Mapping of the central sulcus using non-invasive ultra-high-density brain recordings

Brain mapping is vital in understanding the brain's functional organization. Electroencephalography (EEG) is one of the most widely used brain mapping approaches, primarily because it is non-invasive, inexpensive, straightforward, and effective. Increasing the electrode density in EEG systems provides more neural information and can thereby enable more detailed and nuanced mapping procedures. Here, we show that the central sulcus can be clearly delineated using a novel ultra-high-density EEG system (uHD EEG) and somatosensory evoked potentials (SSEPs). This uHD EEG records from 256 channels with an inter-electrode distance of 8.6 mm and an electrode diameter of 5.9 mm. Reconstructed head models were generated from T1-weighted MRI scans, and electrode positions were co-registered to these models to create topographical plots of brain activity. EEG data were first analyzed with peak detection methods and then classified using unsupervised spectral clustering. Our topography plots of the spatial distribution from the SSEPs clearly delineate a division between channels above the somatosensory and motor cortex, thereby localizing the central sulcus. Individual EEG channels could be correctly classified as anterior or posterior to the central sulcus with 95.2% accuracy, which is comparable to accuracies from invasive intracranial recordings. Our findings demonstrate that uHD EEG can resolve the electrophysiological signatures of functional representation in the brain at a level previously only seen from surgically implanted electrodes. This novel approach could benefit numerous applications, including research, neurosurgical mapping, clinical monitoring, detection of conscious function, brain-computer interfacing (BCI), rehabilitation, and mental health.

Intraoperative language mapping guided by real-time visualization of gamma band modulation electrocorticograms: Case report and proof of concept

Background

Electrocorticography (ECoG) language mapping is often performed extraoperatively, frequently involves offline processing, and relationships with direct cortical stimulation (DCS) remain variable. We sought to determine the feasibility and preliminary utility of an intraoperative language mapping approach guided by real-time visualization of electrocorticograms.

Methods

A patient with astrocytoma underwent awake craniotomy with intraoperative language mapping, utilizing a dual iPad stimulus presentation system coupled to a real-time neural signal processing platform capable of both ECoG recording and delivery of DCS. Gamma band modulations in response to 4 language tasks at each electrode were visualized in real-time. Next, DCS was conducted for each neighboring electrode pair during language tasks.

Results

All language tasks resulted in strongest heat map activation at an electrode pair in the anterior to mid superior temporal gyrus. Consistent speech arrest during DCS was observed for Object and Action naming tasks at these same electrodes, indicating good correspondence with ECoG heat map recordings. This region corresponded well with posterior language representation via preoperative functional MRI.

Conclusions

Intraoperative real-time visualization of language task-based ECoG gamma band modulation is feasible and may help identify targets for DCS. If validated, this may improve the efficiency and accuracy of intraoperative language mapping.

Long-latency gamma modulation after median nerve stimulation delineates the central sulcus and contrasts the states of consciousness

OBJECTIVE

To evaluate the functional use of sub-band modulations in somatosensory evoked potentials (SSEPs) to discriminate between the primary somatosensory (S1) and motor (M1) areas and contrast the states of consciousness.

METHODS

During routine intraoperative cortical mapping, SSEPs were recorded with electrocorticography (ECoG) grids from the sensorimotor cortex of eight patients in the anesthetized and awake states. We conducted a time-frequency analysis on the SSEP trace to extract the spectral modulations in each state and visualize their spatial topography.

RESULTS

We observed late gamma modulation (60-250 Hz) in all subjects approximately 50 ms after stimulation onset, extending up to 250 ms in each state. The late gamma activity enhancement was predominant in S1 in the awake state, where it discriminated S1 from M1 at a higher accuracy (92 %) than in the anesthetized state (accuracy = 70 %).

CONCLUSIONS

These results showed that sensorimotor mapping does not need to rely on only SSEP phase reversal. The long latency gamma modulation can serve as a biomarker for primary sensorimotor localization and monitor the level of consciousness in neurosurgical practice.

SIGNIFICANCE

While the intraoperative assessment of SSEP phase reversal with ECoG is widely employed to delineate the central sulcus, the median nerve stimulation-induced spatio-spectral patterns can distinctly localize it and distinguish between conscious states.

NIMG-64. INTRAOPERATIVE LANGUAGE MAPPING USING GAMMA-BAND MODULATIONS OF ELECTROCORTICOGRAM (ECOG) INDUCED BY WORD/SOUND CATEGORIZATION TASK: VALIDATION WITH REPRODUCIBLE SPEECH ARRESTS DURING LINGUISTIC TASKS

OBJECTIVE

Determination of the correlation between gamma-band modulations of electrocorticogram (ECoG) induced by linguistic tasks and reproducible speech arrests caused by bipolar direct cortical stimulations (DCS).

