Corneal Nerves Function and Structure

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    VA Office of Research and Development
Updated on 10 July 2021


Peripheral neuropathy affects about 50% of the diabetic population and there is no treatment other than good blood glucose control, which is ineffective in subjects with type 2 diabetes. Part of the problem for the lack of an effective treatment is the inability to detect peripheral neuropathy in its early stage. The hypotheses to be addressed in the first phase of this study is that changes in cornea sensitivity (blinking and squinting) following addition of a hyperosmotic solution will provide a novel screening tool for early diagnosis of peripheral neuropathy. For the second phase of the study the investigators will examine the effect of fish oil treatment of diabetic subjects with neuropathy on corneal nerve density and sensitivity. Corneal nerves are the most highly innervated part of the human body with great sensitivity. The first phase will use this property and determine whether sensitivity is lost in diabetic patients with neuropathy. Preclinical studies have supported this hypothesis and now this will be tested in human subjects. Preclinical studies have also shown that treating diabetic rodents with fish oil improves nerve regeneration and outcome measures of peripheral in diabetic rodents. In the second phase the investigators will perform preliminary studies in human subjects with diabetic neuropathy and determine whether treating them with fish oil increases corneal nerve density and sensitivity.


Recruitment has been suspended because of the COVID-19 crisis and the Iowa City VA is just beginning to have face to face appointments.

The hypotheses to be studied in the following specific aims is that changes in cornea sensitivity to a hyperosmotic solution will provide a novel screening tool for early diagnosis of diabetic peripheral neuropathy. The investigators also propose that clinical feasibility studies will demonstrate that treatment with fish oil will improve outcome measures of diabetic peripheral neuropathy.

Specific aims:

  1. Determine if the reflex squinting and blinking response to the application of a hyperosmotic solution to the cornea is impaired in patients with type 2 diabetes with or without peripheral neuropathy.
  2. Determine whether changes in corneal sensitivity correlate with loss of cornea nerves in the sub-epithelial layer of the cornea and symptom scores for PN.
  3. Determine if treating human subjects with type 2 diabetic peripheral neuropathy with an oral fish oil supplement increases circulating levels of resolvin D1, restores cornea sensitivity to a hyperosmotic eye drop challenge and improves corneal nerve density and symptom scores for peripheral neuropathy.

For this first phase the investigators will enroll 75 subjects to participate (Fifty subjects with type 2 diabetes; 25 with clinically detectable peripheral neuropathy and 25 without neuropathy will be enrolled as well as 25 age-matched control subjects). In the second phase of the study the investigators will investigate whether increased intake of fish oil improves diabetic peripheral neuropathy. Subjects will receive 4g (2g twice a day) of fish oil per day in capsule form.

The investigators first introduced fish oil as a successful treatment of peripheral neuropathy in pre-clinical studies with diabetic rodents and will now expand those studies , culminating in this feasibility study of treatment using human subjects with type 2 diabetes and neuropathy. In this study non diabetic subjects (control) and subjects with type 2 diabetes with mild to moderate peripheral neuropathy will be recruited from phase 1 of this study to participate in the phase 2 treatment phase of the study. A baseline determination of their neuropathy (Subjective Peripheral Neuropathy Screen Questionnaire) including evaluation of corneal nerve structure and sensitivity and blood sample will be performed. Tests besides the questionnaire to be used to screen the subjects for peripheral neuropathy will include 10 g mono filament, vibration threshold, ankle reflex and sensitivity to warm and cold stimulus. Subjects will then be instructed to take 2 fish oil capsules twice a day. Subjects will return at 4, 8 and 12 months for re-evaluation. If successful, the feasibility study will provide a proof of concept to advance fish oils to a larger clinical trial for diabetic peripheral neuropathy.

Visual acuity (subject's vision) will be measured by having the subject read the smallest letters on an eye chart with their glasses or best correction. This takes about 5 minutes or less.

