Background: Small-Fiber-Neuropathy describes the degeneration of mildly or unmyelinated nerve fibers and causes neuropathic pain and autonomic dysfunction. Gold standard for the diagnosis is a small skin punch biopsy from the lower leg and the histological quantification of the intraepidermal nerve fiber density (IENFD). In children, the normal IENFD has not been systematically assessed and normal reference values are needed. In Parkinson´s disease, the neurodegeneration also affects the peripheral nerves and SFN is present already in the early stages. Whether neurodevelopmental disorders (NDDs) in childhood are likewise associated with SFN is largely unknown. The IENFD is age-dependent and declines with age.

Aims: In this study, we are establishing the reference values for the physiological IENFD in children from 0-18 years. Moreover, we are investigating if children with NDDs have a reduced IENFD and if SFN is a clinically relevant cause of pain and autonomic dysfunction.

In the first part of the study, skin biopsies will be collected from the leg from children with a normal neurological development (healthy children) in the setting of elective orthopedic surgery (during general anesthesia; from surgical crop margins).

The IENFD is determined by staining and quantification of PGP9.5-positive intraepidermal nerve fibers on 40 µm thick skin sections.

The skin biopsies will be collected from the site of orthopedic surgical procedure. For example, if the child is undergoing corrective osteotomy of genu valgum, the skin biopsy is drawn from the most distal part of surgical incision sides (distal leg); if the child is undergoing orthopedic surgery in the context of epiphyseolysis capitis femoris, the skin biopsy is drawn from the thigh (proximal leg):

In adults, the IENFD is higher in specimens derived from the proximal leg than in skin from the distal leg. Because in small fiber neuropathy the small nerve fiber degeneration often starts distally, the recommended sampling site for the quantification of IENFD in adults is the distal leg. That is why the calculation of the 5th percentile as the cutoff for reduced IENFD in children will be based on skin biopsies collected from the distal leg.

Due to the limited number of surgeries on the distal leg and distal biopsies in certain age groups of healthy participants (e.g. 4-8), cases with skin biopsies taken from the proximal leg will also be included. The IENFD from cases drawn from the proximal leg and cases from the distal leg will be compared. The samples drawn from the distal or proximal leg will be labelled accordingly. If the IENFD is comparable between the distal and proximal cases, a pooled analysis including proximal and distal cases will be performed.

In the second part of the study, skin biopsies from the distal leg will be collected from children with neurodevelopmental disorders of acquired, genetic and initially unexplained etiology in the setting of local anesthesia, whenever possible during an elective intervention with sedation/general anesthesia. IENFD below the predicted age- and sex-specific 5th percentile will be classified "reduced", otherwise as "normal".

A literature review will be performed to identify those neurodevelopmental disorders that have been associated with peripheral neuropathy in the past.

In cases with a yet unexplained etiology of neurodevelopmental disorder (no perinatal asphyxia, postnatal asphyxia, no cardiopulmonary resuscitation, no extreme prematurity, no encephalitis others, no stroke, or cerebral tumor), genetic testing (exome or genome sequencing and microarray analysis) will be performed.

Clinical symptoms of SFN (distal sensory signs and autonomic dysfunction) In adults the diagnosis of SNF is also based on the presence of distal sensory signs. Therefore, all participants or their legal guardians will be requested to answer questionnaires [(SFN-symptom inventory questionnaire (SFN-SIQ) and the SFN-Screening list (SFNSL)] that specifically assess distal sensory and autonomic symptoms related to SFN.

Additional exploratory subgroup analysis 1): In adults the diagnosis of SNF is also based on abnormal thermal perception assessed by quantitative sensory testing (QST). In collaboration with the University Children´s Hospital of the Ruhr University Bochum, QST will be performed in those children that are old enough and who have the cognitive ability to undergo QST. Small nerve fiber function including thermal perception will be assessed.

Additional exploratory subgroup analysis 2): Small fiber neuropathy can be associated with large fiber neuropathy. Therefore, the proportion of abnormal findings of electrophysiological studies in medical record of children with a reduced versus normal IENFD will be assessed.

control patients

Inclusion Criteria:

• Elective surgical intervention from lower leg
• Interest to participate in the study, informed consent
• Unremarkable neurological development

Exclusion Criteria:

• Clinical signs of polyneuropathy, autonomic dysfunction
• Skin inflammation, scar, skin disease, other known chronic diseases that can cause small fiber pathology

Neurodevelopmental patients

Inclusion Criteria:

• Neurodevelopmental problems (developmental delay, muscular hypotonia, seizures, ataxia,...) with age of onset before the age of 18 years
• Interest to participate in the study, informed consent

Exclusion Criteria:

• Known polyneuropathy
• Skin inflammation, scar, skin disease, other known chronic diseases that can cause small fiber pathology

1. Control (no underlying neurological disorder) individuals
2. Neurodevelopmental disorder_Genetic cause known
3. Neurodevelopmental disorder_Acquired (cause known, for example perinatal asphyxia or intracranial bleeding)
4. Neurodevelopmental disorder_Cause unknown

All IPD that underlie results in a publication

After publication

Email to principal investigators explaining why it might be helpful to share the data.