The R61 will perform a four-part double-blind randomized crossover study transitioning from a pretreatment baseline phase, to randomized treatment with either lidocaine or an identical placebo patch, washout, and alternate arm. DLss measures will be obtained before and after each phase. Twice daily report of pain using a visual analogue scale will track severity of ongoing spontaneous pain in participants. The hybrid biomarker will distinguish between placebo and active treatment arms, will significantly correlate with extent of neuropathic pain reduction during lidocaine, but will not change during the placebo phase or no-treatment lead-in. If preset Go/No-Go criteria are met, the subsequent R33 validation will then compare lidocaine patch and placebo treatment in a blinded, randomized parallel arm study.

Pain caused by peripheral neuropathy is very common. Understanding patient response to pain and its treatment is one of the biggest barriers to development of effective medicines to reduce neuropathic pain. This project will develop and validate a non-invasive test that correlates with patients' experience of pain caused by peripheral neuropathy, a "pain biomarker". The biomarker device applies specialized light energy to skin on the top of the foot to selectively stimulate the nerve fibers in skin that sense and transmit pain. The test will determine how much energy is necessary to cause a person to feel the stimulus, or to first feel the pain of the stimulus. Sensory threshold to the stimulus, and transient enhancement of blood flow in the skin (flare) will be correlated with patient report of ongoing foot pain. Use of the experimental device, diagnostic testing, and use of lidocaine have risks that are explained in this document, but can include allergic reactions, discomfort, and temporary numbness.

SUMMARY OF STUDY PROCEDURES This study is being conducted by Drs. J. Robinson Singleton in the Department of Neurology at the University of Utah, and by Mikhail Nemenov PhD, of Stanford University, who developed the use of this technique for stimulating nerves in the skin. Study 1 will correlate the biomarker with patients' reported ongoing neuropathic foot. You may also be asked to participate in the Study 2, in which change in the biomarker will be correlated longitudinally to change in pain experience in a brief, randomized crossover trial of placebo or lidocaine patches applied to the top of the foot.

Optimization of DLss Biomarker in Healthy Subjects (Stanford and Utah)

Healthy subjects will be screened to exclude neuropathy, foot pain and diabetes then will have the DLss measures performed over several days. The visit lasts about 3-4 hours.

Study 1: biomarker correlation with ongoing pain (Utah)

This study consists of a single visit designed to evaluate possible neuropathy and assess its severity with history, standardized brief exam, and specialized testing. All participants will then rate their ongoing foot pain, and biomarker testing will be performed. The entire visit will take 2-3 hours. The purpose is to correlate the biomarker with patients reported ongoing neuropathic foot pain.

People with peripheral neuropathy. A total of 50 people will participate.

Study 2: biomarker correlation with change in pain during lidocaine treatment (Utah)

Study 2 is designed to see if change in neuropathic pain from treatment with an effective pain reducing agent correlates with change in the biomarker. Lidocaine, a locally acting anesthetic, will be applied to the top of the foot using a patch. The study is structured as a 4 week blinded and randomized crossover treatment trial and will five consist of 5 brief weekly study visits, once weekly over the 4 week study period. Crossover means that you will receive both treatments for a week each. Randomized means that the treatment you receive first, placebo or Lidocaine patch, will be chosen at random. Blinded means that you will not be told whether the treatment you receive during the treatment weeks is placebo or Lidocaine. The patches will look similar or identical. Neither you, nor the study coordinator will be able to tell the identity of the patches.

Study Segments: Each weekly segment of the study is proceeded and followed by a visit, in sequence. Each visit will include review of neuropathic pain and performance of QST and Biomarker Device assessment. There are four segments:

1. Baseline neuropathic pain evaluation. Each participant will be screened in clinic, then record daily pain severity for 7 days.
2. Treatment period 1. After this baseline period, each participant will be randomized to 7 days of treatment with a patch (either lidocaine or placebo) chosen by the study at random.
3. A 7-day wash out period without treatment.
4. Treatment period 2. Daily treatment with patch not received in the first treatment period.

A total of 44 people with neuropathy and associated neuropathic pain will participate

Inclusion Criteria:

Inclusion criteria for Objective 1 (Healthy Controls)

1. 18-70 years of age
2. no complaints of peripheral neuropathy or other foot pain
3. no medical history of disease or medication use associated with peripheral neuropathy (e.g. diabetes)
4. DLss 3-questions and sharp sensation testing indicate neuropathy is not present.

4a Do you have persistent numbness or tingling in your feet? 4b Do you have pain in your feet that is not due to an orthopedic injury or lesions to skin/toenails? 4c Do you have problems with your balance? 5) no known allergy to lidocaine

Inclusion Criteria for Objective 2 - Neuropathy Correlation

1. . 18 years of age and older
2. Length dependent, sensory predominant, peripheral neuropathy from any non-acute acquired cause (e.g. diabetes, pre-diabetes, chemotherapy induced), OR musculoskeletal pain from plantar fasciitis or ankle sprain.

Inclusion Criteria for Cross Over

1. 18 years old or older:
2. Pain rating on Average Visual Analog Scale (VAS) > 30mm or Worst Visual Analog Scale (VAS) > 40mm
3. UENS score greater than or equal to 4 or a length dependent, sensory predominant, peripheral neuropathy from any non-acute acquired cause based on the opinion of the investigator:

Exclusion Criteria:

1. Current peripheral neuropathy that is rapidly changing (e.g. Guillain Barre syndrome or glucose correction neuropathy) :
2. Severe liver disease (ie: untreated/active hepatitis) :
3. Currently receiving chemotherapy:
4. Recent or planned change in neuropathic pain medication during study (approximately 7 weeks) :
5. Structural defects in foot that result in participant being unable to differentiate between neuropathic pain and another source of pain (ie: plantar fasciitis, arthritis) :
6. Unable to comply with protocol requirements or is at undue risk as judged by the investigator :

Exclusion Criteria for Patch Cross Over

1. Known allergy to topical lidocaine or other patch ingredients (ie adhesive, butylated hydroxytoluene, dipropylene glycol, silicon dioxide, styrene/isoprene/sytrene copolymer):
2. Use of topical lidocaine patch or other topical neuropathic pain creams on feet (e.g. capsaicin) :
3. Taking medications similar to lidocaine or with anti-arrhythmic properties (ie: tocainide or mexiletine) :
4. Known allergy to lidocaine or other para-aminobenzoic acid derivative (e.g. procaine, tetracaine, benzocaine) :

Unblinded team member will dispense patch masked to patient and investigator