Acute ischemic stroke is the second leading cause of acquired disability. Type 2 diabetes, hypertension, smoking, dyslipidemia, and metabolic syndrome are the most common risk factors for stroke. Data mining has revealed that stroke patients are becoming younger and don't necessarily have the prevailing risk factors for stroke. So we need new ways to identify those who are more likely to suffer from stroke.

Magnetic resonance imaging (MRI) can show asymptomatic infarcts, cerebral microbleeds, periventricular white matter hyperintensities, and perivascular depression, all of which have been linked to stroke risk. It also predicts stroke by subtle micro alterations in the shape of healthy-looking white matter, independent of the widespread vascular lesions. Thus, noninvasive imaging technology is missing from the equation to detect subclinical neuronal damage and hence people at increased risk of stroke.

Corneal confocal microscopy (CCM) is a fast, non-invasive imaging procedure that image corneal nerves in resolution up to 600x magnification and has been used to assess for nerve degeneration in everything from diabetes mellitus to neurodegenerative diseases. CCM might even predict diabetic neuropathy, for which corneal nerve anatomy changes have been linked to stroke risk factors (adnormal blood glucose and dyslipidemia). In addition, intraepidermal nerve fiber loss has been reported in stroke victims, especially those with post-stroke central pain. CCM is noninvasive and a potential substitute for white matter hyperintensities.

Introduction to the CCM Technique

CCM Instruments

Laser scanning CCM HRTIII (Heidelberg Retina Tomograph III Rostock Corneal Module, Heidelberg Engineering GmbH, Heidelberg, Germany) is the most widely used instrument that uses a red Helium-Neon Diode laser of 670 nm to produce ultra-resolution images of corneal epithelial cells, keratocytes, endothelial cells, sub-basal nerve plexus, and dendritic cells. Other slit scanning in vivo commercial CCMs are manufactured by Tomey Corporation, Nidek Technologies, and Helmut Hund but are too low resolution to image the sub-basal nerve plexus.

CCM Image Acquisition

CCM image acquisition can occur in section, volume, or sequence modes. The section mode allows for manual focusing on specific areas, making it ideal for experienced users, while sequence and volume modes enable quicker automated image capture but sacrifice some image quality. Most studies have used 5-8 high-quality images from the central cornea, although the small field of view of CCM has led some centers to utilize wide-field imaging for mapping the sub-basal nerve plexus.

CCM Image Quantification

In terms of quantification, key corneal nerve parameters include corneal nerve fiber density (CNFD), corneal nerve branch density (CNBD), corneal nerve fiber length (CNFL), and inferior whorl length (IWL). CNFD measures total nerve fibers per area, CNBD counts primary branches per area, CNFL assesses total nerve length, and IWL measures nerve length at the inferior whorl. The corneal nerve fractal dimension can help differentiate between neuropathies of various etiologies. Software tools like CCMetrics and ACCMetrics facilitate manual and automated nerve quantification, and novel AI algorithms have been developed for fully automated assessments, including the identification of diabetic neuropathy.

Fig.1. HRT III-RCM Corneal confocal microscope. (Petropoulos, et al., 2021)

Applications of CCM in Ischemic Stroke

Performing CCM and measuring corneal nerve fiber pathology and stroke risk factors in a group of patients with normal glucose tolerance (NGT), impaired glucose tolerance (IGT), and Type 2 diabetes admitted with acute ischemic stroke. They identified a loss of corneal nerve fibers in the acute ischemic stroke patient, regardless of glycemic status. It also found weak associations between corneal nerve function and biomarkers like HbA1c and triglycerides.

CCM is a quick and noninvasive replacement for skin biopsies for assessing nerve damage, requiring little to no technical knowledge for taking photographs and being powered by computer-generated analysis software. They report profound corneal nerve loss in acute ischemic stroke patients and modest progression in patients with impaired glucose tolerance and type 2 diabetes, suggesting that hyperglycemia isn't the underlying cause of stroke nerve loss.

These trials increase the clinical potential of the CCM in the diagnosis of neurodegenerative patients and call for longer-term cohort studies to determine the prognostic value of the CCM as a surrogate marker of neuronal damage in acute ischemic stroke.

Fig.2. Representative images of the sub-basal nerve plexus of a healthy control participant. (Khan, et al., 2017)

1. Petropoulos, I. N., et al. (2021). Corneal confocal microscopy to image small nerve fiber degeneration: ophthalmology meets neurology. Frontiers in Pain Research, 2, 725363.
2. Khan, A., et al. (2017). Corneal confocal microscopy detects corneal nerve damage in patients admitted with acute ischemic stroke. Stroke, 48(11), 3012-3018.