METHODS

3 subjects (age 39, 56, 64 years) with left temporal lobe glioma underwent surgery involving awake craniotomy. A 4x8 ECoG electrode grid (2.3mm contact exposure, 1cm contact spacing) was placed above the respective tumor area. A MATLAB-Simulink based real-time software system running on a portable laptop computer was used to map gamma-band modulations as a 2D heat map while the subjects engaged in different linguistic tasks: word/sound categorization by pressing a button, object naming, action naming, written descriptive naming, and auditory descriptive naming. Auditory stimulus was applied during word/sound categorization task (duration 300 – 500ms), auditory descriptive naming ( > 1s); other tasks involved visual stimulus only. The subjects repeated the four naming tasks while bipolar DCS (2/4/6 mA, 60Hz, 2s) was applied at different electrode pairs.

RESULTS

The electrodes having stronger gamma-band modulations were distinct for different tasks. Reproducible speech arrests occurred during object, action, auditory naming tasks while stimulating specific electrode pairs, even though not all these electrodes had strong activations during these tasks. Across all subjects these electrodes had strong activations consistently during word/sound categorization tasks, starting as early as 250ms and lasting even after the auditory stimuli were terminated (~ 650ms). The longer activations can be associated with word recognition process. The subjects self-reported about having difficulty in comprehension rather than speech production during speech arrests. 3D brain rendering using MRI images showed that the speech arrest electrodes were identically located on the superior temporal gyrus, inferior to central sulcus for all 3 subjects.

CONCLUSION

Intraoperative language mapping guided by gamma-band ECoG modulations induced by word/sound categorization tasks can be utilized to localize eloquent cortex associated with auditory processing.

NIMG-100. THE MEDIAN NERVE STIMULATION INDUCES LONG LATENCY GAMMA MODULATIONS IN THE PRIMARY SOMATOSENSORY CORTEX, WHICH DIFFERENTIATES THE CONSCIOUS STATE

OBJECTIVE

While the intraoperative assessment of the short-latency somatosensory evoked potentials (SSEPs) phase reversal in electrocorticogram (ECoG) is routinely used to locate the central sulcus (CS), its spatial-spectral patterns in the primary sensorimotor cortex and the relation to consciousness is not well characterized. Our goal was to evaluate the functional utility of using median nerve stimulation-induced ECoG sub-band modulations in pre-and post-CS to differentiate between anesthetized and awake states. Method: SSEPs were recorded from the sensorimotor cortex with ECoG during routine intraoperative cortical mapping of 8 patients in the anesthetized and awake states. We conducted a time-frequency analysis on the SSEP trace in each state to extract the spectral modulations up to 900Hz and contrasted their intensity in M1 and S1 to discern awake from anesthetized states.

RESULTS

We observed late gamma activity ranging from 60-250Hz, starting approximately 50ms after stimulation onset and extending up to 250ms. The late gamma activity was suppressed in the anesthetized state in both M1 (p< 0.01) and S1 (p=0.0369). Whiles in the awake state, the late gamma power increased significantly with respect to baseline both in M1 (p=0.0180) and S1 (p< 0.01). The difference in late gamma power between the anesthetized and the awake state was highly significant for M1 (p< 0.01) and for S1 (p< 0.01).

CONCLUSION

The results show that besides CS delineation, SSEPs long latency gamma modulations can serve as an additional biomarker to monitor the level of consciousness in neurosurgical practice.

NCOG-14. INTRAOPERATIVE COGNITIVE-LINGUISTIC MAPPING GUIDED BY VISUALIZATION OF GAMMA BAND MODULATION ELECTROCORTICOGRAMS: PROOF OF CONCEPT IN A PATIENT WITH LEFT TEMPORAL AND OCCIPITAL LOW-GRADE ASTROCYTOMA

OBJECTIVE

Determine the feasibility and preliminary utility of a novel approach to intraoperative brain mapping guided by visualization of electrocorticography (ECoG) heat maps.

METHODS

A 39-year-old male with a biopsy-proven left posterior temporal and occipital WHO grade II IDH-mutant astrocytoma underwent awake craniotomy with intraoperative language mapping. Language mapping utilized a dual iPad stimulus presentation system (NeuroMapper) coupled to a portable real-time neural signal processing system capable of both recording cortical activity and delivering direct cortical stimulation in a closed-loop fashion. An ECoG grid (4x8 with 1cm pitch) which covered the majority of the left temporal lobe was used to assess oscillatory cortical activity during administration of language paradigms including object, action, auditory descriptive, and written descriptive naming. ECoG recording and cortical stimulation were synchronized with stimulus presentation via a photosensor attached to the patient-facing tablet. Gamma band modulations in response to language paradigms at each electrode were processed in real-time and visualized as heat maps in MATLAB/Simulink. Following recording and visualization, bipolar direct cortical stimulation from the grid was conducted for each neighboring electrode pair (up to an intensity of 6 mA) during administration of language tasks.

