Tibial somatosensory evoked potential intraoperative monitoring: recommendations based on signal to noise ratio analysis of popliteal fossa, optimized P37, standard P37, and P31 potentials.

OBJECTIVE: To compare the intraoperative signal-to-noise ratio (SNR), reproducibility and rapidity of popliteal fossa (PF), optimized P37, standard P37 and P31 potentials. METHODS: Raw sweeps and 11 averages doubling sweep number from 2 to 2048 were compared in 37 patients undergoing scoliosis surgery. Optimized (highest amplitude or SNR) P37 derivations were Cz-CPc (22), CPz-CPc (27), Pz-CPc (7), iCPi-CPc (8), CPi-CPc (1), Cz-Pz (2) or Pz-FPz (3), and in two patients with non-decussation, Cz-CPi (1) or CPz-CPi (3). Standard P37 and P31 derivations were CPz-FPz and FPz-C5S. Signal amplitude was measured in 2048-sweep averages; peak noise was measured in raw sweeps and +/- averages; SNR was amplitude/noise. Visual superimposability and < 20-30% amplitude variation determined reproducibility. Sweeps to reproducibility determined rapidity. RESULTS: The SNR order was PF >> optimized P37 > standard P37 > P31. Mean optimized P37 SNR advantages over the standard P37 and P31 were 2.1:1 and 4.9:1. SNR had powerful non-linear correlations to reproducibility and rapidity. Median sweeps to reproducibility were PF: 2, optimized P37: 128, standard P37: 512 and P31: 1024. EEG noise was greatest in FPz derivations. Burst-suppression increased scalp potential SNR and rapidity. CONCLUSIONS: Optimized P37 and PF recordings are most rapidly reproducible due to superior SNRs and are recommended. FPz should be avoided. Burst-suppression may be desirable. SIGNIFICANCE: CPz-FPz and FPz-C5S should no longer be standard.

A comparison between derivation optimization and Cz'-FPz for posterior tibial P37 somatosensory evoked potential intraoperative monitoring.

OBJECTIVE: To compare P37 derivation optimization to Cz'-FPz. METHODS: After induction in 120 patients, monitoring derivations optimized by mapping FPz, Cz, Cz', Pz, C4', C2', C1' and C3'-mastoid to determine the P37 and N37 maximums for use as inputs 1 and 2 were compared to Cz'-FPz. This was repeated later in 35 surgeries. RESULTS: Eleven optimal derivations occurred and usually differed between sides. Input 1 was Cz', Pz, Cz, iCi', or Ci' and input 2 was Cc', FPz, Ci' or Pz. Even the most frequent Cz'-Cc' derivation was optimal for both sides of an individual in only 17% and this was true for Cz'-FPz in only 4%. Optimization produced higher amplitudes than Cz'-FPz (P<0.001). The ratio was [squareroot of 2] : 1 in 61% of patients and > or =2:1 in 28%, approximately halving or quartering averaging times. Optimization assessed decussation, disclosing non-decussation in one patient while Cz'-FPz did not. Alterations of P37 topography that reduced initially optimal derivation amplitude and made a different derivation optimal were demonstrated by repeat optimization in 13 of 35 patients, preventing misinterpretation in one. While also affected, Cz'-FPz neither detected nor adjusted for potentially misleading topographic changes. CONCLUSIONS: Higher amplitudes, decussation assessment and topographic adjustment make P37 derivation optimization superior to Cz'-FPz for monitoring this highly variable potential.

Intraoperative neurophysiologic discovery of uncrossed sensory and motor pathways in a patient with horizontal gaze palsy and scoliosis.

