Description of the condition

The peripheral nervous system encompasses the nerves mediating sensory, motor and autonomic functions that are located outside of the brain and spinal cord. Alterations to the anatomical integrity of the peripheral nerves can adversely affect their function, presenting clinically as abnormal or loss of sensation, weakness and/or as changes to autonomic function.1 England describes peripheral neuropathy as ‘a general term that indicates any disorder of the peripheral nervous system’2; this is a broad definition that includes nerve damage due to a variety of aetiologies. The pathophysiological mechanisms underlying both the development and progression of peripheral neuropathy are complex and may depend on the cause. Some of these mechanisms include altered metabolism and intracellular signalling,3 vascular and inflammatory stress4 and reactive oxygen species formation.5

The most common systemic cause of peripheral neuropathy, which is evident in over 50% of individuals affected by the condition, is diabetes mellitus; the risk of peripheral neuropathy increases with longer disease duration6 and may be correlated with the degree of glycaemic control, particularly in type 1 diabetes.7–10 Other causes include hereditary neuropathies (eg, Charcot-Marie-Tooth syndrome), post-infectious and inflammatory neuropathies (eg, Guillain-Barré syndrome) and drug-induced neuropathies (eg, platinum analogues, thalidomide and alcohol).11 Up to one-third of cases do not have an identified aetiology and are thus defined as idiopathic peripheral neuropathies.12

Clinical evaluations of peripheral nerve integrity generally include a combination of symptoms, signs and electrodiagnostic studies, which aim to evaluate the extent of nerve damage.2 Symptoms and signs of nerve damage are often assessed using validated neuropathy scales or composite scores (which combine symptomatology with clinical measures of nerve function). Nerve biopsies are invasive and, as a result, not easily repeatable and are therefore not frequently used as an outcome parameter in longitudinal studies but are instead reserved for diagnostic purposes.13

Electrodiagnostic testing examines the characteristics of the conduction of an electrical signal that travels through a single nerve. These tests are useful in providing diagnostic information and for longitudinally monitoring disease progression.14 Nerve conduction studies (NCSs) are reproducible and correlate well with underlying structural abnormalities,15 but the precision of these tests is limited to detecting changes in large myelinated nerve fibres, as they are not sufficiently sensitive to detect small nerve fibre damage.16 17 Quantitative sensory tests, which quantify thermal and pain thresholds, can be used to evaluate small nerve fibre function.17 Skin biopsies offer an alternative method to accurately diagnose and classify the extent of small fibre neuropathy, even in the absence of large fibre nerve damage.17 18 Cutaneous silent period testing is a reproducible measurement of the nociceptive spinal reflex, where thinly myelinated A-delta fibres are the afferent arm. Quantitative sudomotor axonal reflex testing assesses the function of unmyelinated postganglionic sudomotor C fibres.19 20 These are among several other methods to assess various small fibre types, and as each individual test may have a relatively low sensitivity, a combination of modalities is usually preferable to better assess small nerve fibre function.21

Recently, corneal confocal microscopy has been applied to visualise small nerve fibres in vivo.22 This technique has been shown to correlate well with intraepidermal nerve fibre biopsy results23 and is useful for detecting and documenting various types of small fibre neuropathies.24–27 Corneal confocal microscopy has also been suggested to be useful for monitoring disease progression, and as a marker for improvements in nerve function, in the investigation of therapeutic targets for diabetic peripheral neuropathy.28–30

Peripheral neuropathies are typically treated based on their subtype and/or underlying cause(s). Treatments primarily aim to manage the underlying condition to prevent progressive nerve damage and to treat any associated symptoms.2 16 31 The consequences associated with symptoms of neuropathic impairment do not only affect an individual’s quality of life, but also impart an economic burden in terms of healthcare costs and medical resources.32–34 This is especially true in chronic conditions, such as diabetes, where lifetime care is required.3

Description of the intervention

Omega-3 polyunsaturated fatty acids (PUFAs) are essential fatty acids (EFAs) with multiple double bonds, the first of which is located at the third carbon from the methyl end of the molecule. The short-chain omega-3 PUFA, alpha-linolenic acid, found in plant sources, is a metabolic precursor to the long-chain omega-3 PUFAs docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which are present in high abundance in oily fish. As humans do not have the enzymes to synthesise these fatty acids de novo, omega-3 PUFAs must be obtained from the diet or through supplementation.35 The other major class of EFAs are the omega-6 fatty acids, which derive from the diet in the form of linolenic acid and are elongated in vivo to gamma-linoleic acid and arachidonic acid (AA). Most eicosanoids derived from the omega-6-dependent AA pathway are proinflammatory; in contrast, long-chain omega-3 fatty acids bias prostaglandin metabolism towards the production of anti-inflammatory eicosanoids.

