Somatosensory evoked potential surgical monitoring. Observations during combined isoflurane-nitrous oxide anesthesia.

Somatosensory evoked potentials are used to monitor the integrity of the of the spinal cord during surgical procedures where the spinal cord is at risk. Changes in somatosensory evoked potential latencies and amplitudes can indicate impending structural damage. This use of somatosensory evoked potentials is complicated by the effects of nonsurgical factors, including anesthetics. Fifty patients were studied during surgery to determine the effects of coadministered isoflurane and nitrous oxide on cortical and subcortical somatosensory evoked potentials potentials following stimulation of the tibial and median nerves. Analysis revealed statistically significant correlations between expired isoflurane concentrations and prolongation of scalp and cervical latencies, and a negative correlation between nitrous oxide concentrations and scalp amplitudes. Scalp somatosensory evoked potential latencies were also positively correlated with procedure duration, unexplained by other variables. These data emphasize the complexity of interpreting somatosensory evoked potential changes during surgical monitoring, and indicate that separate, defined physiologic effects of an anesthetic can be manifest even when more than one anesthetic is administered.

Quantitative measurement of delta activity in Alzheimer's disease.

Quantitative measurements of delta activity were made in 10 healthy elderly controls and 31 subjects with Alzheimer's disease. Delta activity did not discriminate between the healthy elderly controls and the early mild Alzheimer's disease subjects. However, delta activity was a significantly greater percentage of total EEG power in the moderate-to-advanced Alzheimer's subjects when compared to either the healthy controls or mild Alzheimer subjects. In the T3 and T4 electrodes, delta activity in the moderate-to-advanced Alzheimer subjects was 78.3% and 47.6% higher, respectively, than in control subjects. Furthermore, delta activity was an excellent predictor of dementia severity within the 31 subjects with Alzheimer's disease.

Somatosensory evoked potentials in lacunar syndromes of pure motor and ataxic hemiparesis.

Syndromes of hemiparetic lacunar infarction include pure motor hemiparesis and ataxic hemiparesis. Twelve such patients were evaluated with somatosensory evoked potentials. Potentials were delayed or absent in all 4 patients with ataxic hemiparesis and in 1 of 8 patients with pure motor hemiparesis. Unlike pure motor hemiparesis, ataxic hemiparesis appears to be associated with abnormal somatosensory evoked potentials. These abnormalities suggest that disturbance of afferent pathways are important in ataxic hemiparesis.

Somatosensory evoked response evaluation of cervical spondylytic myelopathy.

There were 13 patients with cervical spondylytic myelopathy (CSM) evaluated. All had extradural defects with distortion of the cervical cord and partial or complete obstruction of myelographic dye. Posterior tibial scalp (SSEPs) were absent or delayed in all 13, whereas median SSEPs were abnormal in 9. Median SSEP abnormalties were limited to those with cord lesions at C5-C6 or above. There were eight patients with associated radiculopathies confirmed by electromyography. No meaningful differences in SSEPs were noted between those with or without root injury. Surgery confirmed the level of cord injury in 10. CSM is common with a high morbidity. The results indicate SSEPs using leg stimulation can be a sensitive indicator of cord injury in these patients and that the more commonly used arm stimulation is of value primarily for localizing the level of the myelopathy.

N20 and P40 somatosensory evoked potentials: thalamic lesions and subcortical origin.

Three patients with well-defined clinical and radiographic lesions have been studied with somatosensory evoked potentials (SSEPs). The data indicate that the primary scalp potentials from stimulation of both the arm and leg (N20 and P40) can be lost with thalamic lesions and would be consistent with these potentials being generated in either the thalamus or thalamocortical radiations.