RESULTS

Despite mild fluent aphasia, a large set of reliable baseline stimuli were obtained for the language mapping paradigms. All naming paradigms resulted in strongest heat map activation at electrode 12 located in the anterior to mid superior temporal gyrus. During stimulation, consistent speech arrest was observed across all paradigms when stimulating electrode pair 11-12, indicating good correspondence with ECoG heat map recordings. Additionally, this region corresponded well with posterior language network representation via resting-state fMRI.

CONCLUSION

Intraoperative real-time visualization of task-based ECoG gamma band modulation is feasible and may help identify targets for direct cortical stimulation. If validated, this may improve the efficiency and accuracy of intraoperative language mapping.

Translational Organic Neural Interface Devices at Single Neuron Resolution

Recording from the human brain at the spatiotemporal resolution of action potentials provides critical insight into mechanisms of higher cognitive functions and neuropsychiatric disease that is challenging to derive from animal models. Here, organic materials and conformable electronics are employed to create an integrated neural interface device compatible with minimally invasive neurosurgical procedures and geared toward chronic implantation on the surface of the human brain. Data generated with these devices enable identification and characterization of individual, spatially distribute human cortical neurons in the absence of any tissue penetration (n = 229 single units). Putative single-units are effectively clustered, and found to possess features characteristic of pyramidal cells and interneurons, as well as identifiable microcircuit interactions. Human neurons exhibit consistent phase modulation by oscillatory activity and a variety of population coupling responses. The parameters are furthermore established to optimize the yield and quality of single-unit activity from the cortical surface, enhancing the ability to investigate human neural network mechanisms without breaching the tissue interface and increasing the information that can be safely derived from neurophysiological monitoring.

Real-Time Delineation of the Central Sulcus with the Spatial Profile of SSEPs Captured with High-Density Ecog Grid

Cortical mapping is widely employed to define the sensorimotor area and delineate the central sulcus (CS) during awake craniotomies. The approach involves the gold standard somatosensory evoked potentials (SSEPs) recorded with electrocorticogram (ECoG) strip electrodes. However, the evoked response can be misconstrued from the manual peak interpretation due to the poor spatial resolution of the strip electrode or when the electrode does not precisely cover the desired cortical area. This can lead to unintentional damage to the eloquent cortex. We present a soft real-time computer based visualization system that uses recorded SSEPs with a subdural grid to aid in cortical mapping. The neural data during electrical stimulation of the median nerve at 0.6Hz are picked up with a bio-amplifier at 2.4kHz. The stimulation artifact recorded from the bipolar electromyogram (EMG) is used as the stimulation onset. The ECoG data are assessed online with MATLAB Simulink to process and visualize the SSEPs waveform. The visualization system is programmed to display the SSEPs peak activation as a heat map on a 2D grid and projected onto a screen, showcasing the nature of the cortical activities over the contact surface area. Since the grid occupies a large cortical surface, the heatmap is able to delineate the central sulcus. The map can be viewed at any time point along the SSEP trace without the need for peak interpretation. With the goal to provide additional information during cortical mapping and facilitate interpretation of ECoG grid data, we believe that this visualization system will aid in rapid definition of the sensorimotor area during surgical planning. Clinical Relevance- This real-time visualization system can be used to delineate the central sulcus in a short time during awake craniotomies.

Unsupervised machine learning can delineate central sulcus by using the spatiotemporal characteristic of somatosensory evoked potentials

Objective.

Somatosensory evoked potentials (SSEPs) recorded with electrocorticography (ECoG) for central sulcus (CS) identification is a widely accepted procedure in routine intraoperative neurophysiological monitoring. Clinical practices test the short-latency SSEPs for the phase reversal over strip electrodes. However, assessments based on waveform morphology are susceptible to variations in interpretations due to the hand area’s localized nature and usually require multiple electrode placements or electrode relocation. We investigated the feasibility of unsupervised delineation of the CS by using the spatiotemporal patterns of the SSEP captured with the ECoG grid.

Approach.

Intraoperatively, SSEPs were recorded from eight patients using ECoG grids placed over the sensorimotor cortex. Neurosurgeons blinded to the electrophysiology identified the sensory and motor gyri using neuronavigation based on sulcal anatomy. We quantified the most discriminatory time points in SSEPs temporal profile between the primary motor (M1) and somatosensory (S1) cortex using the Fisher discrimination criterion. We visualized the amplitude gradient of the SSEP over a 2D heat map to provide visual feedback for the delineation of the CS based on electrophysiology. Subsequently, we employed spectral clustering using the entire the SSEP waveform without selecting any time points and grouped ECoG channels in an unsupervised fashion.

