Linear interactions between intraocular, intracranial pressure, and retinal vascular pulse amplitude in the fourier domain

Calculation of vessel pulse wave velocities in retinal vein segments within the optic disc centre

The carotid-femoral pulse wave velocity (PWV) method is used clinically to determine degrees of stiffness and other indices of disease. It is believed PWV measurement in retinal vessels may allow early detection of diseases. In this paper we present a new non-invasive method for estimating PWVs in retinal vein segments close to the optic disc centre, based on the measurement of blood column pulsation in retinal veins (reflective of vessel wall pulsation), using modified photoplethysmography (PPG). An optic disc (OD) PPG video is acquired spanning three cardiac cycles for a fixed ophthalmodynamometric force. The green colour channel frames are extracted, cropped and aligned. A harmonic regression model is fitted to each pixel intensity time series along the vein centreline from the centre to the periphery of the OD. The phase of the first harmonic is plotted against centreline distance. A least squares line is fitted between the first local maximum phase and first local minimum phase and its slope used to compute PWV. Five left eye inferior hemi-retinal veins from five healthy subjects were analysed. Velocities were calculated for several induced intraocular pressures ranging from a mean baseline of 14 mmHg (SD 5) to 56 mmHg in steps of approximately 5 mmHg. The median PWV over all pressure steps and subjects was 20.77 mm/s (IQR 29.27). The experimental results show that pulse wave propagation direction was opposite to flow in this initial venous segment.

Utilizing retinal arteriole/venule ratio to estimate intracranial pressure

PURPOSE

Intracranial pressure (ICP) control is important to avoid secondary brain injury in patients with intracranial pathologies. Current methods for measuring ICP are invasive and carry risks of infection and hemorrhage. Previously we found correlation between ICP and the arteriole-venous ratio (A/V ratio) of retinal vessels in an outpatient setting. This study investigated the usability of fundoscopy for non-invasive ICP estimation with the addition of intraocular pressure (IOP) in patients in a neuro-intensive care unit (NICU).

METHODS

This single-center prospective cohort study was conducted at the NICU of Odense University Hospital from September 2020 to May 2021. Adult patients with a Glasgow Coma Score of 8 or less, who underwent invasive pressure neuromonitoring were included. Fundoscopy videos were captured daily and analyzed using deep learning algorithms. The A/V ratio was calculated and correlated with ICP. The data was analyzed using mixed-effect linear regression models.

RESULTS

Forty patients were enrolled. Fifteen were included in the final analysis. ICP ranged from -1 to 31 mmHg (mean: 10.9, SD: 5.7), and IOP ranged from 4 to 13 mmHg (mean: 7.4, SD: 2.1). The A/V ratio showed a significant negative correlation with ICP > 15 mmHg (regression slope: -0.0659, 95%-CI: [-0.0665;-0.0653], p < 0.001). No significant change in A/V ratio was observed for ICP ≤ 15 mmHg. A similar significant correlation was found for ICP > IOP (regression slope: -0.0055, 95%-CI: [-0.0062;-0.0048], p < 0.001). Taking the IOP into account did not improve the model. The sensitivity analysis showed a sensitivity of 80.08% and a specificity of 22.51%, with an AUC of 0.6389.

CONCLUSION

In line with our previous work, non-invasive fundoscopy is a potential tool for detecting elevated ICP. However, challenges such as image quality and diagnostic specificity remains. Further research with larger, multi-center studies are needed to validate the utility. Standardization may enhance the technique's clinical applicability.

Probability density and information entropy of machine learning derived intracranial pressure predictions

Even with the powerful statistical parameters derived from the Extreme Gradient Boost (XGB) algorithm, it would be advantageous to define the predicted accuracy to the level of a specific case, particularly when the model output is used to guide clinical decision-making. The probability density function (PDF) of the derived intracranial pressure predictions enables the computation of a definite integral around a point estimate, representing the event's probability within a range of values. Seven hold-out test cases used for the external validation of an XGB model underwent retinal vascular pulse and intracranial pressure measurement using modified photoplethysmography and lumbar puncture, respectively. The definite integral ±1 cm water from the median (DIICP) demonstrated a negative and highly significant correlation (-0.5213±0.17, p< 0.004) with the absolute difference between the measured and predicted median intracranial pressure (DiffICPmd). The concordance between the arterial and venous probability density functions was estimated using the two-sample Kolmogorov-Smirnov statistic, extending the distribution agreement across all data points. This parameter showed a statistically significant and positive correlation (0.4942±0.18, p< 0.001) with DiffICPmd. Two cautionary subset cases (Case 8 and Case 9), where disagreement was observed between measured and predicted intracranial pressure, were compared to the seven hold-out test cases. Arterial predictions from both cautionary subset cases converged on a uniform distribution in contrast to all other cases where distributions converged on either log-normal or closely related skewed distributions (gamma, logistic, beta). The mean±standard error of the arterial DIICP from cases 8 and 9 (3.83±0.56%) was lower compared to that of the hold-out test cases (14.14±1.07%) the between group difference was statistically significant (p<0.03). Although the sample size in this analysis was limited, these results support a dual and complementary analysis approach from independently derived retinal arterial and venous non-invasive intracranial pressure predictions. Results suggest that plotting the PDF and calculating the lower order moments, arterial DIICP, and the two sample Kolmogorov-Smirnov statistic may provide individualized predictive accuracy parameters.

