Diabetic polyneuropathy (DPN) is one of the most common chronic complications of diabetes mellitus and is characterized by peripheral nerve damage caused by long-term hyperglycemia. Progressive sensory loss and impairment of proprioception may lead to balance disturbances, gait instability, and an increased risk of falls. Neurofilament Light Chain (NfL) has emerged as a potential biomarker of neuroaxonal injury in several neurological disorders.

The aim of this observational cross-sectional study is to investigate the relationship between serum Neurofilament Light Chain (NfL) levels and neuropathy severity, balance performance, and fall risk in patients with diabetic polyneuropathy. Neuropathy severity will be evaluated using the Michigan Neuropathy Screening Instrument (MNSI) and electrophysiological findings, while balance performance and fall risk will be assessed using the Berg Balance Scale and the Falls Efficacy Scale-International (FES-I).

Diabetic polyneuropathy (DPN) is one of the most common chronic microvascular complications of diabetes mellitus and is characterized by peripheral nerve damage associated with long-standing hyperglycemia. Several pathophysiological mechanisms including oxidative stress, accumulation of advanced glycation end products, activation of the polyol pathway, and microvascular dysfunction contribute to distal axonal degeneration. Clinically, DPN commonly presents with distal symmetric sensory loss in a "glove-and-stocking" distribution accompanied by symptoms such as numbness, paresthesia, burning sensation, and neuropathic pain.

As the disease progresses, impairment of vibration and proprioception may occur, leading to deterioration in postural control, balance disturbances, and gait instability. These changes may increase the risk of falls and negatively affect functional capacity and quality of life in patients with diabetes.

Neurofilament Light Chain (NfL) is a structural protein found in neuronal cytoskeleton and is released into the bloodstream following axonal injury. In recent years, serum NfL has been investigated as a potential biomarker reflecting neuroaxonal damage in various neurological diseases. However, limited evidence exists regarding the association between serum NfL levels and clinical manifestations of diabetic polyneuropathy.

The aim of this observational cross-sectional study is to evaluate the relationship between serum Neurofilament Light Chain (NfL) levels and neuropathy severity, balance performance, and fall risk in patients with diabetic polyneuropathy. The study will include patients with Type 2 diabetes mellitus who attend the Physical Medicine and Rehabilitation outpatient clinic.

Participants will be divided into two groups: patients with diabetic polyneuropathy (DPN+) and diabetic patients without neuropathy (DPN-). The diagnosis of diabetic polyneuropathy will be established based on clinical examination, the Michigan Neuropathy Screening Instrument (MNSI), and electrophysiological evaluation using nerve conduction studies. Peripheral nerve conduction studies will include evaluation of motor and sensory nerves of the lower extremities, particularly tibial, peroneal, and sural nerves.

Serum Neurofilament Light Chain (NfL) levels will be measured using venous blood samples obtained after overnight fasting. Blood samples will be centrifuged and serum will be stored at -80°C until analysis. NfL measurements will be performed using a validated ELISA-based immunoassay kit according to the manufacturer's instructions.

Clinical evaluation will include neuropathy severity assessment using the Michigan Neuropathy Screening Instrument (MNSI), balance performance assessment using the Berg Balance Scale, and fall risk assessment using the Falls Efficacy Scale-International (FES-I). These measurements will allow evaluation of the relationship between serum NfL levels and both clinical and functional parameters.

The primary objective of the study is to investigate whether serum NfL levels are associated with the presence of diabetic polyneuropathy. Secondary objectives include evaluating the relationship between serum NfL levels and neuropathy severity, balance performance, and fall risk.

Receiver Operating Characteristic (ROC) analysis will be used to determine the diagnostic performance of serum NfL levels in distinguishing patients with diabetic polyneuropathy. This study focuses on patients with electrophysiologically confirmed large-fiber diabetic polyneuropathy, and patients with suspected small-fiber neuropathy without electrophysiological abnormalities will not be included in the study.

Inclusion Criteria:

Inclusion Criteria Age between 30 and 75 years Diagnosis of Type 2 Diabetes Mellitus for at least 1 year Ability to understand and complete clinical assessments and questionnaires Willingness to participate and provide written informed consent

For the DPN+ group:

Presence of diabetic polyneuropathy confirmed by clinical examination, Michigan Neuropathy Screening Instrument (MNSI), and nerve conduction studies

For the DPN- group:

Type 2 diabetes mellitus without clinical or electrophysiological evidence of diabetic polyneuropathy

Exclusion Criteria:

• Neuropathy due to causes other than diabetes (e.g., vitamin B12 deficiency, hypothyroidism, alcohol abuse, chemotherapy) Renal dysfunction (eGFR < 60 mL/min/1.73 m²) Active infection or malignancy Inflammatory or autoimmune neurological diseases Central nervous system disorders (e.g., stroke, multiple sclerosis) Lumbar radiculopathy or peripheral entrapment neuropathy Severe visual impairment or balance disorders affecting testing Pregnancy Cognitive impairment preventing completion of questionnaires Patients with isolated small fiber neuropathy without electrophysiological evidence of large fiber involvement

Adults with type 2 diabetes mellitus aged 30-75 years attending a Physical Medicine and Rehabilitation outpatient clinic, including patients with diabetic polyneuropathy and diabetic individuals without neuropathy serving as controls.