What condition is characterized by prolonged, involuntary tonic muscle spasms?

The condition characterized by prolonged, involuntary tonic muscle spasms is known as dystonia. Dystonia is a movement disorder that involves sustained muscle contractions, leading to twisting and repetitive movements or abnormal postures. This can affect various parts of the body, including the neck, face, and limbs, and can range from mild to severe.

Tardive dyskinesia, on the other hand, is a condition that involves involuntary, irregular movements, often affecting the face, such as lip smacking or tongue protrusion. While it can be distressing, it is distinct from the muscle spasms seen in dystonia, as it typically involves more choreiform movements rather than sustained muscle contractions.

Neuroleptic malignant syndrome is a life-threatening condition associated with antipsychotic medications, presenting with severe muscle rigidity and autonomic dysfunction, but it is not characterized solely by prolonged tonic spasms; it presents with broader systemic symptoms and acute severity.

Akathisia refers to a state of inner restlessness and an uncontrollable need to be in constant motion, which is also not characterized by prolonged muscle spasms.

Therefore, the defining feature of dystonia aligns it closely with the description in the question, making it the appropriate choice.