Characterization of Ocular Sonography Findings and their Relationship to Clinical Features from a South Indian Cohort of Idiopathic Intracranial Hypertension

Background

Idiopathic intracranial hypertension (IIH) typically manifests with headache, accompanied by papilledema and visual loss, and has a higher prevalence in females. In recent years, ocular sonography, particularly, measurement of optic nerve sheath diameter (ONSD), is being investigated for diagnosis of IIH.

Methods

A total of 35 patients over the age of 18 years, fulfilling the modified Dandy's criteria for diagnosis of IIH were included. Patients underwent assessment with magnetic resonance imaging, lumbar puncture, and ocular sonography to measure ONSD and ocular arterial indices.

Results

The mean ONSD values (in centimeters) in the right eye of patients with IIH was 0.57 ± 0.13, while it was 0.48 ± 0.03 in controls. In the left eye, the mean ONSD value (cm) was 0.59 ± 0.13 in patients with IIH and 0.48 ± 0.03 in controls. ONSD was significantly higher in cases compared to controls (P < 0.001, Welch test). Pulsatility index of the central retinal artery was significantly higher in cases compared to controls (P < 0.001, Welch test). Resistance index of the ophthalmic artery was statistically significant (P < 0.005, Welch test). Receiver operating characteristic curve analysis revealed a cutoff value of 5.1 mm on the right side and 5 mm on the left side had a sensitivity and specificity of more than 80% for IIH diagnosis.

Conclusion

Our study provides insights into the utility of optic nerve sheath measurements and arterial indices in the diagnosis of IIH in a South Indian cohort. Further research is needed to fully understand the longitudinal relationship of these parameters and treatment outcomes in IIH.

Empirical retinal venous pulse wave velocity using modified photoplethysmography

OBJECTIVE

Using the novel imaging method of high-speed modified photoplethysmography we measured the retinal venous pulse wave velocity in a single case.

RESULTS

A healthy 30-year-old subject underwent high-speed modified photoplethysmography (120 frames per second) with simultaneous ophthalmodynamometry at 26 Meditron units. A video of the optic nerve was analyzed using custom software. A harmonic regression model was fitted to each pixel in the time series and used to quantify the retinal vascular pulse wave parameters. Retinal venous pulsation at the optic disc was observed as a complex dynamic wall motion, whereas contraction commenced at a point in the vein at the center of the optic disc, and progressed centrifugally. The empirically estimated retinal venous pulse wave velocity at this segment was approximately 22.24694 mm/s. This measurement provides an estimate for future studies in the field.

Non-Invasive Intracranial Pressure Monitoring

(1) Background: Intracranial pressure (ICP) monitoring plays a key role in the treatment of patients in intensive care units, as well as during long-term surgeries and interventions. The gold standard is invasive measurement and monitoring via ventricular drainage or a parenchymal probe. In recent decades, numerous methods for non-invasive measurement have been evaluated but none have become established in routine clinical practice. The aim of this study was to reflect on the current state of research and shed light on relevant techniques for future clinical application. (2) Methods: We performed a PubMed search for “non-invasive AND ICP AND (measurement OR monitoring)” and identified 306 results. On the basis of these search results, we conducted an in-depth source analysis to identify additional methods. Studies were analyzed for design, patient type (e.g., infants, adults, and shunt patients), statistical evaluation (correlation, accuracy, and reliability), number of included measurements, and statistical assessment of accuracy and reliability. (3) Results: MRI-ICP and two-depth Doppler showed the most potential (and were the most complex methods). Tympanic membrane temperature, diffuse correlation spectroscopy, natural resonance frequency, and retinal vein approaches were also promising. (4) Conclusions: To date, no convincing evidence supports the use of a particular method for non-invasive intracranial pressure measurement. However, many new approaches are under development.

A machine learning approach in the non-invasive prediction of intracranial pressure using Modified Photoplethysmography

The ideal Intracranial pressure (ICP) estimation method should be accurate, reliable, cost-effective, compact, and associated with minimal morbidity/mortality. To this end several described non-invasive methods in ICP estimation have yielded promising results, however the reliability of these techniques have yet to supersede invasive methods of ICP measurement. Over several publications, we described a novel imaging method of Modified Photoplethysmography in the evaluation of the retinal vascular pulse parameters decomposed in the Fourier domain, which enables computationally efficient information filtering of the retinal vascular pulse wave. We applied this method in a population of 21 subjects undergoing lumbar puncture manometry. A regression model was derived by applying an Extreme Gradient Boost (XGB) machine learning algorithm using retinal vascular pulse harmonic regression waveform amplitude (HRWa), first and second harmonic cosine and sine coefficients (an1,2, bn1,2) among other features. Gain and SHapley Additive exPlanation (SHAP) values ranked feature importance in the model. Agreement between the predicted ICP mean, median and peak density with measured ICP was assessed using Bland-Altman bias±standard error. Feature gain of intraocular pressure (IOPi) (arterial = 0.6092, venous = 0.5476), and of the Fourier coefficients, an1 (arterial = 0.1000, venous = 0.1024) ranked highest in the XGB model for both vascular systems. The arterial model SHAP values demonstrated the importance of the laterality of the tested eye (1.2477), which was less prominent in the venous model (0.8710). External validation was achieved using seven hold-out test cases, where the median venous predicted ICP showed better agreement with measured ICP. Although the Bland-Altman bias from the venous model (0.034±1.8013 cm water (p<0.99)) was lower compared to that of the arterial model (0.139±1.6545 cm water (p<0.94)), the arterial model provided a potential avenue for internal validation of the prediction. This approach can potentially be integrated into a neurological clinical decision algorithm to evaluate the indication for lumbar puncture.