As omega-3 PUFAs competitively inhibit the metabolic conversion of omega-6 PUFAs,36 the balance of omega-3 to omega-6 fatty acid consumption can affect systemic inflammatory processes and immune activity. The ratio of consumed omega-6 to omega-3 in typical Western diets is approximately 15 to 1, whereas a ratio of 4 to 1 is considered optimal.37 Increased consumption of omega-3 PUFAs is considered to provide a range of potential general health benefits, including a reduced risk of cardiovascular disease38 39 and lowered systemic triglycerides.40 DHA, as an integral component in cellular membrane structures of the brain and retina, has been implicated in perinatal visual and neural development.41–43 In ocular conditions, omega-3 fatty acids supplements can reduce the symptoms and clinical signs associated with ocular surface inflammation in dry eye disease.44 The American Heart Foundation recommends a daily intake, for adults, of 500 mg of long-chain omega-3 PUFAs45 and up to 4 g/day in hypertriglyceridaemia.46

How the intervention might work

Once consumed, omega-3 PUFAs alter membrane protein activity and cellular signalling response to reduce immune activity and the concentration of systemic lipid inflammatory mediators.47 The incorporation of omega-3 PUFAs into cellular membranes, and their subsequent effect on membrane activity, has been shown to alter vascular function, improve sciatic nerve blood flow and enhance nerve conduction velocity (NCV) in a rat model of experimental diabetic neuropathy.48

Omega-3 PUFAs also affect intracellular signalling pathways and the expression of genes, some of which may be associated with the regulation of neuron growth and neuroprotection.49 In animal models of diabetes, omega-3 PUFA supplementation has been shown to attenuate adverse changes in nerve structure and function.50 51 Mice enriched with genes that increase endogenous profiles of omega-3 PUFAs have been shown to have reduced neuronal cell death and increased recovery to mechanical stress and peripheral nerve injury.52 Omega-3 PUFAs have also been demonstrated to promote neurite growth in rat sensory neurons.53

Derivatives of omega-3 PUFA metabolism, resolvins and protectins, which are oxygenated metabolites from EPA and DHA, respectively, may further promote neuronal function. Neuroprotectin D-1 has been shown to facilitate the regeneration of corneal nerves following refractive surgery and neurite growth from the trigeminal ganglion of mice54 and to prevent neuropathic pain after peripheral nerve injury.55

Why it is important to do this review

This will be the first systematic review to consider the potential effects of omega-3 PUFA supplementation on peripheral nerve integrity. Omega-3 PUFA supplementation has been shown to reduce neuronal damage and enhance recovery following nerve injury in experimental animal models of peripheral neuropathy. Confirmation of these effects in clinical populations would contribute significantly towards enhancing the clinical management of peripheral neuropathy. A therapeutic agent to prevent the pathogenesis of, or slow the progression of, peripheral nerve damage has the potential to greatly improve clinical outcomes. Furthermore, the potential for omega-3 PUFA supplements to alleviate neuropathy-associated symptoms would be predicted to reduce the impairment on an individual’s quality of life and lessen the economic burden of peripheral neuropathy in the community.

Eligibility criteria

All studies published from the date of database inception until 21 November 2017 will be included. Studies will be selected according to the following eligibility criteria.

Types of participants

We will include studies involving adults (ie, aged 18 years or older), recruited from within any study setting, where the structure and/or function of peripheral nerves was assessed. To be eligible for inclusion in the review, studies need to include at least one subjective measure of peripheral neuropathy (eg, symptom score), one composite measure of peripheral neuropathy (ie, combining subjective and objective measures) or one objective measure of peripheral nerve structure (eg, nerve biopsy) or function (eg, NCSs).

