The objective of this study is to develop a rationale for the selective treatment of small fiber neuropathy with immune globulin (IVIG) in the appropriate patients.

The investigators hypothesize that individuals with auto-antibodies targeting neuronal antigens (TS-HDS and FGFR3) and confirmed evidence of small fiber neuropathy (by skin biopsy analysis of intra-epidermal nerve fiber density) will have an improvement in both nerve fiber density and pain after treatment with immune globulin.

The co-primary endpoints will be a change in neuropathic pain (by VAS pain score) and a change in intra-epidermal nerve fiber density (by punch skin biopsy).

The data gained from this pilot study will establish a rationale, with an appropriate screening test, for the use of immune globulin for the treatment of small fiber neuropathy.

Small fiber neuropathies, and mixed small and large fiber neuropathies, have many potential causes including diabetes, vitamin deficiencies, environmental and toxic exposures, HIV, autoimmune and paraproteinemias.

However, despite this broad differential at least 30% of cases of small fiber neuropathies remain idiopathic. There is therefore a growing interest in the potential for using IVIG in small fiber neuropathy without direct proof that the disorder is caused by immune reactions. We have recently uncovered two novel autoantibodies, TS-HDS and FGFR-3, that are targeted again peripheral neural structure. TS-HDS is a disaccharide component of glycosylation of heparin and heparin sulfate.

Patients with elevated levels of IgM against TS-HDS display clear small fiber loss with IgM deposits around the outside of medium- & larger-sized capillaries with C5b-9 complement deposits. FGFR-3 is a secreted cell surface receptor; genetic defects of FGFR-3 are linked to achrondroplasia and other bony abnormalities.

The antibodies to TS-HDS and FGFR-3 are detected in up to 20% of patients with otherwise idiopathic small fiber neuropathy, but are rare in patients without small fiber neuropathy.

Dr. Levine (a co-investigator on this project) recently presented 3 cases of small fiber associated with elevated levels of auto-antibodies to TS-HDS or FGFR-3 who were treated with IVIG at 2 gm/kg/month for 6 months. He examined skin biopsies for intra-epidermal nerve fiber density and patient self-reported pain scores at baseline and after six months of therapy. All 3 cases showed marked improvement in pain scores. The average reduction in pain was 54%. In addition there was a clear increase in the intra-epidermal nerve fiber density (IENFD) after 6 months of therapy. Pre-treatment IENFD was 1.6, 1.7, and 2.4 at the calf. After 6 months of therapy the IENFD was 8.4, 5.7, 3.3 respectively (these are clinically significant improvement in nerve fiber density.

The investigators believe these anecdotal cases suggest that TS-HDS and FGFR-3 antibodies may be a marker for a group of SFN patients that are immune mediated and may respond to IVIG. (This case series was presented as a poster at the American Academy of Neurology meeting in 2017)

Inclusion Criteria:

1. Patient with clinically evident and biopsy proven pure small fiber neuropathy as evidenced by reduced intra-epidermal nerve fiber density seen on skin biopsy using PGP 9.5 as the immunostain.
2. Patients must have a baseline pain score on a VAS scale of Greater or equal to 4/10
3. Patients must have elevated titers of autoantibodies to TS-HDS or FFR3 as measured in Dr Alan Pestronk's lab at Washington University in St Louis.

Exclusion Criteria:

1. Any other known cause for small fiber neuropathy other than the presence of the elevated titers of auto-antibodies. For example patients with diabetes, HIV, Sjogrens, Vitamin deficiency etc.
2. Patients with generalized, severe musculoskeletal conditions other than SFN that prevent a sufficient assessment of the patient by the physician
3. Cardiac insufficiency (New York Heart Association III/IV), cardiomyopathy, significant cardiac dysrhythmia requiring treatment, unstable or advanced ischemic heart disease).
4. Severe liver disease (ALAT 3x > normal value).
5. Severe kidney disease (creatinine 1.5x > normal value).
6. Known hepatitis B, hepatitis C or HIV infection.
7. Patients with a history of deep vein thrombosis (DVT) within the last year prior to baseline visit or pulmonary embolism ever; patients with susceptibility to embolism or deep vein thrombosis.
8. Body mass index (BMI) ≥40 kg/m2.
9. Medical conditions whose symptoms and effects could alter protein catabolism and/or IgG utilization (e.g. protein-losing enteropathies, nephrotic syndrome).
10. Known IgA deficiency with antibodies to IgA.
11. History of hypersensitivity, anaphylaxis or severe systemic response to immuno-globulin, blood or plasma derived products, or any component of Gamunex.
12. Known blood hyperviscosity, or other hypercoagulable states.
13. Use of IgG products within six months prior to enrolment.
14. Use of other blood or plasma-derived products within three months prior to enrollment.
15. Patients with a history of drug or alcohol abuse within the past five years prior to enrollment.
16. Patients unable or unwilling to understand or comply with the study protocol
17. Participating in another interventional clinical study with investigational treatment within three months prior to enrollment.
18. Women who are breast feeding, pregnant, or planning to become pregnant, or are unwilling to use an effective birth control method (such as implants, injectable, combined oral contraceptives, some intrauterine devices (IUDs), sexual abstinence or vasectomized partner) while on study.

6 months

Adverse events captured at 3 week intervals or by interval phone calls for any acute changes in condition.

This was a small pilot study that was underpowered to detect changes to secondary endpoints.

There was a 15% dropout. Despite several publications that note an association between TS-HDS and FGFR-3 autoantibodies in neuropathy, we do not have evidence for pathogenicity (supported by a non-insignificant number of patients excluded from the study with autoantibodies but without evidence of neuropathy). This too may have limited the potential treatment effect of immune globulin.