Rubella

01Pathophysiology

Rubella is inhaled as a droplet and replicates first in the nasopharyngeal epithelium, then drains into the lymphoid tissue immediately behind it. That anatomy is the reason the exam keeps describing the same three node groups. The postauricular, suboccipital and posterior cervical nodes are the regional drainage basin of the posterior scalp and nasopharynx, so they enlarge and become tender while the virus is still confined locally. This is why lymphadenopathy precedes the rash by up to a week and is the single most useful physical finding: it is present before there is anything to see on the skin.

After roughly 5 to 7 days of local replication a primary viremia carries virus to the reticuloendothelial system, and a secondary viremia seeds the skin, the synovium and, in a pregnant patient, the placenta. Peak shedding from the nasopharynx occurs around the time the rash appears, which is why the contagious window runs from about 7 days before to 7 days after rash onset. Exposure history in a vignette is usually placed inside that window on purpose.

The exanthem itself is not a cytopathic lesion. It appears exactly as neutralizing antibody becomes detectable and viremia is being cleared, so the rash is immune-mediated, driven by immune complex deposition and a cell-mediated response in dermal vessels. That mechanism explains its character: sparse, fine, pink, non-confluent, and gone in three days without desquamation. Compare this with measles, where the rash is a more intense endothelial infection and therefore becomes confluent, lasts five or six days, and leaves brownish staining and fine peeling. The same immune complex mechanism deposited in synovium produces the polyarthralgia and polyarthritis of adolescent and adult women, affecting up to 70 percent of them, beginning as the rash fades and resolving over 2 to 4 weeks.

Congenital disease works by a different mechanism, and this is where most of the testable content lives. During maternal viremia the virus infects the placenta and reaches fetal endothelium. Rubella is not primarily lytic in the fetus. Instead it establishes persistent, non-cytolytic infection that inhibits mitosis and causes focal apoptosis and endothelial necrosis. The result is a fetus built from too few cells and supplied by damaged vessels, which is why congenital rubella produces hypoplastic organs, symmetric growth restriction and vascular malformations rather than the destructive necrosis and calcification seen with cytomegalovirus or toxoplasmosis.

Because the injury is a block on cell division, the defect you get is determined by which organ is dividing at the moment of infection. The lens vesicle detaches and completes its critical growth by about week 7, so cataract only occurs when infection happens before that point. The cochlea and organ of Corti differentiate through the first and early second trimester, so deafness has the widest window and is therefore the commonest single defect. The ductus arteriosus and pulmonary arteries depend on endothelium that the virus damages directly, giving the characteristic patent ductus arteriosus and peripheral pulmonary artery stenosis. Marrow suppression with compensatory dermal extramedullary hematopoiesis produces the purple non-blanching papules known as the blueberry muffin rash. Persistent infection also injures pancreatic islets and thyroid, which is why type 1 diabetes and thyroid disease appear years later.

One consequence of persistent infection deserves its own sentence, because it is a classic infection control question: an infant with congenital rubella keeps shedding virus in urine and nasopharyngeal secretions for months, and is treated as infectious until 1 year of age unless two specimens taken a month apart are negative.

02Classification and Clinical Manifestation

Rubella splits cleanly into acquired (postnatal) infection, which is a benign illness with an important complication profile in adults, and congenital infection, which is the reason the disease is still notifiable.

ACQUIRED RUBELLA BY STAGE

COMPLICATION SPECTRUM OF ACQUIRED RUBELLA

CONGENITAL RUBELLA SYNDROME: DEFECT CATEGORIES

GESTATIONAL TIMING DETERMINES THE DEFECT

Read the gestational age in the stem before you read anything else. A vignette that gives "6 weeks pregnant" and one that gives "26 weeks pregnant" are asking two different questions with two different correct answers, even when every other word is identical.

03Diagnostic Workup

Start from the principle that rubella cannot be diagnosed clinically with enough confidence to act on. The rash is nonspecific, half of infections have no rash at all, and the consequence of a missed case is a congenital infection in someone else's pregnancy. Every suspected case therefore gets laboratory confirmation, and every suspected case is notified to public health.

The best initial test in acquired disease is serum rubella IgM, because it is fast, widely available, and positive in over 90 percent of true cases when the sample is drawn at least 5 days after rash onset. The timing is the tested detail: on the day the rash appears only around half of true cases are IgM positive, so a negative IgM drawn on day 0 or day 1 does not exclude rubella and the correct next step is to repeat it, not to abandon the diagnosis. IgM remains detectable for up to about 3 months.