Main results.

Consistently in all patients, two different time points provided almost equal discrimination between anterior and posterior channels, which vividly outlined the CS when we viewed the SSEP amplitude distribution as a spatial 2D heat map. The first discriminative time point was in proximity to the conventionally favored ~20 ms peak (N20), and the second time point was slightly later than the markedly high ~30 ms peak (P30). Still, the location of these time points varied noticeably across subjects. Unsupervised clustering approach separated the anterior and posterior channels with an accuracy of 96.3% based on the time derivative of the SSEP trace without the need for a subject-specific time point selection. In contrast, the raw trace resulted in an accuracy of 88.0%.

Significance.

We show that the unsupervised clustering of the SSEP trace assessed with subdural electrode grids can delineate the CS automatically with high precision, and the constructed heat maps can localize the motor cortex. We anticipate that the spatiotemporal patterns of SSEP fused with machine learning can serve as a useful tool to assist in surgical planning.

Power Modulations of ECoG Alpha/Beta and Gamma Bands Correlate With Time-Derivative of Force During Hand Grasp

It is well-known that motor cortical oscillatory components are modulated in their amplitude during voluntary and imagined movements. These patterns have been used to develop brain-machine interfaces (BMI) which focused mostly on movement kinematics. In contrast, there have been only a few studies on the relation between brain oscillatory activity and the control of force, in particular, grasping force, which is of primary importance for common daily activities. In this study, we recorded intraoperative high-density electrocorticography (ECoG) from the sensorimotor cortex of four patients while they executed a voluntary isometric hand grasp following verbal instruction. The grasp was held for 2 to 3 s before being instructed to relax. We studied the power modulations of neural oscillations during the whole time-course of the grasp (onset, hold, and offset phases). Phasic event-related desynchronization (ERD) in the low-frequency band (LFB) from 8 to 32 Hz and event-related synchronization (ERS) in the high-frequency band (HFB) from 60 to 200 Hz were observed at grasp onset and offset. However, during the grasp holding period, the magnitude of LFB-ERD and HFB-ERS decreased near or at the baseline level. Overall, LFB-ERD and HFB-ERS show phasic characteristics related to the changes of grasp force (onset/offset) in all four patients. More precisely, the fluctuations of HFB-ERS primarily, and of LFB-ERD to a lesser extent, correlated with the time-course of the first time-derivative of force (yank), rather than with force itself. To the best of our knowledge, this is the first study that establishes such a correlation. These results have fundamental implications for the decoding of grasp in brain oscillatory activity-based neuroprosthetics.

A portable platform to collect and review behavioral data simultaneously with neurophysiological signals

This paper presents a portable platform to collect and review behavioral data simultaneously with neurophysiological signals. The whole system is comprised of four parts: a sensor data acquisition interface, a socket server for real-time data streaming, a Simulink system for real-time processing and an offline data review and analysis toolbox. A low-cost microcontroller is used to acquire data from external sensors such as accelerometer and hand dynamometer. The micro-controller transfers the data either directly through USB or wirelessly through a bluetooth module to a data server written in C++ for MS Windows OS. The data server also interfaces with the digital glove and captures HD video from webcam. The acquired sensor data are streamed under User Datagram Protocol (UDP) to other applications such as Simulink/Matlab for real-time analysis and recording. Neurophysiological signals such as electroencephalography (EEG), electrocorticography (ECoG) and local field potential (LFP) recordings can be collected simultaneously in Simulink and fused with behavioral data. In addition, we developed a customized Matlab Graphical User Interface (GUI) software to review, annotate and analyze the data offline. The software provides a fast, user-friendly data visualization environment with synchronized video playback feature. The software is also capable of reviewing long-term neural recordings. Other featured functions such as fast preprocessing with multithreaded filters, annotation, montage selection, power-spectral density (PSD) estimate, time-frequency map and spatial spectral map are also implemented.

