Previously Healthy 3-Week-old Infant with Limited Right Arm Mobility.

A previously healthy, 3-week-old, term girl presents with a 6-day history of decreased right arm movement.She had previously been using both arms equally; however, a week before presentation, after waking her up from a nap her parents noticed that the infant's right arm was hanging limp.There was no reported history of trauma, and the infant was otherwise well appearing.The patient initially presented to an outside emergency department.There she was diagnosed as having a right elbow subluxation, and a reduction was subsequently attempted.When symptoms persisted 2 days later, her primary pediatrician directed the family to a referred medical center for further evaluation.On presentation the infant is afebrile, with normal vital signs.Examination reveals minimal movement of her right fingers, and her arm is held in an adducted, extended, and internally rotated position.Notably, palpation and manipulation of the right arm appear to induce pain, without any focal areas of tenderness.There is no swelling, warmth, or erythema of the soft tissues or joints.An asymmetrical Moro reflex is elicited and appears painful.The remainder of her physical examination findings are normal.Given the patient's apparent pain with manipulation of her right arm, a plain radiograph of the extremity is obtained to rule out a fracture.Imaging reveals her diagnosis and prompts further laboratory studies and evaluation.