The most accurate confirmation is detection of viral RNA by RT-PCR from a throat or nasopharyngeal swab, which also yields the genotype that public health laboratories need to link cases and prove elimination has not been lost. Its own timing constraint mirrors the biology: RNA is detectable from about 2 days before to 4 days after rash onset and the yield is best in the first 3 days, so the swab is collected at the same visit as the serology rather than later. Where PCR is unavailable, a $\ge 4\times \backslash geq\; 4\backslash times$ rise in IgG between acute and convalescent sera taken 2 to 3 weeks apart is an accepted confirmation.

The order of testing changes in pregnancy, and this is where questions get built. A pregnant woman who has been exposed and has no documented immunity gets IgG and IgM immediately. Detectable IgG with negative IgM in a sample drawn shortly after exposure means pre-existing immunity and reassurance. Negative IgG means she is susceptible, so serology is repeated in 2 to 3 weeks to catch seroconversion. Isolated positive IgM in an asymptomatic pregnant woman is the trap: false positives arise from rheumatoid factor, heterophile antibodies, parvovirus B19, Epstein-Barr virus and cytomegalovirus, and IgM can persist from a remote infection. The correct next step there is IgG avidity testing or repeat testing by a different assay format, not counselling on the basis of a single positive IgM. Low avidity supports infection within the last 3 months; high avidity effectively excludes recent primary infection.

In the newborn, infant IgM is the best initial test because maternal IgM does not cross the placenta, so anything detected belongs to the baby. When it is negative and suspicion persists, follow serial IgG: transplacental maternal IgG falls with a half-life near 30 days and should be undetectable by 6 to 12 months, so persistent or rising IgG past that point confirms congenital infection. RT-PCR of nasopharyngeal secretions, urine, cerebrospinal fluid or a lens aspirate confirms the diagnosis and, just as importantly, tells you whether the infant is still shedding and therefore still a hazard to susceptible pregnant contacts.

04Management and Treatment

Acute care. There is nothing to treat. Acquired rubella in a child is fever control, fluids and reassurance, and the illness is over in about a week. In an adult, the part that lingers is joint disease, and it is managed with a non-steroidal anti-inflammatory drug for 2 to 4 weeks. If a stem describes a young woman with a symmetric polyarthritis of the hands and wrists that began days after a transient rash, the answer is not rheumatoid arthritis and not a rheumatology referral. It is supportive treatment and reassurance that this resolves.

The real acute management is containment, and this is where "next best step" questions live. Place the patient on droplet precautions, isolate until 7 days after rash onset, and notify public health. Because the contagious period begins 7 days before the rash, the contact list reaches back a full week, and it must be screened for two groups above all others: pregnant women without documented immunity and susceptible healthcare personnel. Susceptible contacts are excluded from day 7 through day 23 after exposure, which is the incubation window plus a margin.

The pregnant patient. If a pregnant woman is exposed or develops a compatible rash, the next best step is always serology, never vaccine and never immunoglobulin as a reflex. Document IgG and IgM, repeat in 2 to 3 weeks if she is susceptible, and use avidity testing to resolve an isolated positive IgM. Once infection is confirmed, counselling is driven entirely by gestational age using the timing table above: before 8 weeks the risk of defects is 85 to 90 percent, by 13 to 16 weeks it is around one in five and is mostly deafness, and after 20 weeks structural defects are not expected. Options are discussed within the framework of local law, and continued pregnancies get serial ultrasound and a newborn evaluation regardless of how normal the scans look, because deafness is not visible on ultrasound.

Immunoglobulin at 0.55 mL/kg intramuscularly is a narrow, second-line consideration: it may reduce maternal symptoms, it has not been shown to prevent fetal infection or congenital rubella syndrome, and infants have been born with the syndrome to mothers who received it. It is reserved for a susceptible woman who has been exposed and for whom termination is not an option.

Prevention, which is the actual answer to most questions. The vaccine is live attenuated, and every rule follows from that. Two doses of measles-mumps-rubella at 12 to 15 months and 4 to 6 years, given subcutaneously, with a minimum interval of 28 days when catching up. A single dose seroconverts about 95 percent of recipients and protection is lifelong. Adults born after routine vaccination began who lack evidence of immunity get one dose, or two if they are healthcare personnel, students in post-secondary education, or international travelers.

Contraindications worth memorizing: pregnancy, with conception avoided for 28 days afterwards; severe immunodeficiency, including high-dose corticosteroid therapy and advanced untreated human immunodeficiency virus infection; anaphylaxis to neomycin or gelatin; and recent receipt of immunoglobulin or blood products, which requires deferring the vaccine by 3 to 11 months depending on the product. Note carefully what is not a contraindication: breastfeeding, egg allergy, a household contact who is pregnant, and a household contact who is immunocompromised. Vaccine virus is not transmitted from a vaccinated person to others, which is why vaccinating the household is the correct way to protect a susceptible pregnant woman.

