Congenital Cytomegalovirus (CMV) Diagnosis

Aug. 17, 2020

Cytomegalovirus (CMV) is the most common congenital infection in the United States, affecting 40,000 infants annually, and is one of the classic perinatal “TORCH” infections (Toxoplasmosis, Other (including syphilis), Rubella, CMV, Herpes simplex virus). The most common manifestation of congenital CMV infection is sensorineural hearing loss (i.e., hearing loss due to inner ear or vestibulocochlear nerve defects; SNHL), but a smaller percentage of infants also present with liver failure and neurologic abnormalities. Treatment of symptomatic congenital CMV can improve long-term outcomes in neonates, so a prompt diagnosis is important. In order to accurately identify CMV disease in a neonate, we must understand the role and implications of different diagnostic modalities in the infant and mother. 
 
Overview of diagnostic methods for congenital CMV.
Test subject Sample type Test method Limitations
Pregnant woman Serum Serology Positive IgM/IgG establish seropositivity, but IgG avidity is often needed to determine timing of primary infection
Fetus Amniotic fluid PCR  
Neonate Urine or saliva PCR or shell vial culture Must be performed in first 3 weeks of life to diagnose congenital (vs. postnatal) infection
Blood Quantitative PCR Used to monitor response to treatment, but not to make initial diagnosis

Transmission of Congenital CMV

To understand how to test for congenital CMV, we first have to understand how infection is acquired and transmitted from mother to baby. Primary CMV infection in children and adults can cause a spectrum of clinical presentations ranging from asymptomatic infection to a mononucleosis-like syndrome with fever, fatigue, swollen lymph nodes, sore throat and transient hepatitis (inflammation of the liver); in people with immunocompromising conditions, it can also cause severe inflammation of the colon, lungs or retina. CMV infection is frequently acquired in childhood, but in the U.S., about 50% of women of childbearing age remain seronegative, meaning they are still at risk of primary CMV infection. Like other herpesviruses, CMV establishes lifelong latency following primary infection and can reactivate in the setting of immunosuppression or physiologic stress. The risk of congenital CMV infection is highest in women who acquire primary infection during pregnancy, although viral reactivation (or even infection with a different strain of CMV) can also result in congenital infection.    

Transmission of CMV to the fetus occurs in about one-third of primary maternal infections. Although congenital infection is most likely to occur in women who are infected in the third trimester, symptomatic disease and long-term outcomes for the child, including SNHL, are more likely when infection occurs early in pregnancy. Among children with congenital CMV infection, only about 10% have symptoms at birth. These symptoms can include SNHL, as well as hepatosplenomegaly and jaundice due to liver failure, a petechial or purpuric rash classically described as having a “blueberry muffin” appearance, chorioretinitis, microcephaly, intracranial calcifications and intrauterine growth retardation. In addition, about 10-15% of babies who have no symptoms at birth will later go on to develop SNHL.

Diagnosing Primary CMV Infection in Pregnant Women 

Women in the U.S. are not routinely screened for CMV infection during pregnancy, although the potential role of prenatal screening is an area of debate. Primary maternal CMV infection may be suspected if a woman has symptoms consistent with CMV, while congenital CMV infection may be suspected on the basis of characteristic prenatal ultrasound abnormalities. Because symptomatic congenital CMV is most likely when a woman acquires primary infection during the first trimester, there are important prognostic implications in assessing not only whether a woman has had CMV infection, but when she first acquired it. 

Maternal antibody testing plays a role in the diagnosis of primary maternal or congenital CMV infection, but standard IgM and IgG assays have significant limitations for this purpose. IgM may be falsely positive, may persist long after primary infection and may return during periods of reactivation, while a positive IgG result does not usually allow determination of the timing of primary infection (unless a woman happens to have had a recent prior negative IgG result). In order to determine when a woman with a positive CMV IgM and IgG may have acquired the infection, a CMV IgG avidity test can be performed. CMV IgG avidity for antigens increases over time following initial infection, so low CMV IgG avidity is indicative of infection within the preceding 3-4 months. When this test is performed during the first or early second trimester, a low avidity result is therefore suggestive of infection early in pregnancy. Suspected congenital CMV infection can be confirmed by PCR testing of amniotic fluid obtained by amniocentesis. While there is at present no established method for treatment of prenatal infection, prenatal diagnosis can help parents to understand the range of possible prognoses for the fetus, consider enrollment in treatment trials and plan for appropriate management at the time of delivery.

Diagnosing Congenital CMV in Neonates 

The diagnosis of congenital CMV should be considered in neonates who have signs or symptoms suggestive of CMV infection, including liver dysfunction, low platelet counts, microcephaly and low birth weight, as well as in babies who fail their hearing test. CMV in pregnant women is typically diagnosed by serology, because the diagnosis is usually considered when abnormal findings are noted on prenatal ultrasound, at which point active virus may no longer be present in the mother. In contrast, congenital CMV in infants is diagnosed by identifying the virus itself, either by culture or PCR, as IgG testing in a neonate reflects maternal antibody but does not provide information about whether the virus has infected the infant. Traditional CMV cell culture is slow, requiring 1-3 weeks for observation of cytopathic effect. The shell vial culture technique, in which the patient sample is centrifuged onto a shell monolayer, with CMV subsequently identified by fluorescent antibody staining, significantly reduces the turnaround time of culture-based testing for CMV, and can be used to diagnose CMV in urine and saliva from neonates. Where available, however, PCR of a saliva or urine sample is typically the test of choice due to its rapidity and excellent sensitivity. While detection and quantification of CMV DNA in blood is not a component of the initial diagnosis, quantitative PCR measurements of viral load in the blood can play a role in monitoring response to treatment

Detection of CMV-specific early-antigen fluorescent foci in the shell vial assay indicates the presence of the virus.
Detection of CMV-specific early-antigen fluorescent foci in the shell vial assay indicates the presence of the virus.
Testing for congenital CMV should be performed within the first 3 weeks of life; beyond this point, CMV detected in urine or saliva may indicate postnatal infection, which rarely causes long-term adverse outcomes in term infants. In preterm infants, by contrast, postnatal CMV infection, which is most often acquired via breastmilk, can present much like congenital CMV. Treatment with ganciclovir or valganciclovir has been shown to improve audiologic and neurodevelopmental outcomes in infants with symptomatic congenital CMV. There remains uncertainty about the role of these drugs for treatment of asymptomatic congenitally infected infants or infants with isolated SNHL.

To make a diagnosis of congenital CMV, we must use the right test at the right time. The clinical microbiologist can play a critical role clarifying the roles of different test modalities and the limitations of each test type. 
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Author: Thea Brennan-Krohn

Thea Brennan-Krohn
Thea Brennan-Krohn is a diplomate of the American Board of Medical Microbiology at Beth Israel Deaconess Medical Center (BIDMC). She is an attending in Pediatric Infectious Diseases at Boston Children's Hospital and a postdoctoral fellow at Beth Israel Deaconess Medical Center,