Diverging pathophysiology in superficial siderosis with proximal upper limb amyotrophy

•Fourteen percent of patients with superficial siderosis in this study have upper limb amyotrophy.•The pathophysiology of upper limb amyotrophy associated with duropathy is diverse.•A novel pathophysiology of amyotrophy is associated with nerve root compression.•Concurrent compartment pressure and spondylosis may compress the nerve root.•Muscle damage associated with nerve root compression is recoverable by surgery.