Types of interventions

We will consider interventions where participants were randomised to oral supplementation with short-chain and/or long-chain omega-3 PUFAs. We will accept studies that administered omega-3 supplements in any form or dosage. We will exclude studies where the intervention was administered in the form of dietary manipulation (ie, a food-based intervention), and where omega-3 PUFA supplements were administered in combination with another intervention (including other nutritional interventions), unless the intervention was administered in the same dose and frequency in the comparator group. We will consider studies where omega-3 PUFA supplements were compared with placebo or no treatment.

Types of outcome measures

There are no gold standard or universally accepted outcome measures for peripheral nerve assessment. In selecting the outcome measures for this review, we considered the recommendations provided by the European Neuromuscular Centre (ENMC) International workshop: Selection of Outcome Measures for Peripheral Neuropathy Clinical Trials (10–12 December 2014), taking into account both subjective and clinical measures.58

We will assess all outcome measures at 6 months of follow-up, with an acceptable follow-up range of between 3 and 9 months from baseline. If studies do not report the change from baseline, we will use data reported at the end of the follow-up period.

Primary outcomes

The primary outcome will be the change, from baseline, in peripheral neuropathy impairments, as quantified by validated, composite (ie, combining symptoms plus objective measures) neuropathy measures. We have not been prescriptive in our selection of particular scales as there are no universally agreed scoring systems; examples of validated, composite neuropathy assessment scales include the Michigan Diabetic Neuropathy Score,59 Neuropathy Impairment Score (NIS)60 and Total Neuropathy Score.61 For the purpose of this review, we define a ‘validated’ measure as a survey instrument that has been psychometrically tested.

Secondary outcomes

We will consider the following secondary outcomes:

1. Symptoms: change, from baseline, in symptoms of peripheral neuropathy, measured by a validated, patient-assessed symptom score.

2. Pain: change, from baseline, in mean scores of pain, measured by a validated, patient-assessed pain scale. Examples of validated scales include the visual analogue scale,62 Likert scales63 64 and the McGill Pain Questionnaire.65

3. Disability: change, from baseline, in the mean score of a patient-reported disability measure. Examples of validated disability measures include the Overall Neuropathy Limitation Scale66 and the Overall Disability Sum Score.67

4. Anatomical markers:

   1. change, from baseline, in central corneal nerve fibre length (CNFL), defined as the total length of nerves in a given area, measured in mm/mm², using a laser-scanning in vivo confocal microscope;

   2. change, from baseline, in intraepidermal nerve fibre density (IENFD).

5. NCSs: change, from baseline, in NCS parameters, as recommended by England (2005)68:

   1. sensory nerve action potential (SNAP) amplitudes of the sural, median and ulnar nerves;

   2. SNAP latencies of the sural, median and ulnar nerves;

   3. sensory NCV of the sural, median and ulnar nerves;

   4. distal compound motor action potential (CMAP) amplitude of the peroneal, tibial, median and ulnar nerves;

   5. CMAP latency of the peroneal, tibial, median and ulnar nerves;

   6. motor NCV of the peroneal, tibial, median and ulnar nerves;

   7. minimum F-wave latency of the peroneal, tibial, median and ulnar nerves.

6. Sensory function in the cornea: change, from baseline, in corneal sensation, as quantified using:

   1. contact aesthesiometry: to quantify mechanical detection thresholds using the Cochet-Bonnet aesthesiometer (measured in millimetres);

   2. non-contact aesthesiometry: to quantify corneal sensation quantified using an air-based aesthesiometer (measured in millibars).

7. Sensory function in the skin: change, from baseline, in sensory function test scores:

   1. mechanical detection thresholds, measured using pressure aesthesiometry (eg, von Frey hair aesthesiometer);

   2. cold detection thresholds measured using quantitative sensory testing (QST) methods;

   3. warm detection thresholds measured using QST methods;

   4. thermal pain thresholds, for cold and hot stimuli, measured using QST methods.

8. Adverse events: we will consider all adverse events and will analyse them in the following categories: (1) any adverse events, (2) adverse events leading to discontinuation of the interventions and (3) serious adverse events, being those leading to hospitalisation or prolonged admission, a life-threatening event or death.