Slit lamp examination: A standard slit lamp examination will be performed to insure that no exclusion criteria are present and no corneal abrasion is present.

The corneal sensitivity procedure will consist of applying a drop of 5% Muro 128 to the eye and recording the blinking and squinting response over the next 5 minutes. Based on preclinical studies it is anticipated that normal subjects will have an increased blinking and squinting response seconds after the addition of the hyperosmotic solution (5% Muro 128), whereas diabetic subjects with neuropathy will have a attenuated response.

Corneal sensation threshold: Corneal sensation will also be measured in each eye using a handheld Luneau Cochet -Bonnet aesthesiometer. This device contains a thin, retractable, nylon monofilament in which the length (stiffness) is varied between 6.0 mm and 1.0 mm until the subject can detect the touch on their central cornea, resulting in a blink reflex. In each eye, testing will begin with the 6.0 mm filament, after which the length will be progressively shortened by 0.5 mm until the patient reports sensing the corneal contact. Each eye will be tested three times and the best performance (in terms of longest filament that is perceived to touch the cornea) will be recorded as the corneal sensation threshold.

Ocular Coherence Tomography (OCT):

The thickness of the optic nerve and macula will also be measured inside of the eye using a special camera that forms an image of the layers of the retina. The imaging is harmless and measures the thickness or structural health of retinal layers and optic nerve. A new feature of OCT is eye-tracking. Eye tracking does not take any longer since it is done simultaneously with the imaging. Eye tracking during OCT may give us new insight into some eye disorders. This test takes approximately 10 minutes.

A hand-held pupillometer/electroretinogram device (RETeval, LKC) will be held in front of the subject s eye, but will not touch the eye. The device will provide a brief, a series of brief light stimuli and then record the pupil response and the elicited electrical response from the retina from a surface skin patch (electrode) placed below each eye, from the light as a measure of whether the inherent sensitivity of the eye in the retina is normal. The investigators will repeat this in the left eye. The visible light stimulus is safe and is given at an intensity experienced in normal daily light exposures. The test takes about 2 minutes per eye.

Condition Diabetic Peripheral Neuropathy
Treatment Blackmores Omega Daily (4 1g capsules per day)
Clinical Study IdentifierNCT04222660
SponsorVA Office of Research and Development
Last Modified on10 July 2021


Yes No Not Sure

Inclusion Criteria

Type 2 diabetes patients diagnosed based on the opinion of an Endocrinologist (Dr. Correia), the absence of a history of ketoacidosis, and a C-peptide > 0.8 ng/ml
At least 5 year known duration of diabetes
HbA1c < 9.0%
None or mild to moderate peripheral neuropathy based on medical record, Michigan Neuropathy Screening Instrument including response to monofilament test, ankle reflex test, vibration perception threshold examination using a 128 Hz tuning fork as well as hot and cold sensation evaluation

Exclusion Criteria

Any neurologic, muscular, genetic, or other condition known to affect nerve or muscle function
Electrolyte abnormalities, untreated hypothyroidism, abnormalities in calcium, phosphate, or magnesium concentrations, or any other metabolic disturbance affecting neural function
Cigarette smoking in the past year
Peripheral vascular, cardiac, pulmonary, or any other disorder affecting blood or tissue oxygenation
Risk of bleeding disorder or on medication known to increase risk of bleeding
History for use of aspirin
Any other medical or psychological condition judged to limit compliance with the protocol or interpretation of results
History of diabetic foot ulcer
LDL > 100 mg/100ml, HDL < 30 mg/100ml, Triglyceride > 400 mg/100ml. 10) BP < 140 systolic and 80 diastolic
BMI 45
Insulin use to manage diabetes 200 units
History of taking fish oil supplements
The investigators are limiting subject population to those over 50 thus pregnancy should not be an issue nonetheless females who are pregnant, looking to become pregnant or are breast feeding will be excluded ( After overnight fast of 8-12 h; Average of three determinations in the sitting position after at least 5 minutes rest.)
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