spasticity vs rigidity, dystonia, and contracture

When a limb feels stiff, it is tempting to call it all 'spasticity', but four very different things can make a joint hard to move, and they call for different treatments. The way to tell them apart is to feel how the stiffness behaves: does it depend on speed, is it steady throughout the range, does it twist the limb into a posture, or does the joint simply refuse to move at all? Each answer points to a different cause.

Spasticity is velocity-dependent: stiff when moved fast, easy when moved slow, and it comes from an overactive stretch reflex after upper motor neuron injury. Rigidity is constant: it resists evenly no matter how fast or slow you move, often with a smooth 'lead-pipe' feel or a ratchety 'cogwheel' quality, and it points to a basal-ganglia problem such as Parkinson's disease. Dystonia is an active twisting: muscles pull the limb into an abnormal posture or repetitive movement, often worse with effort or stress, and it is driven by faulty motor commands, not a stretch reflex. Contracture is mechanical: the muscle and surrounding tissue have shortened and stiffened permanently, so the joint will not reach its full range even under anaesthesia — there is nothing for a nerve-acting drug to switch off.

Getting this distinction right is one of the highest-value judgments in tone management, because the wrong label leads to the wrong treatment. Botulinum toxin or baclofen can quiet spastic or dystonic overactivity but does nothing for a fixed contracture, which needs stretching, casting, or surgery. And the four can coexist — untreated spasticity over months can produce a contracture on top of it — so the examiner must ask, at each joint: how much of this stiffness is neural and reversible, and how much is mechanical and fixed?