Differential Diagnosis for Progressive Rigidity in Lower Limb
Single most likely diagnosis
- Parkinson's disease: A 3-month history of progressive rigidity in the lower limb, leading to dislocation of a total hip replacement, suggests a condition affecting motor control. Parkinson's disease is a common cause of rigidity and could lead to increased risk of falls and subsequent hip dislocation, especially in an elderly patient. The normal MRI head and spine reduce the likelihood of structural causes.
Other Likely diagnoses
- Stroke or cerebral vasculopathy: Although the MRI head is normal, small vessel disease or previous strokes not evident on MRI could lead to spasticity and rigidity. The history of progressive worsening suggests a possible vascular cause.
- Spinal stenosis or lumbar spondylosis: Despite a normal MRI spine, it's possible that the imaging missed dynamic or subtle structural issues contributing to lower limb rigidity, especially if the patient has a history of back pain or radiculopathy.
- Dystonia: Focal dystonias can cause significant rigidity and abnormal posturing in the affected limb, potentially leading to dislocation of a prosthetic joint.
Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
- Intracranial infection or abscess: An infectious process in the brain could present with focal neurological signs, including rigidity, and would require urgent treatment. A normal MRI does not entirely rule out this possibility, especially if the infection is early or located in a area difficult to visualize.
- Subdural hematoma: Even with a normal MRI, a subdural hematoma could be a consideration, especially in an elderly patient with a history of falls. It's a medical emergency that requires immediate intervention.
- Cancer with brain or spinal metastases: Metastatic disease to the brain or spine could cause focal neurological deficits, including rigidity. Although the MRI is normal, metastatic disease can sometimes be subtle or located in areas not well visualized by standard MRI sequences.
Rare diagnoses
- Wilson's disease: A rare genetic disorder that can cause neurological symptoms, including rigidity and dystonia, due to copper accumulation in the brain. It's less likely given the patient's age and the normal MRI head.
- Creutzfeldt-Jakob disease: A rare, degenerative, fatal brain disorder that could present with rapidly progressive neurological symptoms, including rigidity. The normal MRI and the relatively slow progression make this less likely.
- Tetanus: Although extremely rare in immunized populations, tetanus infection can cause severe muscle rigidity. The clinical context and lack of typical tetanus symptoms (e.g., lockjaw, generalized spasms) make this diagnosis unlikely.