OBJECTIVE: To report the intraoperative neurophysiologic discovery of clinically unsuspected non-decussation of the somatosensory and motor pathways. METHODS: We performed somatosensory evoked potential (SEP) and transcranial electric stimulation (TES) muscle motor evoked potential (MEP) monitoring during scoliosis surgery for a 16 year old patient with familial horizontal gaze palsy and progressive scoliosis. Our routine procedures included optimizing tibial cortical SEP monitoring derivations through saggital and coronal (C4', C2', Cz', C1', C3'-mastoid) P37 mapping, which surprisingly indicated non-decussation. Consequently, we also obtained coronal median nerve SEPs and simultaneous bilateral muscle recordings to lateralized TES (C3-Cz, C4-Cz) intraoperatively and focal hand area transcranial magnetic stimulation (TMS) postoperatively. RESULTS: For each nerve, tibial P37/N37 distribution was contralateral/ipsilateral and median N20 ipsilateral. For each hemisphere, ipsilateral TES MEPs had lower thresholds and TMS MEPs were exclusively ipsilateral. Accurate monitoring required reversed montages. Reevaluation of an MRI (previously reported normal) disclosed a ventral midline cleft of the medulla. CONCLUSIONS: The results indicate uncrossed dorsal column-medial lemniscal and corticospinal pathways due to brain-stem malformation with absent internal arcuate and pyramidal decussations. SIGNIFICANCE: Simultaneous bilateral recording to unilateral stimulation demonstrates SEP/MEP hemispheric origin and is important for accurate interpretation and monitoring because decussation anomalies exist.

Median somatosensory evoked potential intraoperative monitoring: recommendations based on signal-to-noise ratio analysis.

OBJECTIVE: To form median somatosensory evoked potential (SEP) monitoring recommendations based on signal-to-noise ratio (SNR). METHODS: Two 1024-sweep right median SEP trials were recorded in 35 patients undergoing spine surgery. The SNR (signal power/noise power) and sweeps to reproducibility (<30% and <20% signal variation) were compared between the following derivations: cubital fossa (CF), Erb's point (EPi-EPc, EPi-M, EPi-Fz), cervical (C5S-EPc, C5S-AC, C5S-M, C5S-Fz), subcortical (CPi-EPc, CPi-M), and cortical (CPc-EPc, CPc-M, CPc-FPz, CPc-Fz, CPc-CPi, CPc-CPz), where M was the mastoid. RESULTS: Higher SNR produced markedly faster reproducibility. The CF derivation had very high SNR and single-sweep reproducibility. Of cortical derivations, CPc-CPz had highest mean SNR and fastest overall reproducibility (median 50 and 120 sweeps to <30% and <20% signal variation); occasionally CPc-Fz or CPc-CPi was better. Of Erb's point and cervical derivations, EPi-M and C5S-M had highest mean SNR and fastest reproducibility. Subcortical derivations had very low mean SNR and slow or non-reproducibility. High voltage EEG degraded cortical and subcortical derivation SNR and reproducibility in young children. CONCLUSIONS: The highest SNR derivations should be used to speed surgical feedback; slower low-SNR derivations should be omitted. Consequently, the CF is the best technical control and CPc-CPz should be the standard cortical derivation, with CPc-Fz and CPc-CPi as alternates. EPi-M and C5S-M are the best Erb's point and cervical derivations, but are optional. Subcortical derivations should be omitted. A presence/absence criterion or SEP omission may be indicated for some young children. SIGNIFICANCE: The results should influence future guidelines.

Neurologic features of horizontal gaze palsy and progressive scoliosis with mutations in ROBO3.

OBJECTIVE: To review the neurologic, neuroradiologic, and electrophysiologic features of autosomal recessive horizontal gaze palsy and progressive scoliosis (HGPPS), a syndrome caused by mutation of the ROBO3 gene on chromosome 11 and associated with defective decussation of certain brainstem neuronal systems. METHODS: The authors examined 11 individuals with HGPPS from five genotyped families with HGPPS. Eight individuals had brain MRI, and six had electrophysiologic studies. RESULTS: Horizontal gaze palsy was fully penetrant, present at birth, and total or almost total in all affected individuals. Convergence, ocular alignment, congenital nystagmus, and vertical smooth pursuit defects were variable between individuals. All patients developed progressive scoliosis during early childhood. All appropriately studied patients had hypoplasia of the pons and cerebellar peduncles with both anterior and posterior midline clefts of the pons and medulla and electrophysiologic evidence of ipsilateral corticospinal and dorsal column-medial lemniscus tract innervation. Heterozygotes were unaffected. CONCLUSIONS: The major clinical characteristics of horizontal gaze palsy and progressive scoliosis were congenital horizontal gaze palsy and progressive scoliosis with some variability in both ocular motility and degree of scoliosis. The syndrome also includes a distinctive brainstem malformation and defective crossing of some brainstem neuronal pathways.