Is autonomic neuropathy a type of polyneuropathy?

Autonomic Neuropathy as a Type of Polyneuropathy

Yes, autonomic neuropathy is a type of polyneuropathy, but it can exist as a distinct phenotype or as one component of a generalized polyneuropathy. 1

Relationship Between Autonomic Neuropathy and Polyneuropathy

• Autonomic neuropathy occurs in several distinct phenotypes in relation to polyneuropathy 1:

  1. As one component of a generalized polyneuropathy (such as distal symmetric polyneuropathy in diabetes), where autonomic dysfunction accompanies "large fiber" sensory and motor involvement 1

  2. As a distinct autonomic neuropathy phenotype (as in amyloidosis and autoimmune autonomic neuropathy), where autonomic nerves are affected disproportionately relative to somatic nerves 1

  3. As part of distal small fiber sensory polyneuropathy (SFSN), where involvement of autonomic and somatic C fibers usually occurs concurrently 1

• In distal symmetric polyneuropathy (DSP) with autonomic involvement, the most common clinical findings are abnormalities of sweating and circulatory instability in the feet 1

Important Clinical Distinctions

• In some autonomic neuropathies, autonomic fibers can be affected in isolation and their involvement may precede somatic fiber involvement 1

• Peripheral somatic and autonomic neuropathies do not invariably coexist in diabetes mellitus - studies have shown that they can develop independently 2:

  • In type 1 diabetes, pure autonomic neuropathy (without peripheral neuropathy) was found in 28.8% of patients 2
  • In type 2 diabetes, pure autonomic neuropathy was found in 20.7% of patients 2

Diagnostic Considerations

• Autonomic testing may be considered in the evaluation of patients with polyneuropathy to document autonomic nervous system dysfunction (Level B evidence) 1

• Such testing should be considered especially for:

  • Evaluation of suspected autonomic neuropathy (Level B evidence) 1
  • Evaluation of distal small fiber sensory polyneuropathy (Level C evidence) 1

• A battery of validated autonomic tests is recommended to achieve the highest diagnostic accuracy (Level B evidence) 1

• Heart rate variability testing is a simple and reliable test of cardiovagal function that detects the presence of diabetic polyneuropathy with nearly the same sensitivity as nerve conduction studies 1

Causes of Autonomic Neuropathy

• Diabetes is the most common cause of autonomic neuropathy in developed countries 3

• Other causes include:

  • Amyloid deposition 3
  • Autoimmune conditions 3, 4
  • Paraneoplastic syndromes 3
  • Exposure to neurotoxins including therapeutic drugs 3
  • Infections 3
  • Hereditary disorders 5, 4

Clinical Implications

• Autonomic neuropathies affect multiple systems including cardiovascular, gastrointestinal, urogenital, sudomotor, and pupillomotor systems 3

• Droxidopa is FDA-approved for the treatment of orthostatic dizziness in adult patients with symptomatic neurogenic orthostatic hypotension caused by primary autonomic failure and non-diabetic autonomic neuropathy 6

• Careful monitoring is required when treating autonomic neuropathy, as medications like droxidopa may cause or exacerbate supine hypertension 6

In summary, autonomic neuropathy represents a specific type of polyneuropathy that can occur either as part of a more generalized polyneuropathy or as a distinct entity where autonomic nerves are predominantly affected. Understanding this relationship is crucial for proper diagnosis and management of patients with neuropathic symptoms.