Sensory Neuropathy Scores Validated by Quantitative Sensory Testing and Nerve Conduction Studies for Kidney Transplant Recipients

Sensory polyneuropathy is one of the most prevalent neurological disorders. A Dutch study observed an incidence of all types of polyneuropathy of 77 per 100.000 person-years, which increases with age. Besides, previous studies evaluated the impact of polyneuropathy on quality of life in different patient populations and showed that accompanying symptoms affect daily living and influence morbidity and mortality of patients adversely.

We can classify sensory polyneuropathies broadly into either small fibre neuropathies, large fibre neuropathies or mixed small and large fibre neuropathies. In small fibre neuropathies the thinly myelinated A delta and unmyelinated C fibres are affected. Therefore, the transmission of thermal and noxious sensory input is disturbed translating to symptoms of sharp, painful, or burning paraesthesia, sensory loss or numbness, and the inability to discriminate between hot and cold sensation. Disorders affecting the large myelinated A delta fibres lead to an impaired vibration sensation and proprioception. Patients will complain about a combination of symptoms of sensory loss, paraesthesia, and gait imbalance.

Sensory polyneuropathy is a common finding in kidney transplant recipients (KTR), but little is known about prevalence, course, or underlying aetiology in this specific group. It is known that patients are at a high risk to develop polyneuropathy before kidney transplantation due to chronic kidney disease. Therefore, related to the previous state of chronic kidney disease uremic and diabetic polyneuropathy are probable underlying aetiologies in KTR. Furthermore, the use of immunosuppressive medication after transplantation, in particular tacrolimus, may result in polyneuropathic symptoms and signs. These circumstances may explain the high prevalence of polyneuropathy in KTR but in order to gain more understanding, it is necessary to diagnose and stage polyneuropathy in an easy and convenient way in KTR. Furthermore, early diagnosis and treatment of certain types of polyneuropathy can prevent further worsening of neuropathic symptoms and signs, or even reverse the neurologic deficits.

The diagnostic approach to polyneuropathy differs depending on the type of affected nerve fibre and the distribution of nerve damage in the respective type of polyneuropathy. The diagnosis of polyneuropathy is generally based on the presence of distal sensory and sensorimotor symptoms and signs, and loss of function, which is consistent with the clinical picture. Nevertheless, to reveal an underlying large fibre aetiology, nerve conduction studies (NCS) are needed. With the help of NCS, it is possible to categorize polyneuropathy as primary axonal or primary demyelinating. This standard diagnostic test, however, exclusively enables the measurement of large fibres and patients with pure small fibre neuropathies will not show any nerve conduction abnormalities. Hence, quantitative sensory testing (QST) is required to assess the function of small sensory nerve fibres. However, NCS and QST are time-consuming, fairly expensive, and the investigator needs a certain level of expertise. Alternatively, different clinical scores exist that aim to detect and stage polyneuropathy based on symptoms and signs. The Toronto Clinical Neuropathy Score (TCNS) was designed for diabetic sensorimotor polyneuropathy but it was shown that its usefulness extends beyond diabetic polyneuropathy. These results were based on correlations of the TCNS with clinical, electrophysiological and disability parameters in non-diabetic polyneuropathies. The same study confirmed that reliability and accuracy were excellent. Eventually, the TCNS was modified (into the mTCNS) to better capture a scale of simple sensory tests representative of early-stage dysfunction and reflex testing was removed due to its high inter-rater variability. The mTCNS was demonstrated to maintain an acceptable correlation with the precursor score. In the University Medical Center Groningen (UMCG) a large population of KTR is followed within the scope of the TransplantLines cohort study and biobank. For this purpose, the mTCNS was further adapted by omitting testing of temperature sense due to practical reasons (adapted into the amTCNS). The E-PSS was designed to diagnose chronic polyneuropathy and was previously validated in a cohort of patients with chronic idiopathic axonal polyneuropathy.

