Background Each year, syphilis complicates over 2 million pregnancies worldwide and continues to rise. Whilst still uncommon, rates of congenital syphilis have been rising in developed countries over recent years. In 2018 alone, 1,306 cases were reported in the United States; constituting a 185% increase since 2014.

This disease can lead to peri-/neonatal death, prematurity and low birth weight. Untreated, spontaneous abortion can occur in 40% cases. Affected individuals often suffer long-term sequelae such as visual loss, sensorineural hearing loss, bone/joint deformity and neurodevelopmental delay.

Cases of congenital syphilis are uncommon in the UK; with an estimate of 10 cases annually in 2015, meaning most paediatricians will have little to no experience with the disease. Many affected individuals may be asymptomatic at birth and symptoms can often mimic other conditions. In this respect, and with rising incidence in adult disease over recent years, congenital syphilis poses a true diagnostic challenge for clinicians; as demonstrated in this case report.

Objectives In this report, we describe a rare case of a 4-month-old girl presenting with a several month history of reduced movement to the left arm accompanied by a maculopapular rash to the limbs. X-Ray findings included inflammatory periosteal changes to the radius and ulna. Blood test results included raised inflammatory markers (CRP and ESR) with a slight microcytic anaemia, raised ALP and suppressed TSH.

Results Treponema pallidum immunoglobin M (IgM) was detected in both baby and mother and a diagnosis of congenital syphilis was made.

Conclusions This case is an interesting clinical picture with a variety of important differential diagnoses including non-accidental injury, malignancy, autoimmune disease and other congenital infections. With an increasing rate of congenital syphilis infection in the developed world, it is vital that clinicians are able to recognise symptoms to ensure prompt diagnosis and treatment. In this respect, we can attempt to avoid the chronic and potentially life-threatening complications of untreated infection.