Inadvertent vaccination in early pregnancy, or conception within 28 days of a dose, is a scenario written to see whether you overreact. No case of congenital rubella syndrome has been attributed to the vaccine strain in decades of surveillance. The answer is counselling and reassurance with continued routine prenatal care, not termination.

NEXT BEST STEP, CONDENSED Rash plus tender postauricular nodes, patient not pregnant: send IgM and a throat swab for RT-PCR, isolate 7 days, notify public health. Pregnant and exposed: serology first. Pregnant and non-immune on booking bloods: vaccinate postpartum before discharge. Newborn with cataract and a murmur: infant IgM plus RT-PCR, contact isolation, audiology.

05Differential Diagnosis and Distractors

ACQUIRED RUBELLA AGAINST ITS LOOK-ALIKES

CONGENITAL RUBELLA SYNDROME AGAINST THE OTHER CONGENITAL INFECTIONS

06Traps and High-Yield Pearls

The single most common way to lose a rubella question is to answer measles. Both are unvaccinated patients with a maculopapular rash spreading from the face down, and the stem is written so that the two look alike until you read the details that were placed there on purpose. Three things separate them and all three appear in almost every rubella vignette: the patient is well-appearing with a low-grade fever rather than toxic with a fever near 40 C, the rash is non-confluent and gone by day 3 rather than confluent for 5 or 6 days with brownish staining, and there is tender postauricular and suboccipital lymphadenopathy rather than cough, coryza, photophobic conjunctivitis and Koplik spots. If the stem never mentions the nodes and does mention cough and conjunctivitis, you are being asked about measles.

The second trap is vaccinating the wrong person at the wrong time. A pregnant woman who is found to be rubella non-immune on her booking bloods is not vaccinated during pregnancy, because this is a live attenuated vaccine. The correct answer is postpartum vaccination before hospital discharge, and it is the step most often forgotten, which is exactly why it is written into questions. The mirror-image trap is over-reacting to an inadvertent dose given just before or during early pregnancy: no case of congenital rubella syndrome has been attributed to the vaccine strain, so the answer there is counselling and routine prenatal care.

Third, serology timing decides the answer more often than serology itself. A negative IgM drawn on the day the rash appeared is not a negative result, because only about half of true cases are positive that early. The next best step is to repeat testing after day 5, or to send an acute and convalescent IgG pair looking for a $\ge 4\times \backslash geq\; 4\backslash times$ rise. Running the other way, an isolated positive IgM in an asymptomatic pregnant woman is more likely a false positive from rheumatoid factor or cross-reacting viral antibody than a true infection, and the correct next step is IgG avidity testing or confirmation on a different assay, not counselling about termination. Getting a woman to a decision on the strength of one unconfirmed IgM is the mistake the question is testing for.

Fourth, gestational age is not background detail. It is usually the variable the question turns on. Under 8 weeks the risk of defects is 85 to 90 percent and the full syndrome is on the table. By 13 to 16 weeks the risk is roughly one in five and it is essentially deafness. After 20 weeks structural defects are not expected. Two vignettes with identical wording and different gestational ages have different correct answers.

Fifth, know which congenital infection you are looking at from the calcification pattern and the eye. Rubella is cataract plus patent ductus arteriosus or peripheral pulmonary artery stenosis plus deafness, with a salt and pepper retinopathy and no calcifications. Cytomegalovirus is periventricular calcifications with microcephaly and deafness, no cataract. Toxoplasmosis is diffuse scattered calcifications with hydrocephalus and chorioretinitis. The blueberry muffin rash belongs to none of them exclusively and is never the discriminator.

Finally, remember that a congenitally infected infant is a public health problem, not just a patient. Shedding continues for months, so the infant is treated as infectious until 1 year of age unless two specimens a month apart are negative, and every caregiver needs documented immunity. A question that ends with a pregnant nursery nurse or a pregnant relative visiting the baby is asking about isolation and immunity documentation, not about treatment.

The core competency being tested across this topic is recognizing a mild illness whose importance lies entirely outside the patient in front of you: identifying rubella from lymphadenopathy and rash character rather than severity, confirming it with correctly timed serology or polymerase chain reaction, translating gestational age into fetal risk, and knowing that the intervention that matters is vaccination given to the right person at the right moment.