Local Anesthetic Systemic Toxicity (LAST): This is the most likely diagnosis because myoclonic jerks can be an early sign of systemic toxicity from local anesthetics like bupivacaine, especially when administered intrathecally. The progression from lower limbs to the whole body suggests a systemic effect.

Epidural or Subarachnoid Hemorrhage: Although less common, bleeding into the spinal canal can cause neurological symptoms, including myoclonus, due to pressure effects on the spinal cord.
Infection (Meningitis or Abscess): Introduction of bacteria during the procedure could lead to infection, which might present with neurological symptoms including myoclonus, although this would typically be accompanied by other signs of infection like fever and elevated white blood cell count.
Cerebrospinal Fluid (CSF) Leak: A dural puncture could lead to a CSF leak, potentially causing changes in intracranial pressure that might result in myoclonic jerks, though this is less directly related.

Spinal Cord Infarction: Although rare, this is a potentially devastating complication that could occur due to vascular injury or spasm during the procedure. Early recognition is crucial for potential intervention.
Anaphylaxis: A severe allergic reaction to the local anesthetic or other substances used during the procedure could present with systemic symptoms including myoclonus, and is a medical emergency.
Status Epilepticus: If the myoclonic jerks are part of a seizure activity, recognizing and treating status epilepticus promptly is critical to prevent long-term neurological damage.

Acute Transverse Myelitis: An inflammatory process affecting the spinal cord, which could be triggered by the procedure, though this is exceedingly rare.
Toxicity from Contaminants: Although quality control measures are in place, the possibility of contamination of the anesthetic solution with a neurotoxic substance cannot be entirely ruled out, especially in cases where the reaction is unexpected and severe.