Differential Diagnosis for a 4-year-old with Forearm Pain and Weakness

Single most likely diagnosis

• Radial head subluxation (nursemaid's elbow): This condition is common in young children and can occur when a child is lifted or pulled by the hand, causing the radial head to partially dislocate. The symptoms of pain, weakness, and swelling in the proximal forearm, along with the mechanism of injury (carrying a bag of rocks and hearing a pop), support this diagnosis. However, the presence of a "pop" and significant weakness might also suggest other injuries.

Other Likely diagnoses

• Fracture of the radial shaft or neck: Given the mechanism of injury and the symptoms, a fracture in this area is possible, especially if the child heard a "pop," which could indicate a break.
• Muscle strain or contusion: The swelling and pain could be due to a strain or contusion of the muscles in the forearm, possibly from overuse or direct trauma when carrying the bag of rocks.
• Elbow or forearm sprain: A sprain, which involves the ligaments, could also present with pain, swelling, and weakness, especially if the injury involved a twisting motion.

Do Not Miss (ddxs that may not be likely, but would be deadly if missed)

• Supracondylar fracture of the humerus: Although less likely given the specific location of pain and swelling in the proximal forearm, this type of fracture can lead to serious complications, including nerve damage and compartment syndrome, if not promptly diagnosed and treated.
• Compartment syndrome: This is a medical emergency that can result from increased pressure within the compartments of the forearm, leading to muscle and nerve damage. The presence of significant pain, swelling, and weakness could be indicative of this condition.
• Vascular injury: Any injury that compromises blood flow to the forearm or hand is an emergency. While less common, the mechanism of injury and symptoms could potentially mask a vascular injury.

Rare diagnoses

• Osteomyelitis or septic arthritis: Infection within the bone or joint could present with pain, swelling, and decreased mobility. However, these conditions would typically be accompanied by systemic signs of infection such as fever.
• Tumor or cyst: Although extremely rare in this context, a pathological fracture through a bone tumor or cyst could present with pain and swelling following a minor injury.