An empirical ratio accounting for all TcMEP parameters seems to drop more than the amplitude during an intraoperative event. As few cases were analyzed, further studies in larger series of patients will be necessary to assess empirical ratio sensitivity/specificity and to determine whether this may be a useful monitoring warning criterion not leading to unnecessary interference with surgical treatment.

A 35-year-old man presented with progressive right face, arm, and leg weakness, and diffuse headache. He lived in rural northwest Argentina. He had a past medical history of sexually transmitted diseases. On examination, he was alert and fully oriented, and had a right hemiparesis with hyperreflexia and an extensor plantar reflex. Apart from low grade fever, the rest of the physical examination was unremarkable. The complete blood count revealed leukopenia (3,300 leukocytes/mL); renal function, liver tests, electrolytes, erythrocyte sedimentation rate, and glucose level were normal. He tested positive for HIV with a CD4 count of 18 cells/mm 3 and viral load of 133,400 copies/mm 3 . Brain CT showed a nonenhancing left temporoparietal lesion with surrounding edema and midline shift.

Level IV.

Purpose:(1) To determine probabilities of immediate postoperative new motor deficits after no, reversible, and irreversible motor evoked potentials (MEP) deteriorations and (2) to calculate the same outcome considering whether MEP deteriorations were followed by surgical interventions in the absence of confounding factors.Methods: We analyzed MEPs from 513 surgeries. Four-limb MEPs were evoked by transcranial electrical stimulation. Baseline recordings were obtained before skin incision and updated before instrumentation. Motor evoked potentials deteriorations were considered significant whenever they showed a persistent, reversible, or irreversible amplitude decrease of .80% of the baseline values.Results: Nine patients showed postoperative new motor deficits. Probabilities of postoperative new motor deficits were null, 2.8%, and 36.8% with no, reversible, and irreversible MEP deteriorations, respectively. The risk of immediate postoperative new motor deficits was significantly lower (P ¼ 0.0002) in reversible MEP compared with irreversible MEP deteriorations. In patients showing reversible/irreversible MEP deteriorations in the absence of confounding factors, surgical interventions compared with nonsurgical interventions significantly decreased the risk of immediate postoperative new motor deficits (P ¼ 0.0216).Conclusions: This study shows that probabilities of immediate postoperative new motor deficits increase with the severity of intraoperative MEP changes. In addition, our results support the value of surgical interventions triggered by MEP deteriorations to reduce postoperative adverse motor outcomes.