The use of motor-evoked potentials to monitor sciatic nerve status during revision total hip arthroplasty.

Sciatic nerve palsy is an uncommon complication after total hip arthroplasty (THA) but can cause permanent functional impairment in the extremity. The goal of this study was to identify specific intraoperative maneuvers that may increase the risk of sciatic nerve injury during revision THA. Motor-evoked potentials (MEPs) were used in combination with electromyography (EMG) monitoring during revision THA in 27 consecutive patients to identify intraoperative events that cause conduction abnormalities through the sciatic and peroneal nerves. MEP monitoring required general anesthesia and a neurophysiologist in the operating room throughout the procedure. Significant electrical events occurred, most commonly during acetabular reconstruction. Hip flexion should be avoided during posterior acetabular retraction when using the posterior approach with posterior dislocation. The position of the sciatic nerve should be clearly identified when complex structural acetabular augmentation with allograft is performed during revision THA. MEP monitoring was used as a research tool for this study. However, we do not use MEP monitoring in our revision THA clinical practice.

Reversed latissimus dorsi muscle flap for repair of recurrent congenital diaphragmatic hernia.

BACKGROUND/PURPOSE: Neonates with large congenital diaphragmatic hernias (CDH) require prosthetic patch closure of the defect because of the paucity of native diaphragmatic tissue. As the child grows, patch separation can occur necessitating reoperation. Use of vascularized autologous tissue may decrease the incidence of reherniation as tissue incorporation and growth may be improved. The authors report our early experience using a local muscle advancement flap with microneural anastomosis for those children in whom reherniation develops after prosthetic patch placement. METHODS: Seven patients with CDH (6 left and 1 right) whose synthetic diaphragmatic patch separated from the chest wall resulting in a clinically significant recurrent hernia were followed up with prospectively. After dissecting the ipsilateral latissimus dorsi off the chest wall and dividing the thoracodorsal neurovascular bundle (based on its lumbar blood supply), the synthetic patch was removed via an eighth intercostal incision. The muscle flap was placed into the hemithorax through the bed of the tenth rib and sutured in place over a Vicryl mesh scaffold. The thoracodorsal nerve was anastomosed to the phrenic nerve. Functional analysis of the flap was performed in 4 patients. RESULTS: Age at placement of the muscle graft ranged from 2 months to 48 months (median, 24 months). There has been no evidence of reherniation after placement of the muscle graft. Long-term outcome and functional analysis of the flap was available in 4 patients (mean, 19 months). Two infants had fluoroscopic and sonographic evidence of nonparadoxical neodiaphragmatic motion. In one of these, electromyographic evidence of function was documented with a phrenic nerve conduction velocity of 22 meters per second. The third infant showed no evidence of neodiaphragmatic motion, and the fourth infant had paradoxical motion. CONCLUSIONS: This is the first direct documentation of phrenic nerve function in an infant with CDH. An innervated reversed latissimus dorsi (RLD) flap reconstruction for recurrent CDH provides an alternative to prosthetic patch repair. This technique offers the advantages of autologous vascularized tissue with potential phrenic nerve innervation and physiologic neodiaphragmatic motion.

Facial nerve monitoring in middle ear and mastoid surgery.

HYPOTHESIS: Intraoperative electromyographic facial nerve monitoring, long accepted as the standard of care in surgery for acoustic neuroma and other cerebellopontine angle tumors, may be of aid in middle ear and mastoid surgery. STUDY DESIGN: Retrospective series of 262 cases of middle ear/mastoid surgery in which monitoring was performed by a neurophysiologist. METHODS: Neurophysiological monitoring events were classified as mechanical or electrical. The voltages producing facial nerve stimulation were compiled and compared with observed facial nerve dehiscence. RESULTS: The most common use of monitoring was localization of the facial nerve by electrical stimulation (60%) or identification of mechanically evoked activity (39%). In 57 cases (36%), the first electrical stimulation event evoked a facial nerve response at less than 1 V threshold, indicating little or no bony covering. The minimum stimulation threshold throughout each of these cases was less than 1 V in 88 of the 159 cases (55%) in which stimulation was attempted. In contrast, the facial nerve was visibly dehiscent in only 35 cases (13%). Neurophysiological monitoring confirmed aberrant facial nerve course through the temporal bone in four cases resulting in cancellation of surgical treatment in two cases. Postoperative facial nerve function was preserved in all cases when present preoperatively. CONCLUSIONS: An electrical stimulation threshold of less than 1 V is a more useful criterion of dehiscence than observation under the operating microscope. The absence of monitoring events allows safe dissection. Monitoring can help locate the facial nerve, guide the dissection and drilling, and confirm its integrity, thereby allowing more definitive surgical treatment while preserving neural function.

Identification of motor pathways during tumor surgery facilitated by multichannel electromyographic recording.

