuses, 62 while a normal neurosonographic examination performed by expert sonographers is a good predictor of normal neurodevelopmental outcome. 63,64 MRI has also been used in multiple studies concerning CMV, with a high negative predictive value (96.8%-99%) for neurological impairment and SNHL. 65,66 As such, familiarity with the typical findings is essential. Furthermore, there is sparse literature regarding body findings in cCMV on fetal MRI. Below we aim to summarize and illustrate the most relevant MRI findings.

| Central nervous system MRI
Prenatal imaging in cCMV is primarily focused on the brain, given the neurotropism of the virus. 67 On a cellular level, cCMV may infect an array of cell types, including neurons, astrocytes, radial glia and endothelial cells, and may result in a number of insults to neuronal proliferation, migration, and cortical cell organization. [67][68][69][70] From an imaging point of view, visible (macroscopic) damage is a late finding that can show variable and progressive (and some regressive) features on prenatal imaging.
MRI findings in cCMV are often unspecific, with ventriculomegaly and white matter (WM) signal abnormalities being the most commonly described CNS anomalies. 71 More characteristic (but less frequently observed) features include temporal lobe lesions (abnormal WM, cysts, and enlargement of the temporal horns), ventriculitis and intracranial calcification. 66,72 A summary of the main findings and their MRI characteristics can be found in Table 1.
Ventricular anomalies are often found in cCMV patients ( Figure 1). Ventriculomegaly is mild to moderate (<15 mm) in most patients. 72 Severe ventriculomegaly (>15 mm) on fetal MRI is associated with worse prognosis ( Figure 2). 73 Extreme forms of ventriculomegaly may mimic acqueductal stenosis-related hydrocephalus, or less common presentations such as mega-cisterna magna. 74 Ventricular septations/adhesions, also called pseudocysts, are another nonspecific finding. 72,74,75 They are more common in the occipital horns and can be identified as thin strands of tissue crossing the ventricles on T2-weighted images ( Figure 3); longer echo times may be needed to detect these structures, and T2w FLAIR may improve visualization of intraventricular lesions. 76,77 Pseudocysts in the caudothalamic groove or in the lateral aspect of the frontal horns have also been described 66 and all seem to carry a good prognosis when isolated. 66,73 Other cystic lesions particularly in the temporal pole region or when involving the WM are associated with neurological sequelae. 13,65 T2-weighted (T2w) hyperintensities of the WM are a subjective diagnosis and difficult to interpret, particularly in the third trimester, when T2 inhomogeneities are often found in the normal brain ( Figure 4, Supporting Information Figure S1). When interpreting T A B L E 1 List of possible magnetic resonance imaging (MRI) findings in fetuses with congenital cytomegalovirus (cCMV) F I G U R E 1 Fetal magnetic resonance imaging (MRI) at 29 gestational weeks referred for microcephaly and splenomegaly. There is parenchymal loss, with global thinning of the cerebral mantle and consequent bilateral ventriculomegaly. Small areas of focal signal anomaly can be detected on T2WI (A, B, black arrows), as well as frontal polymicrogyria (A, white dashed arrow). Calcifications can be identified on T1WI (c, white arrows). Enlarged spleen (gray arrow) and liver (asterisk) can be identified on T2W steady state free precession (D), echo planar imaging (E) and T1W images (F), with slight signal intensity anomaly on the latter positions, resulting in an appearance of too many infoldings for gestational age 80 ; a thickened cortical ribbon with irregular gyration and possible blurry gray/WM margins may also be identified on T2WI ( Figures 1 and 5). 80 Vascular injury (by direct endothelial infection or placental involvement) at critical time points of development is associated with the fetal brain disruption sequence. 68 Severe injury may also lead to clefts. 83 Both schizencephaly and porencephaly appear as a transmantle cleft extending from the ventricular lining to the cerebral surface. The former is lined by a cortical ribbon, usually polymicrogyric, 80 while the latter results from destructive changes occurring after the completion of neuronal migration and thus the cleft is not lined with gray matter. 84 Calcifications are a classical finding in cCMV, typically described in US. Intracranial calcifications are more common in the periventricular region, but can also occur in the basal ganglia, as a manifestation of lenticulostriate vasculopathy, and in the brain parenchyma. They present as areas of low T2w and high T1w signal, and low signal in echo planar imaging (EPI) (Figures 1 and 2). Punctate calcifications may be difficult to detect on fetal MRI. 72 The suspicion of cCMV infection should be raised when aforementioned lesions occur in the anterior temporal lobe, also termed "polar temporal lesions". 66,71 There may be WM T2 hyperintensity ( Figure 1) with or without swelling, cysts and/or pseudocysts, and mild dilation of the temporal horn, 81 which may reflect hippocampal dysplasia reported on postnatal MRI examinations. 85 Decreased volume of the temporal lobes compared with unaffected fetuses has also been described. 86 These anterior temporal lobe features can be seen alone or, more often, in combination and are commonly bilateral. These findings are better evaluated with fetal MRI than with US. 66  and diffusion tensor imaging (DTI) have promising applications in cCMV. MRS allows assessment of the metabolic profiles of tissues in vivo, with an estimated success rate of 65%. 87 In cCMV, MRS may show increased concentration of lipids, myo-inositol (mI), and possibly alanine, in cases of T2w WM hyperintensities ( Figure 6) and even normal-appearing WM, which may correlate with viral brain infection. 88 mI may also be a marker for increased astroglia/gliosis, 87,88 and is increased in neonates with hypoxic-ischemic injuries 89 which is one of the mechanisms through which the brain is thought to be involved in cCMV, secondary to placental involvement. 67

