Neurotoxin therapy is an effective component of comprehensive spasticity management. The two cases presented demonstrate adverse effects.

Patient S is a 60-year-old female with right spastic hemiparesis due to left hemispheric stroke (age 46) treated with therapy, bracing, oral baclofen. Patient R is a 28-year-old female with left spastic hemiparesis due to hemispherectomy (age 6).

For patient S, onabotulinumtoxinA (ONA) was initiated at 300 units (u) to the right upper extremity (RUE), advanced to 500u. She received injections every 3–3.5 months for over 9 years and continued to function independently. At routine follow-up, she related left upper extremity (LUE) weakness that she noted but did not report with her two prior injections. She denied any other symptoms. Examination revealed mild, diffuse LUE weakness, imaging was unremarkable and EMG demonstrated a chronic LUE axonal polyradiculopathy. ONA was continued at 300u to the RUE with somewhat lesser but maintained benefit without adverse effects.

For patient R, ONA was initiated at 400u to the LUE every three months for 13 rounds, advanced to 500u. Contralateral (RUE) weakness developed after the second round of 500u dosing. Diagnostic evaluation was notable only for increased insertional activity, fibrillation potentials and decreased recruitment with subsequent long duration polyphasic motor unit potentials. Unchanged with high dose steroids, weakness improved with IVIG. Repeat ONA with 500u resulted in good local effect, recurrent contralateral upper and lower extremity weakness. EMG demonstrated contralateral cervical, thoracic and lumbosacral axonal polyradiculopathy. Symptoms improved with IVIG. Patient declined subsequent ONA.

Both patients were treated with neurotoxin therapy for 3–9 years with good clinical response before developing the adverse reaction. Weakness distant to the injection sites is supported by electrodiagnostic findings of contralateral axonal polyradiculopathy. The clinical presentations suggest the possibility that the adverse effect of distant weakness may be immune-mediated and dose-related.