Increased TRAb and/or low anti-TPO titers at diagnosis of graves' disease are associated with an increased risk of developing ophthalmopathy after onset

Patients with low thyroid peroxidase antibodies (anti-TPO) and increased TSH-receptor antibodies (TRAb) at diagnosis of Graves' disease (GD) have been suggested to have an increased risk to develop Graves' ophthalmopathy (GO). The aim was to evaluate if GO development can be predicted.This is an observational study with registration of possible GD and GO risk factors.399 patients with GD were registered 2003-2008 in Malmö, Sweden and out of these 310 were retrospectively followed up to 6 years. The main outcome measures were anti-TPO titer, TRAb titer, smoking habits, radioiodine treatment and GO development.TRAb was assessed with a third generation assay at GD diagnosis in 231 patients. The proportion of patients with GO increased above the median 6.3 IU/L both at diagnosis of GD (p=0.001) and at follow-up (p=0.0001).The distribution of GO patients anti-TPO above or below 20 kIU/L at diagnosis of GD was similar between groups (p=0.239). However at follow-up anti-TPO<20 kIU/L was associated with an increased proportion of newly developed GO as compared to the cohort with anti-TPO>20 kIU/L (p=0.018).87% of patients who developed GO after GD diagnosis had TRAb above 6.3 IU/L and/or anti-TPO below 20 kIU/L. The proportion of GO was doubled in GD patients treated with radioiodine but could not explain the described findingsAnti-TPO<20 kIU/L and/or TRAb>6.3 IE/L at the time of GD diagnosis were associated with an increased risk to develop GO after diagnosis of GD.

Comparison of ranked segment analysis (RSA) and cup to disc ratio in computer-assisted optic disc evaluation

PURPOSE

Ranked segment analysis is a new method for evaluation of optic nerve head topography with the Heidelberg Retina Tomograph. This analysis ranks sector measurements around the optic disc and compares these measurements with rank specific significance limits. We evaluated the diagnostic precision of the ranked segment analysis in a large clinical sample and compared with vertical cup/disc ratio measurements. Vertical cup/disc ratio measurements were not corrected for disc size.

METHODS

We analysed optic disc images from 153 normal and 75 glaucomatous eyes using the Heidelberg Retina Tomograph (software version 2.01). Ranked segment analyses and vertical cup/disc ratio measurements were obtained from each optic disc image, and ROC curves were plotted.

RESULTS

At the 95% specificity level, sensitivity was significantly lower for ranked segment analysis (70%) compared to vertical cup/disc ratio measurements (85%). Almost 30% of the discs in the glaucoma group were classified as normal using the ranked segment analysis.

DISCUSSION

Ranked segment analysis yielded unacceptably poor discrimination, in fact, worse than cup/disc ratio with its known clinical limitations Glaucomatous disc damage typically occurs at the vertical poles of the disc. High rank measurements (low sector values) in normal eyes, on the other hand, are more common in the temporal disc sectors. Important spatial information is lost during the ranking procedure. This may partly explain the low sensitivity of the ranked segment analysis observed in our study. The results raise serious concerns regarding the clinical usefulness of ranked segment analysis.

Kinetic and static fixation methods in automated threshold perimetry

PURPOSE

Static fixation is the standard method for stabilizing the eye during automated perimetry. Kinetic fixation is an alternative for fixation control in which the eye follows a moving target. This study was conducted to evaluate the fixation accuracy of static and kinetic fixation perimetry and to determine their ability to detect the absolute scotoma of the physiologic blind spot.

METHODS

The 71 patients with early glaucomatous field loss (mean age 65 years) and 45 control subjects (mean age 57 years) recruited from five clinical sites underwent threshold testing on the Dicon perimeter (kinetic fixation; Vismed, San Diego, CA) and Humphrey Field Analyzer (static fixation). The frequency of Heijl-Krakau fixation catch-trial errors was used as an indicator of fixation accuracy, and the measured sensitivity at the physiologic blind spot was used as an indicator of perimetric accuracy.

RESULTS

In patients with glaucoma, the frequency of fixation errors was significantly greater for kinetic fixation (17.2%) than for static fixation (10.2%). In the control group, the frequency of fixation errors also was significantly greater for kinetic fixation (27.5%) than for static fixation (12.6%). The threshold at the presumed location of the blind spot (15 degrees temporal, 3 degrees inferior from fixation) was 14.8 dB using kinetic fixation versus 4.0 dB with static fixation in patients with glaucoma, and 18.5 dB using kinetic fixation versus 2.5 dB using static fixation in the control group.

CONCLUSION

Relative to static fixation, kinetic fixation was associated with fixation inaccuracy and underestimation of the absolute scotoma at the physiologic blind spot.

A perimetric learner's index

PURPOSE

Certain individuals need earlier perimetric experience before producing normal fields on automated static threshold perimetry. Mid-peripheral depressions and an unaffected central field are typical findings in such cases. We have devised a learner's index, to detect defect patterns that may be due to such perimetric inexperience.

METHODS

The central visual field was partitioned into 5 concentric zones and averages of deviations from the age-corrected normal threshold values were studied in each zone. The third test session in 74 randomly selected normal subjects provided an experienced reference material. Visual field results typical of normal individuals lacking perimetric experience were represented by the first field test obtained from each of 7 subjects ('learners') from the same group, who showed significant learning during three test sessions. A linear discriminant function, learner's index, was constructed that discriminates between typically experienced and typically inexperienced field results in normals.

