Differential Diagnosis

The patient's presentation of sudden onset tingling in bilateral lower extremities, progressing to burning in the thighs, along with lymphocytes in the CSF and specific complement level abnormalities, suggests an immune-mediated or inflammatory process. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Guillain-Barré Syndrome (GBS): This condition is characterized by rapid-onset muscle weakness caused by the immune system damaging the peripheral nervous system. The presence of lymphocytes in the CSF and the progression of neuropathy are consistent with GBS. The negative MRI and specific antibody tests help rule out other causes of neuropathy.

• Other Likely Diagnoses

  • Multiple Sclerosis (MS): Although the MRI is negative, MS can sometimes present with minimal or no lesions on initial imaging. The presence of lymphocytes in the CSF and the clinical presentation could still suggest MS, particularly if this is the first episode (clinically isolated syndrome).
  • Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): This is an autoimmune disorder where the body's immune system mistakenly attacks part of its peripheral nervous system, similar to GBS but with a more chronic course. The progression of symptoms and CSF findings could be consistent with CIDP.
  • Neuropathy associated with Systemic Lupus Erythematosus (SLE): Despite negative ANA and DsDNA, SLE can have a wide range of presentations, and some patients may not have typical antibodies at the onset. The low C3a level could suggest an alternative complement pathway activation, which is seen in SLE.

• Do Not Miss Diagnoses

  • Spinal Cord Infarction or Arteriovenous Malformation (AVM): Although the MRI is reported as negative, it's crucial to ensure that the imaging was of high quality and specifically looked for these conditions. Spinal cord infarction or AVM could present with acute onset of neurological deficits and would be catastrophic if missed.
  • CNS Lymphoma: This is a type of cancer that could present with neurological symptoms and lymphocytes in the CSF. It's less likely given the acute onset and lack of other systemic symptoms but is critical not to miss due to its severe prognosis.
  • Sjögren's Syndrome: Although SSA/SSB antibodies are negative, Sjögren's can occasionally present with neurological manifestations, including neuropathy. The low C4a level might suggest an alternative pathway of complement activation, which could be seen in Sjögren's.

• Rare Diagnoses

  • Neuromyelitis Optica Spectrum Disorder (NMOSD): This condition affects the optic nerve and spinal cord, leading to vision loss and neurological symptoms. Although less likely given the lack of optic neuritis and a negative MRI, it's a rare condition that could present with longitudinally extensive spinal cord lesions and CSF lymphocytosis.
  • POEMS Syndrome: A rare, multisystem disorder characterized by polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy, and skin changes. The presentation with progressive neuropathy could be consistent, but other systemic features are typically present.