OBJECT: The goal of this study was to determine the usefulness of electromyographic (EMG) recording in locating motor pathways near the central sulcus or internal capsule during surgery. METHODS: Multichannel EMG recordings were compared with visual observation of contralateral body movement that was elicited by direct cortical or subcortical stimulation used to identify motor pathways before and during tumor resection. The EMG recordings were more sensitive than visual observation alone in identifying motor responses: in 30% of cases, responses were identified by EMG recording alone at some point during the operation and, in 9% of cases, EMG responses were the only responses observed. Additionally, EMG recordings often detected seizure activity resulting from electrical stimulation of the cortex that could not be appreciated on visual inspection. No new motor deficits were seen postoperatively in 88% of the patients in this series. CONCLUSIONS: Using EMG recording in addition to motor pathway mapping results in greater sensitivity, allowing the use of lower stimulation levels and facilitating detection of stimulation-induced seizure activity.

Assessment of sensory function in neonatal sheep with somatosensory evoked potentials: methodology and normative data.

Fetal sheep are increasingly used as animal models for fetal surgical interventions such as repair of myelomeningocele. Since behavioral observations cannot provide objective information about preservation of sensory function, we have developed a technique for reliably recording somatosensory evoked potentials in neonatal sheep. We determined anatomic criteria for placement of recording electrodes over the somatosensory cortex using external landmarks, and recorded normative data for both ulnar and posterior tibial nerve stimulation in a series of normal neonatal sheep. The methodology and normative data are presented in this report; a companion paper demonstrates the utilization of this technique in a variety of experimental fetal interventions.

Experimental fetal neurosurgery: effects of in-utero manipulations on somatosensory evoked potentials.

Somatosensory evoked potentials (SEP) were used to objectively evaluate sensory function in neonatal sheep after experimental fetal surgery. Posterior tibial (PTN) and ulnar (UN) nerves were stimulated electrically and averaged SEP were recorded from scalp electrodes placed over the somatosensory cortex. Animals with experimentally-created myelomeningocele (MMC) showed no SEP to PTN stimulation, but normal SEP to UN stimulation. In-utero repair of the MMC resulted in preservation of neurologic function and normal PTN SEP. In-utero thoracic spinal-cord transection resulted in no regeneration, and no SEP to PTN stimulation. In-utero unilateral transection of the sciatic nerve, even with attempted repair, resulted in little or no regeneration and absent or grossly abnormal PTN SEP from the affected side. In summary, the SEP technique provides valuable information concerning preservation of sensory function in a variety of experimentally created neurologic abnormalities and can aid in functional evaluation of experimental therapeutic fetal interventions.

Very late pain-related activity identified with topographically mapped frequency domain analysis of evoked potentials.

OBJECTIVE: To identify low-frequency activity in the pain-evoked potential at very late latencies, consistent with C-fiber transmission velocities. METHODS: Brief (1 ms) painful (intracutaneous) and two levels of non-painful (mild and strong) electrical pulses were applied to the index and middle fingers of the left hand. Evoked potentials (EPs) were recorded from 30 electrodes covering the entire scalp. Data from the 3 stimulus conditions (approximately 60 trials per condition per subject) were compared using the frequency domain technique of complex demodulation applied to single trial data. Subjects were 14 normal right-handed male human volunteers, aged 19-36 years. RESULTS: Using descriptive probability mapping, pain versus strong non-pain differences were found in grand average data as well as in 8 of 14 subjects, consisting of greater low-frequency power at latencies from 700 to 1100 ms at electrodes near the contralateral central sulcus and at the vertex. CONCLUSIONS: There are topographically focal, pain versus non-pain differences in the 700-1100 ms latency range that can be seen using frequency-domain analytic techniques. These differences were not seen with traditional time domain analyses. They may be due to a C-fiber-related mechanism or to very late activity triggered by faster fibers.

Comparison of response amplitude versus stimulation threshold in predicting early postoperative facial nerve function after acoustic neuroma resection.