RESULTS

Average deviation from age-corrected normal threshold increased with increasing eccentricity in the learner's initial fields. Clinical examples illustrate the intended use of the new index.

CONCLUSIONS

The learner's index highlights those first field tests from eyes with normal visual fields, that deviate significantly from a normal experienced result in the direction of a learner's result. Patients showing significant learner's index are candidates for repeated visual field testing.

Comparison of diagnostic performance and fixation control of two automated perimeters

BACKGROUND

The Humphrey perimeter and its Statpac (analysis programs have been widely used and studied. Another statistical analysis program, FieldView, is used with the Dicon perimeter. The purpose of this study was to compare the diagnostic performance of the two perimeters and their statistical analysis packages.

METHODS

Twenty-three normal subjects (age range, 27 to 83 years) and 31 patients with glaucoma or cerebrovascular disease (age range, 28 to 87 years) experienced in automated perimetry were examined using the Dicon and the Humphrey perimeters.

RESULTS

The total number of significant points identified on the Humphrey total deviation probability maps was in close agreement with statistical expectations, while the Dicon total deviation probability maps yielded significantly more false-positive defects than expected for normals. Fixation loss ratios were almost twice as high with the Dicon perimeter (mean, 16%) as compared with the Humphrey perimeter (mean, 9%). The Humphrey perimeter was more reliable than the Dicon in measuring the defect depth of the physiological blind spot.

CONCLUSION

The Dicon perimeter appears to yield excessive false-positive findings in normal subjects, resulting in poor sensitivity/specificity combinations, while at the same time failing to properly measure defect depth in scotomas.

Giant cell arteritis presenting with oculomotor nerve palsy

We present a case of histologically proven giant cell arteritis presenting as an acute unilateral oculomotor nerve palsy without pupillary dilatation. The etiology and mechanisms involved in this uncommon phenomenon are discussed. It is emphasized that a giant cell arteritis with cranial nerve involvement is a medical emergency, and that swift diagnosis and treatment is necessary to avoid permanent disability.

Perimetric probability maps to separate change caused by glaucoma from that caused by cataract

We describe a new method for analysis of change in glaucomatous visual fields with the object to differentiate between changes caused by glaucoma from those caused by cataract. New pattern deviation change probability maps were developed from a prospectively collected glaucoma material and designed to be sensitive to changes in localized field loss, but to be unaffected by media-induced perimetric change. We compared the new change probability maps with the commercially available total deviation change probability maps in series of Humphrey perimetric tests in a glaucoma material of 43 eyes of 35 patients, who had undergone cataract surgery. When using the total deviation maps, considerable differences were seen between fields obtained before and after cataract surgery. Much smaller differences were seen when using the new change probability maps, that almost eliminated the common and disturbing effect of increasing cataract. This new tool could be of considerable help in differentiation between progressive glaucomatous visual field loss and deterioration caused by increasing media opacities.

Physiology of cumulative defect curves; consequences in glaucoma perimetry

Purely diffuse field loss is a controversial sign of early glaucoma. Cumulative defect curves have been proposed to distinguish between diffuse and localized visual field loss. In such curves, pointwise deviations from the normal reference field are ranked. The deviations are then compared with normative limits for each rank. The ranking leads to a complete loss of spatial information and ignoration of pointwise physiological threshold variability. We believed that this might impair the curves' intended ability to distinguish diffuse and localized types of defects. The lower 5% normative limit was estimated for each rank in the cumulative defect curve using Humphrey 30-2 tests of 88 subjects from the Statpac normal database. For each rank we identified those two test points (among the total of 6512 (74 test points in each of 88 subjects)) from which the 5% limit was calculated. The average eccentricity, eccrank, of those two points was then determined at each rank. For lower to intermediate ranks eccrank was spread out from center to periphery. When instead the 20 highest ranks (most depressed points) were considered eccrank was much concentrated peripherally. Our normative limits were subsequently used to evaluate field results in 101 patients with glaucoma. In this glaucoma evaluation group, the largest deviations were often encountered in central or paracentral scotomas, in sharp contrast to the more peripheral position in the normative group. These deviations often failed to reach the cumulative defect curve limits which originated from points in the mid-periphery where normal variability is large. At present, cumulative defect curves are not optimized to distinguish diffuse and localized field loss.

