IVIG in Painful Sensory Neuropathy

Inclusion Criteria: Painful sensory neuropathy fulfilling diagnostic criteria for idiopathic small fiber neuropathy or sensory neuronopathy * Idiopathic small fiber neuropathy (all of the following) 1. Neuropathic pain (length-dependent/non-length dependent/focal) and at least two of the following clinical signs 1. Hypoesthesia (tactile, pinprick, or thermal) 2. Allodynia (tactile, dynamic, thermal, pressure) 3. Hyperalgesia 4. Aftersensation 2. Normal nerve conduction studies: 3. Reduced intra-epidermal nerve fiber density at the distal leg (IENFD) at the distal leg or Reduced thermal threshold assessed at the foot by QST 4. Causal investigation according to clinical guidelines has not revealed a cause of the neuropathy * Sensory neuronopathy (all of the following) 1. Clinical pure sensory neuropathy 2. A score \>6.5 on the following 1. Ataxia in the lower or upper limb: 3.1p 2. Asymmetrical distribution of sensory loss: 1.7p IVIGSeN Protocol number 1.2 3. Sensory loss not restricted to the lower limb at full development: 2.0 p 4. At least 1 sensory action potential absent or 3 sensory action potentials \<30% of the lower limit of normal in the upper limbs not explained by entrapment neuropathy: 2.8p 5. Less than two nerves with abnormal motor nerve conduction studies in the lower limbs: 3.1p 3. Causal investigation according to clinical guidelines has not revealed a cause of the neuropathy Pain intensity \> 5.0 on a pain score from 0-10, lasting at least 4 weeks Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: Medical Conditions 1. Previous allergic reaction to IVIG or other blood products) 2. Selective IgA deficiency 3. Cardiac insufficiency (NYHA III/IV ie marked or severe limitation in activity due to symptoms, even during less-than-ordinary activity) 4. Cardiac dysrhythmia requiring treatment 5. Unstable or advanced ischemic heart disease 6. Severe hypertension (diastolic \>120 or systolic \> 170) 7. Known hyperviscosity 8. Renal insufficiency (GFR \< 30 ml/min/1,73m2) or nephrotic syndrome 9. Previous thromboembolic event 10. Smoking 11. Diabetes 12. Prolonged immobilization 13. Hypercoagulable state Prior/Concomitant Therapy 14. Change of pain medication in the 30 days prior to inclusion (unchanged pain medication is allowed, provided dosages stay equal during the study) 15. Treatment with IVIG the last 4 weeks or other ongoing immunomodulatory treatments 16. Use of loop diuretics, Dinutuksimab beta, or Imlifidase Diagnostic assessments 17. Predominant clinical or neurographic features of motor nerve fiber involvement Other Exclusions 18. Females who are breastfeeding, pregnant or unwilling to practice contraception throughout the study 19. Unable to give independent informed consent