OBJECTIVE: This study aimed to better predict the early postoperative facial nerve (FN) function after acoustic neuroma (AN) resection. STUDY DESIGN: This study was a prospective series. SETTING: The surgery was conducted in a tertiary referral center. PATIENTS: A total of 44 patients undergoing AN resection with cranial nerve monitoring were observed for at least 1 year after surgery. MAIN OUTCOME MEASURES: The predictive value of amplitude of the FN stimulus response on the early postoperative FN function was measured. RESULTS: Cranial nerve monitoring in AN surgery was used to obtain the stimulation threshold and facial electromyograph response amplitudes to FN stimulation proximal and distal to the tumor at 0.2 V above threshold. Thirty-eight of forty-four patients studied had a low postresection threshold (< or = 0.1 V). Of these (10), 26% sustained a postoperative FN dysfunction of House-Brackmann (HB) grades 3-6. In an effort to improve the predictive value from cranial nerve monitoring, the response amplitude to suprathreshold stimulation was compared with the threshold and FN function. Eighty-nine percent of patients with an amplitude of > or =200 microV had a grade 1-2 early postoperative FN function, whereas only 41% of patients with < 200 microV had a grade 1-2 early postoperative FN function (p = 0.00035). Eighty-eight percent of patients with both a low threshold and high amplitude had a grade 1-2 early postoperative FN function, whereas the remaining 12% of patients had a grade 3-6 FN function (p = 0.0032). The false-positive rate of threshold alone in predicting a grade 1-2 FN function was 26% compared to 12% for low threshold and high amplitude combined. CONCLUSIONS: The use of FN threshold and amplitude together is superior to threshold alone as a predictor of early postoperative FN function.

Midgestational sciatic nerve transection in fetal sheep results in absent nerve regeneration and neurogenic muscle atrophy.

In order to test whether fetal nerve healing and regeneration result in complete functional recovery, we transected the sciatic nerve at trunk level in 13 midgestational sheep fetuses. In 10 fetuses immediate microsurgical nerve coaptation was performed. The neonatal lambs were evaluated clinically, electrophysiologically, and histologically. On the transected side, the 10 surviving lambs showed a sensorimotor sciatic nerve paralysis and atrophy of the muscles innervated by the sciatic nerve. Somatosensory evoked potentials were weakly present in 5 animals and absent in 5 animals. Histologically, minimal signs of axonal regeneration, massive degeneration of the entire nerve, and a marked neurogenic muscle atrophy were found. These unexpected results differ from the findings after peripheral nerve transections in late gestational sheep fetuses and also from the classic wallerian degeneration-regeneration pattern that follows adult nerve injury. We speculate that the almost absent regenerative potential at midgestation is related to axotomy-induced neurotrophic factor deprivation during a developmental phase where the neurons are critically dependent on growth factor for survival.

Auditory P3 responses to name stimuli.

Auditory evoked potentials (AEPs) were recorded from 10 normal adults in response to their own first names and to other first names spoken on tape. The following experimental conditions were used: 30 repetitions of the subject's first name; 80 other assorted first names from the same gender; 30 repetitions of a first name other than the subject's name. A P3 component was recorded from all ten subjects in response to their own first name, but not to other first names. Utility of this procedure could include assessment of cognitive processing of nonresponsive populations such as comatose patients, stroke patients, demented patients, autistics, infants, and children.

The fetal spinal cord does not regenerate after in utero transection in a large mammalian model.

OBJECTIVE: Regeneration and functional recovery after spinal cord transection do not occur in mammalian animals and humans postnatally. The goal of this study was to test whether in utero transection of the fetal spinal cord is succeeded by anatomic healing and functional recovery. METHODS: In five sheep fetuses, at 60 days of gestation and 75 days of gestation (term = 150 d), the spinal cord was completely transected at T10. The animals were delivered near term by cesarean section for clinical evaluation, measurement of cortical somatosensory evoked potentials, and morphological assessment. RESULTS: The newborn lambs demonstrated sensory-motor paraplegia, were incontinent of urine and stool, and exhibited a spinally generated, ambulatory pattern of the hindlimbs. No cortical somatosensory evoked potentials could be recorded in response to posterior tibial nerve stimulation, although potentials from the ulnar nerve, which enters the cord rostral to the lesion, were normal in all animals. Histologically, no neuronal connections across the transection site were identified. The cord proximal to the lesion was grossly normal, whereas distal to the transection, it appeared slightly smaller but with the cytoarchitecture preserved. CONCLUSIONS: Unlike in lower vertebrate and avian species, the fetal ovine spinal cord has no detectable spontaneous regenerative capabilities when transected during midgestation. Gap formation after transection, secondary posttraumatic cell death, and missing guiding channels for sprouting axons may be factors involved in the absence of any regenerative response.

Steady-state analysis of somatosensory evoked potentials.

We report the development of a new method for frequency domain analysis of steady-state somatosensory evoked potentials (SEPs) to amplitude-modulated electrical stimulation, which can be recorded in significantly less time than traditional SEPs. Resampling techniques were used to compare the steady-state SEP to traditional SEP recordings, which are based on signal averaging in the time domain of cortical responses to repetitive transient stimulation and take 1-2 min or more to obtain a satisfactory signal/noise ratio. Median nerves of 3 subjects were stimulated continuously with electrical alternating current at several modulation frequencies from 7 to 41 Hz. Amplitude modulation was used to concentrate the power in higher frequencies, away from the modulation frequency, to reduce the amount of stimulus artifact recorded. Data were tested for signal detectability in the frequency domain using the T(circ)2 statistic. A reliable steady-state response can be recorded from scalp electrodes overlying somatosensory cortex in only a few seconds. In contrast, no signal was statistically discriminable from noise in the transient SEP from as much as 20 s of data. This dramatic time savings accompanying steady-state somatosensory stimulation may prove useful for monitoring in the operating room or intensive care unit.

