Decreased MEPs during subcutaneous dissection for untethering surgery of a 'true' lipomyelomeningocele: aggravated traction of the spinal cord by release of the sac from the original nest

It is well known that the preoperative enlarged sac in occult spinal dysraphism, in which the spinal cord is attached to its dome, can be associated with neurological deficits by aggravation of spinal cord traction. We experienced a similar phenomenon during the early stage of untethering surgery for “lipomyelomeningocele in a strict sense (LMMC).” We report a case of a 3-month-old girl with LMMC, which showed decreased motor evoked potentials (MEPs) in the lower extremities during the early stage of untethering surgery. The sac was released from the original nest by incision with separation of the overlying skin and dissection from the muscle fascia. The sac became rounder, and the spinal cord attached to the dome of the sac was further stretched. After aspiration of cerebrospinal fluid from the sac, the MEPs recovered. Postoperatively, the patient had no neurological deficits. Although this phenomenon rarely occurs, release of the herniated sac of LMMC from the original nest during the early stage of untethering surgery may result in neurological changes. Application of intraoperative neurophysiological monitoring helps to detect electrophysiological deterioration and can prevent neurological deficits.