Pitfalls of automated perimetry in glaucoma diagnosis

Perimetry is a cornerstone in glaucoma management. The detection of glaucomatous visual field loss is of crucial importance for diagnosing the disease. Automated threshold perimetry makes thorough testing of the central visual field very possible and high-quality data are often achieved. Certain factors, however, may hamper the visual field examination or visual field interpretation. Many diseases other than glaucoma are known to influence the visual field in more or less predictable ways. Of interest is also low patient reliability, learning and fatigue effects, as well as test artifacts and suboptimal test strategies or parameters. Finally, statistical aids provided by the built-in computer of the perimeter must be judged keeping in mind the population from which the patient is derived. We describe such pitfalls in glaucoma perimetry, how they can be identified, and dealt with clinically.

Diffuse visual field loss and glaucoma

It is often claimed that general reduction, or diffuse loss, of perimetric sensitivity is an early sign of glaucoma. Our clinical experience and the results of a few other studies led us to believe otherwise. To investigate factors associated with diffuse field loss we reviewed 4222 Humphrey 30-2 threshold tests from 1582 eyes of 862 patients followed at our department. Most of these patients had ocular hypertension or glaucoma. Each field test was evaluated with the Glaucoma Hemifield Test of the Statpac 2 program. The Glaucoma Hemifield Test classifies field test results as within or outside normal limits regarding localized field loss and general shifts in sensitivity. General reduction of sensitivity without concomitant localized loss was found in 117 tests from 81 eyes of 69 patients. Corresponding patient records were found for 60 eyes of 60 patients. Media opacities or miotic therapy were noted in 46 eyes (77%), 10 eyes (17%) had end-stage field loss, and in 2 eyes (3%) other non-glaucomatous ocular pathology explained the diffuse loss of sensitivity. Thus, general reduction of differential light sensitivity as an isolated finding was almost always associated with reasons other than early to moderate glaucoma in this material mainly consisting of patients with glaucoma or increased intraocular pressure. Since only 2 out of the 1582 eyes had general reduction of sensitivity that was not explained by non-glaucomatous reasons, we conclude that purely diffuse field loss was not a sign of glaucoma.

Arcuate cluster analysis in glaucoma perimetry

Typical glaucomatous visual field defects are often contiguous areas of diminished differential light sensitivity presenting as clusters of abnormally depressed points in the visual field chart. We investigated the value of recognizing arcuate cluster patterns, corresponding to the anatomy of the normal retinal nerve fiber layer. Cluster sizes were quantified using cluster volume and surface area. Clusters were analyzed separately in different regions of the visual field. Central 30 degrees static threshold fields from 87 eyes of 87 normal subjects and 101 eyes of 101 patients with glaucoma were studied. The two groups were discriminated with logistic regression. Central and nasal clusters were more indicative of glaucoma than were equally large clusters in other areas. Discrimination of normal and glaucoma eyes was significantly better with arcuate cluster analysis than with a traditional cluster analysis, which did not take cluster shape into account. Thus, arcuate cluster analysis was more sensitive to early central, paracentral, or nasal glaucomatous field loss and at the same time capable of deemphasizing common test artifacts and nondiagnostic field disturbances. Arcuate cluster volume gave better classification than did arcuate surface area. The results indicate that analysis of arcuate clusters offers substantial advantages as compared with traditional cluster analysis for recognition of early glaucomatous visual field loss.

Weighting according to location in computer-assisted glaucoma visual field analysis

In recent years several aids for automated interpretation of visual field data have been suggested. We believed that incorporation of thorough knowledge of normal visual field variability would allow improvements in the performance of such aids since more attention would be paid to field results in areas with low physiological variability. Two visual field models for classification of fields in glaucoma based on comparisons of sensitivity values in the upper and lower hemifields and on analysis of test point clusters with diminished sensitivity were compared. Both models were constructed using logistic regression analysis in 101 normal eyes and 101 eyes with glaucoma. The first, more traditional model assumed Gaussian distributions of deviations from age-corrected normal thresholds and constant variability across the field (non-weighted model). The second model took into account empirically determined variability of pointwise threshold results and of cluster volumes in various visual field regions (weighted model). The two models were subsequently tested on an independent material of 163 normal eyes and 76 eyes with glaucoma. The weighted model gave significantly better classification of the fields in both materials. Accounting for physiological threshold variability can offer significant advantages in the construction of perimetric analysis aids for detection of glaucoma.