To be able to study prevalence, course, and underlying aetiology in this specific population of KTR it is crucial to reveal whether the E-PSS and amTCNS correlate with the golden standards of diagnosing polyneuropathy, NCS and QST. In the long term, both clinical scores have great potential to help clinicians from various disciplines without the specific expertise needed for NCS and QST to diagnose polyneuropathy.

In conclusion, the aim of this research proposal is to validate the E-PSS and amTCNS with NCS and QST for a population of KTR.

The primary objective is to validate the Erasmus Polyneuropathy Symptoms Score (E-PSS) and adapted modified Toronto Clinical Neuropathy Score (amTCNS) with QST and NCS.

The secondary objective is to define reference values of the amTCNS dependent on predictors in a healthy subject population. This healthy subject population consists of potential kidney donors that were already seen and examined within the TransplantLines cohort study and biobank prior to this study. These healthy subjects will not undergo QST or NCS.

This research protocol contains a cross-sectional study or can alternatively be described as a validation study. The aim is to examine the validity of the E-PSS and amTCNS, clinical scores that were designed to capture symptoms and signs of sensory polyneuropathy. The E-PSS and amTCNS will be validated with QST and NCS which are defined as golden standards for diagnosing small fibre neuropathies and large fibre neuropathies, respectively.

The aim is to validate the E-PSS and amTCNS for a population of KTR to be able to subsequently identify and follow up on polyneuropathies in this specific population in a more convenient way.

KTR will be recruited retrospectively after their participation in the TransplantLines cohort and biobank study in the UMCG by telephone. Every possible future participant will receive written information concerning this study (aim, design and duration of the study, disadvantages and risks of participation). The subjects will be required to sign the written informed consent form within two weeks if they want to take part in this study. The subjects will be informed that if they give consent for participation in the study they may withdraw consent at any time. This withdrawal of consent may be given to the investigator in oral or written form. After receiving the written informed consent a screening via telephone will be planned to assess whether the potential participant meets the inclusion criteria.

The amTCNS result will be calculated based on a questionnaire and neurological examination. This includes a symptoms score assessing foot pain, numbness, tingling, weakness, ataxia and upper limb symptoms, and a sensory test score including testing of pinprick sensation, light touch, proprioception and vibration sense. QST consists of thermal threshold testing in which the thresholds of warm sensation and cold sensation will be determined following the method of limits and the method of levels. NCS will include testing of the CMAP amplitude and distal motor latency of the tibial, peroneal and ulnar nerve; the SNAP amplitude of the sural and ulnar nerve; nerve conduction velocity and the soleus H-reflex. Participants will be asked to fill in the questionnaire of the E-PSS prior to their study visit via mail.

Prior to this study the amTCNS was already carried out in 200 healthy subjects which were potential kidney donors. Clinical characteristics that function as independent predictors of the amTCNS in a healthy subject population will be determined by means of ordinal logistic regression analyses. The 95th percentile of the amTCNS final score will be calculated to define reference values according to the identified independent predictors. The outcome of the polyneuropathy measures of the current study will be dichotomized as follows: presence of polyneuropathy (both aberrant QST and NCS), presence of small fibre neuropathy (aberrant QST), presence of large fibre neuropathy (aberrant NCS). These outcomes will be opposed to the dichotomous outcome of presence of polyneuropathy according to the reference value of the amTCNS that was previously established in a healthy subject population. Sensitivity, specificity, positive predictive value and negative predictive value will be calculated.

To compare amTCNS cut-off points for diagnosis, the reference values determined in the cohort of healthy subjects without polyneuropathy will be compared with the data in KTR. The accuracy of the amTCNS in KTR will be determined by the area under the receiver operating characteristic curve, with the presence of polyneuropathy as the dichotomous outcome variable and amTCNS as the test. The optimal diagnostic threshold will be identified by the point on the receiver operating characteristic curve closest to the point of perfect discrimination.