| Extra-CNS assessment
As neurodevelopmental sequelae are the main long-term consequence of cCMV, fetal scans tend to focus on brain imaging. However, cCMV is a global process that involves not only the whole fetus, but also the placenta and AF. 51 Hepato-and splenomegaly are common findings, readily identified both by US and fetal MRI (Figures 1 and 7). 65 While focal liver F I G U R E 3 Fetal magnetic resonance imaging at 24 gestational weeks (GW) referred for maternal cytomegalovirus seroconversion and normal brain ultrasound. On T2 weighted images (WI) slight periventricular caps can be identified frontally (A, black arrow) as well as a small periventricular cyst. PRESS spectroscopy performed with a short TE (35 ms) depicts a myoinositol peak (myo-inositol, white circle, C) and a lactate peak (Lac, yellow arrow, C), raising the suspicion of more extensive brain involvement. Images should be compared to T2w single shot fast spin echo (ssFSE).
They may cause secondary organ compression of the heart and lungs ( Figure 5) with prognostic implications. 104 Etiology may be multifactorial: anemia due to the combined effect of liver insufficiency and bone marrow infection, anoxia, endothelial cell damage, increased capillary permeability, and myocarditis. 13,105,106 Nonimmune fetal hydrops may ensue in severe cases.
Cardiomyopathy is a rare finding. While cardiomegaly is readily identifiable on T2w ssFSE/SSFP images, for identification of the thickened myocardium specific cardiac gated images may be necessary.
This may be a contributing factor to fetal hydrops and could be associated with tachyarrhythmia. 96,106 After reaching the fetal circulation, CMV preferentially involves the kidneys which may cause transient oligohydramnios and less frequently polyhydramnios, 13,107 although there are no specific descriptions of kidney signal anomalies on fetal MRI in this context. FGR may develop as a result of either fetal or placental infection, or both. 65 Further, FGR may be secondary to CMV placental involvement in the absence of fetal transmission. 13,108 Infants with FGR have a higher perinatal morbidity and mortality than infants with normal birth weights, as well as long term sequelae, and testing for cCMV may be warranted even if this is the sole finding. 109 The placenta acts as a barrier against CMV but also as a reservoir, and poor outcome is also associated with viral replication, inflammation, edema, and fibrinoid development in the placenta. 109 Placentitis, defined as a placental thickness of 4 cm or more, associated with a heterogeneous appearance for gestational age and possibly calcifications, is better identified on US. 73 Although there is evidence that CMV-related pathology is mediated by direct fetal infection, CMV infection of placental cells may also contribute to the pathogenesis of cCMV infection by altering placental formation and function, inducing placental damage and ultimately resulting in placental insufficiency. 67  Further sequences should be obtained as deemed appropriate. A run-through of the sequences and their applicability in the context of CMV can be found in Table 2.

| Timing of imaging
Determining the ideal timing for imaging is not always straightforward. Although fetuses as young as 18 weeks can be successfully imaged, the physiological lissencephalic aspect of the brain at this point in development limits the ability to detect migration. 72  Another factor to be considered is the legal framework of termination of pregnancy that in many countries is limited to 24 weeks. Doneda et al have shown that detection of CMV-associated brain anomalies is possible before this gestational age. 66 63 Notably, all false positive findings consisted of dubious WM hyperintensities (see below), and there was no postnatal imaging follow-ups confirming or excluding these findings. The same authors noted that although MRI did not change counseling or outcome, it provided further reassurance for parents. 63 It is also worth mentioning that although neurosonography provides a high NPV when the operator is aware of fetal infection (as high as 93% 73 ), systematic ultrasound performed as part of routine antenatal care has a poor (as low as 35%) sensitivity. 112 Beyond the discussion of what technique is superior, attaining an accurate diagnosis and as much prognostic information as possible is the ultimate goal for each patient. Decision on the need for fetal MRI should be made on a case-to-case basis and taking into consideration the objective or subjective need for further information or reassurance of referring physicians and parents.

| Further considerations
In terms of MRI findings, anterior temporal lobe lesions are the most specific finding, and an area where MRI may add information to US. Other findings albeit unspecific should raise the suspicion of T A B L E 2 Magnetic resonance imaging sequences and possible application in the context of congenital cytomegalovirus

| CONCLUSION
More awareness is needed regarding cCMV among parents, clinicians and radiologists. Given the paucity of unspecific symptoms and the lack of structured screening, many of these patients will not be diagnosed if the hypothesis of infection is not raised by findings on fetal imaging. This has severe consequences in treatment and patient counseling for present and future pregnancies. Anterior temporal lobe lesions are the most specific MR finding, and an area where MRI may add information to US. Other findings albeit unspecific should raise the suspicion of fetal infection, particularly when found in combination. This highlights the need for a full fetal assessment, as these findings may be outside the SNC. Advanced MR sequences may help in determining prognosis, but further studies are needed to assess their significance.