Spatial analyses of glaucomatous visual fields; a comparison with traditional visual field indices

Interpretation of numeric automated threshold visual field results is often difficult. A large amount of data is obtained for every single field tested. Various approaches to summarize this data have been suggested, most commonly the mean and standard deviation of departures from age-corrected normal threshold values. These visual field indices differ substantially from subjective field interpretation where spatial relationships are important. We have previously devised two methods for automated field interpretation which take spatial information into account--regional up-down comparisons and arcuate cluster analysis. We now studied the merits of using these new spatial methods and compared them to traditional visual field indices for discrimination between normal and glaucomatous field results. Central static 30 degree field results in 101 eyes of 101 normal subjects and 101 eyes of 101 patients with glaucoma were discriminated using logistic regression analysis. The best field classification was obtained with a spatial visual field model combining up-down differences and arcuate clusters. The advantages of the spatial model were confirmed in an independent material of 163 eyes of 163 normal subjects and 76 eyes of 76 patients with glaucoma where eyes with large field defects had been removed. In this material the spatial model gave 87% sensitivity and 83% specificity while the best non-spatial model gave 82% sensitivity and 80% specificity. Visual field interpretation in glaucoma may be significantly enhanced if detection is focused on circumscribed field loss rather than on averages of differential light sensitivities and similar indices which do not take spatial relationships into consideration.

Evaluation of methods for automated Hemifield analysis in perimetry

A new aid to perimetric analysis, the Glaucoma Hemifield Test, primarily evaluates up-down differences in automated static visual field tests. We analyzed the visual fields of 163 eyes of 163 normal subjects and 77 eyes of 77 patients with glaucoma diagnosed on bases other than perimetry using the Glaucoma Hemifield Test and a similar, previously developed, hemifield analysis method. The performance of the Glaucoma Hemifield Test was compared with that of the earlier method and the differences in test design were evaluated individually. The Glaucoma Hemifield Test allowed significantly improved separation between the normal group and the group with glaucoma than did the earlier method. This improvement was due to an increase in sensitivity, and was associated with the use of test point significances instead of threshold values, and a large normal database alone in the determination of normal limits.

Glaucoma Hemifield Test. Automated visual field evaluation

We have developed an algorithm, the Glaucoma Hemifield Test (GHT), for automated evaluation of single static threshold visual field test results in glaucoma. The GHT uses empirically determined limits of normality for up-down differences in the Statpac probability maps of the Humphrey Field Analyzer to detect localized visual field loss. It is also constructed to detect field loss that is symmetric around the horizontal meridian. Analysis is done in five corresponding pairs of sectors that are based on the normal anatomy of the retinal nerve fiber layer. Deviations from the age-corrected normal threshold in the most sensitive portions of the visual field are used to detect general reductions of sensitivity or abnormally high sensitivities. The GHT provides brief visual field evaluations printed on the field chart as plain text. The aim of this article is to describe the fundamentals of the analysis program and to provide clinical examples.

A clinical study of perimetric probability maps

Perimetric probability maps depict visual field results in terms of the frequency with which the measured findings are seen in a normal population. We tested clinically the importance of the model of the normal visual field used to calculate such maps. Forty-one eyes of 41 normal subjects and 58 eyes of 46 glaucomatous patients were studied. Probability maps were calculated by means of two different models of the normal visual field. The first model assumed gaussian threshold distributions with constant variability across the field. The second used empirically determined nongaussian location-dependent threshold distributions. Probability maps using the empiric model allowed better separation between glaucomatous and normal eyes, and the number of significant points in normal subjects was in better agreement with the theoretically expected number. The gaussian model yielded an unacceptably high frequency of significant points in normal fields, particularly in the midperiphery. The clinical usefulness of perimetric probability maps depends critically on the choice of normal visual field model.

Visual field interpretation with empiric probability maps

Automated visual field charts may be difficult to interpret partly because of the magnitude and complex nature of normal threshold variability. We devised two types of empiric probability maps in which this variability is taken into account and the significances of measured threshold values are displayed. These maps are highly sensitive to nonobvious but significant paracentral field loss but will at the same time deemphasize false-positive patterns commonly found more peripherally. They also frequently show field defects before these are obvious in conventional threshold printouts. In addition, they differentiate between generalized loss of sensitivity and localized field defects.

Evaluation of adaptive spatial enhancement in suprathreshold visual field screening

Sixty-three normal subjects and 94 abnormal patients, most of whom had glaucoma, were tested in the central visual field using a threshold-related, eccentricity-compensated, spatially adaptive suprathreshold screening program and a full-threshold program on the Humphrey field analyzer. The initial stimulus locations on the screening test were identical to those of the threshold test; additional screening stimuli were presented surrounding each missed initial stimulus. Surprisingly, this spatial enhancement strategy did not improve sensitivity or specificity rates of the screening beyond that achieved by considering the initial stimulus locations alone. Points missed during screening often showed a depressed sensitivity rate (measured threshold greater than 6 dB below the age-corrected normal reference value) in the same area of the threshold field. This was true in fields from abnormal and normal subjects. This finding of persistent shallow defects in the same test session among otherwise normal persons has disturbing implications for the importance of "confirmed" defects in the diagnosis of disease.