Search methods for identification of studies

We will conduct comprehensive electronic database searches in: the Cochrane Central Register of Controlled Trials (CENTRAL) (Issue 10, October 2017), Ovid MEDLINE, Ovid MEDLINE Epub Ahead of Print, In-Process & Other Non-Indexed Citations, Ovid MEDLINE Daily (January 1946–November 2017) and Embase (January 1947–November 2017). We will also search the US National Institutes of Health Clinical Trials Registry (www.clinicaltrials.gov) and the WHO International Clinical Trials Registry Platform (www.who.int/ictrp/en/). We will not impose any restrictions on date or language in our search strategies. Search strategies for all electronic searches are included in online supplementary appendices 1–5. We will additionally search the bibliographies of included RCTs to identify any other potentially relevant studies.

Data collection and analysis

After the search strategies are performed within each electronic database, the reference lists will be imported into EndNote. Duplicate entries will be identified and removed. The final reference library will be imported into Covidence,69 the standard production platform for Cochrane systematic reviews, for the study selection process.

Selection of studies

The titles, keywords and abstracts of all unique articles, identified by the search strategies, will be independently reviewed by two review authors to identify potentially eligible studies. The full-text papers of articles identified as relevant or potentially relevant, by at least one reviewer, will be retrieved for full-text screening. Full-text articles will be independently screened by two review authors and assessed for eligibility according to the prespecified inclusion and exclusion criteria. Reasons for exclusion will be identified and recorded for ineligible studies that progress to the full-text screening stage. Any disagreements in eligibility assessment will be adjudicated between the two reviewers; if consensus cannot be achieved, a third independent author will be consulted to reach consensus. We will include a PRISMA flow diagram (summarising the article selection process) and a ‘Characteristics of excluded studies’ table (with reasons for study exclusion).

Data extraction and management

Two review authors will independently extract outcome data and key study characteristics for all included studies. Any discrepancies will be resolved by discussion and consensus. We will contact the study authors of relevant trials, by email, if further information or clarification is required. If we fail to receive a response from the corresponding author within 4 weeks, or if the authors are unable to provide us with the requested information, we will use the information that is available.

For each study, we will extract the following information:

1. article details: year of publication, journal of publication, language and publication status;

2. study details: dates study conducted, trial registration number, country, study setting, corresponding author details (name, institution, e-mail and address) and whether the study investigators were contacted for further information;

3. methods: exclusions after randomisation, losses to follow-up, how missing data were handled and whether a sample size calculation was reported;

4. participants: number of participants in each intervention group, participant baseline characteristics (ie, age, gender and/or sex (as specified by the study authors), underlying conditions, peripheral neuropathy diagnostic criteria), participant inclusion criteria and exclusion criteria and comparison of groups at baseline;

5. interventions: intervention(s) and comparator (type (long chain or short chain), dose (milligrams/day), duration of treatment), concomitant medications or treatments and compliance measures (ie, whether compliance was assessed, and the method used, eg, returned capsule counts and red blood cell fatty acid profiles);

6. outcomes: prespecified primary and secondary outcome measures and time points of assessments;

7. other: sources of funding statement (ie, present or absent), actual source of funding (eg, industry funding), conflicts of interest statement (ie, present of absent) and nature of conflict of interest (eg, industry employee).

Assessment of risk of bias in included studies

Two review authors will independently assess the risk of bias for each included study using the Cochrane Handbook for Systematic Reviews guidelines. Assessment of bias will be considered using the following domains:

1. selection bias (random sequence generation and allocation concealment);

2. performance bias (blinding of participants and all study personnel);

3. detection bias (blinding of outcome assessors);

4. attrition bias (incomplete outcome data);

5. reporting bias (selective reporting of outcomes);

6. other sources of bias (funding source and conflicts of interest).

Each review author will judge the risk of bias in each domain as: (1) low risk, (2) unclear risk or (3) high risk. Disagreements in bias assessment will be resolved by consensus between the two review authors.

Measures of treatment effect

We will analyse data according to the methods described in Chapter 9 of the Cochrane Handbook for Systematic Reviews of Interventions.56

As all of the predefined outcomes are continuous measures, we will extract information on the change in mean from baseline, and standard deviation (SD) of change, for the intervention and comparison groups. If change from baseline is not reported, we will extract information on the mean and SD of the outcome, for the intervention and comparator groups, at the specified follow-up period. The effects of the interventions will be expressed as the mean difference, with 95% confidence intervals (CIs), between the intervention and comparator groups for significant outcomes (P<0.05); exact p values will be reported.