In utero repair of experimental myelomeningocele saves neurological function at birth.

In a previous series of fetal sheep experiments, the authors demonstrated that midgestational exposure of the normal spinal cord to the amniotic space leads to a myelomeningocele (MMC) at birth that closely resembles human MMC phenotypes in terms of morphology and functional deficit. The present study tested whether delayed in utero repair of such evolving experimental MMC lesions spares neurological function. In 12 sheep fetuses, a spina bifida-type lesion with exposure of the lumbar spinal cord was created at 75 days' gestation (full term, 150 days). Four weeks later, the developing MMC lesions were repaired in utero for seven fetuses (five fetuses died before this time). Of those that had repair, three were delivered near term by cesarean section, and four died in utero or were aborted. All survivors had healed skin wounds and near-normal neurological function. Despite mild paraparesis, they were able to stand, walk, and perform demanding motor tests. Sensory function of the hindlimbs was present clinically and confirmed electrophysiologically. No signs of incontinence were detected. Histologically, the exposed and then covered spinal cord showed significant deformation, but the anatomic hallmarks as well as the cytoarchitecture of the spinal cord essentially were preserved. These findings show that timely in utero repair of developing experimental MMC stops the otherwise ongoing process of spinal cord destruction and "rescues" neurological function by the time of birth. Because there is evidence that a similar secondary damage to the exposed neural tissue also occurs in human MMC, we propose that in utero repair of selected human fetuses might reduce the neurological disaster commonly encountered after birth.

Delayed onset facial nerve dysfunction following acoustic neuroma surgery.

Delayed onset facial nerve dysfunction following acoustic neuroma surgery is an under-appreciated phenomenon. The authors have recently reviewed long-term (> 1 year) facial nerve outcome in 129 patients who underwent acoustic neuroma removal with the aid of cranial nerve monitoring between 1986 and 1990. The facial nerve was anatomically preserved in 99.2% of the patients, and at one year, 90% of all the patients had House-Brackmann (H-B) grade I or II facial nerve function. Delayed onset worsening of facial nerve function was noted in 38 of 129 (29%) patients, most of which occurred in the first few postoperative days. The incidence increases to 41% (38 of 93) when corrected for those with immediate H-B grade VI weakness, and who therefore could not manifest further deterioration. The facial nerve function either deteriorated from normal to abnormal or increased in severity of weakness. Delayed facial palsy was not related to the size of tumor or the surgical approach. The most common occurrence was that of a patient with H-B grade I or II facial nerve function worsening to H-B grade VI in the postoperative period. The prognosis for recovery of facial nerve function following delayed palsy was excellent. In the majority of cases, the recovery was complete within the first 6 months without specific treatment. Comparable to the patients without delayed palsies, 89% (34 of 38) of the cases had H-B grade I or II and 97% (37 of 38) had H-B grade III or better facial nerve function at 1 year. This review suggests a surprisingly high incidence of delayed facial palsy following acoustic neuroma surgery, which fortunately has an excellent prognosis for spontaneous recovery.

Creation of myelomeningocele in utero: a model of functional damage from spinal cord exposure in fetal sheep.

A recent study in human fetuses with myelomeningocele (MMC) suggested that the primary malformation is not neural but a failed closure of the posterior vertebral column and paraspinal soft tissue, which leads to exposure and secondary destruction of the spinal cord. The goal of this study was to test whether chronic exposure of the normal spinal cord to the amniotic space produces a lesion similar to human MMC. In fetal sheep at 75 days' gestation (group A) and 60 days' gestation (group B) (term = 150 days), the lumbar spinal cord was exposed to the amniotic cavity by excising skin and paraspinal soft tissues, and by performing a laminectomy. Some animals from both groups were fetectomized and assessed morphologically at 100 days' gestation. The remainder were delivered near term and assessed clinically, electrophysiologically, and morphologically. In group A, all animals showed MMC-type pathology. The exposed spinal cord was herniated out of the spinal canal and rested on the dorsal membranes of a cystic sac. The neural tissue was stretched and flattened out. Histologically, the hallmarks of the spinal cord were not discernable and the cytoarchitecture was lost. These changes were less severe at 100 days than at term. The three survivors in group A were paraplegic. In group B, the two survivors and two fetuses harvested at 100 days had healed skin wounds and near normal spinal cord histology. The other animal harvested at 100 days had a MMC-type lesion with less severe histological changes. The two survivors had a mild paraparesis.(ABSTRACT TRUNCATED AT 250 WORDS)