Unit of analysis issues

The unit of analysis for this review will be the individual participant. In studies where outcomes were measured in ocular tissues, the unit of analysis will be the enrolled study eye of the participant. In studies where participants were randomly allocated to treatment, there will be no unit of analysis issues if only one eye per person is included in the trial, or if both eyes per person are included and the average value of both eyes are reported. In studies where participants were randomly allocated to treatment and both eyes were included, but reported separately, we will analyse this as clustered data (ie, adjusted for within-person correlation). We may have to contact the trial investigators for further information to do this.

Dealing with missing data

For any studies where missing outcome data (eg, omitted SD, standard errors) are identified, we will attempt to contact the study authors. If we fail to receive a response from the corresponding authors in 4 weeks, or if the authors are unable to provide us with the requested information, we will use the information that is available. We will use imputed data, if this has been derived by the trial investigators using an appropriate method, but will not impute missing data ourselves.

Assessment of heterogeneity

We will assess clinical and methodological heterogeneity in the included studies by examining differences in the intervention (eg, type, dose and form), participant characteristics at baseline (eg, age, gender/sex, cause of neuropathy, eligibility criteria) and risk of bias. We will quantify statistical heterogeneity using the I² statistic, as outlined in Chapter 9 of the Cochrane Handbook for Systematic Reviews of Interventions 56; we will consider an I² statistic of 60% or more as consistent with a moderate level of heterogeneity. In measuring heterogeneity, we will also consider the: (1) magnitude and direction of the effects of individual studies and (2) strength of evidence for heterogeneity (using a p<0.10 from the χ² test as the criterion for significant heterogeneity).

Assessment of reporting biases

If at least 10 studies are included in a meta-analysis, we will use a funnel plot to assess for any potential publication bias. We will interpret any asymmetries in the funnel plot in association with the trial characteristics, considering relevant factors such as sample size.

Data synthesis

We will undertake meta-analyses, for the primary and/or secondary outcomes, when this would be clinically meaningful (ie, for studies where the treatment, participants and the underlying clinical questions are similar). If fewer than three RCTs are to be included in a meta-analysis, we will use a fixed-effect model; otherwise, we will use a random-effects model.

If there is inconsistency between individual study results, such that the pooled results may not provide a fair representation of the trial findings (eg, the effects are in opposite directions or I² >60% or the χ² test p value is <0.10), we will not pool the study data but will instead describe the pattern of the individual study results. If there is statistical heterogeneity but all of the effect estimates are in the same direction, such that a pooled estimate would seem to provide a good summary of the individual trial results, we may pool the data.

If a meta-analysis is not deemed appropriate, we will provide a descriptive or tabulated results summary.

Subgroup analysis and investigation of heterogeneity

If sufficient data are available, we will perform subgroup analyses by prognostic factors (eg, type of disease (including subtype of diabetes), severity of peripheral neuropathy at baseline and age) and by potential intervention effect modifiers (eg, dose, duration and type of omega-3 PUFA supplement), as these factors are potentially important to any observed treatment effects.

Sensitivity analysis

Provided there are sufficient data available, we will perform a sensitivity analysis for the primary outcome (ie, change in peripheral NIS), to assess the impact of excluding studies that: (1) were appraised as having a high risk of bias due to lack of allocation concealment or lack of blinding of participants and study personnel, (2) had more than 20% of participants that were lost to follow-up, (3) were unpublished and (4) were funded by industry.

Summary of findings table

Provided that sufficient data are available, we will provide a ‘Summary of findings’ table for the primary outcome (change in peripheral neuropathy as measured using a validated composite neuropathy assessment) and the following secondary outcomes (as previously defined): (1) symptoms, (2) pain, (3) CNFL, (4) IENFD, (5) SNAP amplitudes of the sural nerve and (6) motor NCV of the peroneal nerve. We will follow the recommendations specified in chapter 11 of the Cochrane Handbook for Systematic Reviews of Interventions.56 The strength and quality of the evidence for each outcome will be assessed using the Grading of Recommendations Assessment, Development and Evaluation approach.70