Hyponatremia from Lyme disease can signal underlying neuroborreliosis causing syndrome of inappropriate antidiuretic hormone (SIADH) secretion. An 83-year-old woman developed persistent low sodium levels (125-126 mmol/L) initially attributed to blood pressure medication, but after one month of diagnostic confusion and medication changes, a lumbar puncture finally revealed Lyme meningitis as the cause. Her SIADH resolved completely after three weeks of antibiotics, demonstrating that unexplained hyponatremia — particularly with neurologic symptoms like back pain, cognitive difficulties, and unsteadiness — warrants investigation for central nervous system infection including Lyme disease, not just medication adjustment.

Hyponatremia: An Overlooked Sign of Lyme Meningitis

There are many causes of hyponatremia. Any disorder of the central nervous system, including infections, can trigger it. However, only a few case reports of Lyme meningitis or Lyme neuroborreliosis have been published with a focus on hyponatremia, according to the authors.

Hyponatremia is a condition that occurs when the level of sodium in the blood is too low. With this condition, the body holds onto too much water. This dilutes the amount of sodium in the blood and causes levels to be low.

The lack of case reports doesn’t mean hyponatremia from Lyme disease is rare — it means it’s under-recognized and under-reported. When patients present with low sodium, clinicians reflexively look for common causes: medications (diuretics, SSRIs), adrenal insufficiency, hypothyroidism, heart failure, cirrhosis. Infection, particularly Lyme meningitis, rarely makes the initial differential.

One Month of Shoulder and Back Pain

One month prior to being admitted to the hospital, the 83-year-old woman had presented to the emergency department because of stabbing back pain, localized to the left shoulder.

“The shoulder pain gradually subsided but lower back pain ensued, worsening at night,” wrote the authors. “In parallel, she noticed difficulties in concentrating, unsteadiness, and poor appetite.”

The symptom progression is classic for early disseminated Lyme disease: radicular pain (shoulder pain radiating along nerve roots), nocturnal worsening (characteristic of neuroborreliosis), cognitive difficulties, and systemic symptoms (poor appetite). But without considering Lyme disease, these symptoms appear unrelated.

Admitted for Unexplained Weight Loss

The woman was admitted for an evaluation of unspecific gastrointestinal symptoms and weight loss.

Her sodium was low (hyponatremia 125 mmol/L) consistent with Syndrome of Inappropriate Antidiuretic Hormone (SIADH) secretion.

SIADH causes the body to retain water despite low sodium, diluting blood sodium concentration. The kidneys fail to excrete free water appropriately because excessive antidiuretic hormone (ADH) signals water retention. Causes include CNS disorders (tumors, stroke, meningitis, encephalitis), lung disease, medications, and malignancy.

Medication Blamed for Low Sodium

The doctors could not find a cause. Drug-related hyponatremia was suspected in the absence of another diagnosis. And her blood pressure medication was changed.

The antihypertensive was held. As sodium levels were slightly higher when controlled 5 days later, amlodipine was prescribed instead.

However, a month later, her sodium levels were still low (126 mmol/L).

This is a common diagnostic pattern: when no obvious cause is found, attribute symptoms to medications. Her blood pressure medication was changed, sodium improved slightly (likely fluctuation or volume status change), reinforcing the medication theory. But persistent hyponatremia one month later proved medication wasn’t the cause.

Lumbar Puncture Reveals the Answer

“In view of the history, nocturnal back pain and obscure hyponatremia, she was admitted for a lumbar puncture,” wrote the authors.

Her spinal tap was diagnostic for Lyme meningitis.

SIADH resolved after a 3-week course of antibiotics.

The lumbar puncture — delayed one month — finally provided the diagnosis. CSF analysis showing lymphocytic pleocytosis, elevated protein, and positive Lyme antibodies confirmed neuroborreliosis. Treating the underlying infection with antibiotics resolved the SIADH completely, proving the hyponatremia was infection-driven.

Bannwarth Syndrome in Retrospect

“In hindsight, the lancinating shoulder pain prompting the patient’s first hospital visit likely represented Bannwarth syndrome, a radiculoneuritis occurring early in the course of Lyme disease,” the authors pointed out.

Bannwarth syndrome is a European term for early neuroborreliosis presenting with painful radiculoneuritis (nerve root inflammation), often affecting the shoulder girdle or trunk. The pain is typically severe, burning or stabbing, and worse at night. It can occur with or without facial palsy or meningitis.

This patient’s initial presentation — stabbing shoulder pain — was Bannwarth syndrome, the first sign of neurologic Lyme disease. Had it been recognized at that first ED visit, she could have been diagnosed and treated one month earlier, preventing progression to meningitis and SIADH.

Why Lyme Meningitis Causes SIADH

Lyme meningitis causes SIADH through several mechanisms:

• Direct CNS inflammation: Meningeal inflammation affects hypothalamic-pituitary axis regulation of ADH
• Increased intracranial pressure: Meningitis can elevate ICP, triggering inappropriate ADH release
• Hypothalamic involvement: Spirochetal invasion or inflammatory cytokines affect ADH-producing neurons
• Stress response: Severe systemic infection triggers ADH release as stress hormone
• Cytokine-mediated: Inflammatory cytokines (IL-6, TNF-alpha) can stimulate ADH secretion

The result: excessive ADH despite low serum sodium, causing water retention, dilutional hyponatremia, and potential neurologic complications from low sodium itself (confusion, seizures, coma).

The Diagnostic Delay

This case illustrates a one-month diagnostic delay from first symptoms to diagnosis. Timeline:

1. Day 1 (ED visit): Stabbing shoulder pain (Bannwarth syndrome) — sent home
2. Week 2-4: Back pain worsens, cognitive difficulties, unsteadiness, poor appetite develop
3. Week 4 (admission #1): Hyponatremia found (125 mmol/L), attributed to medication, BP med changed
4. Week 5: Sodium rechecked, slightly improved (likely coincidental)
5. Week 8: Sodium still low (126 mmol/L)
6. Week 8 (admission #2): Lumbar puncture finally performed, Lyme meningitis diagnosed

This month-long delay exposed her to preventable risks: worsening meningitis, severe hyponatremia complications (seizures, altered mental status), and progression of neurologic damage.

Clinical Perspective

This case demonstrates how hyponatremia from Lyme disease gets missed when physicians don’t consider CNS infection in the differential. The patient had multiple red flags: nocturnal back pain (classic for neuroborreliosis), cognitive difficulties, unsteadiness, and persistent unexplained hyponatremia. Yet Lyme wasn’t considered until one month later.

The initial attribution to medication is understandable — drug-induced SIADH is common, particularly with diuretics, SSRIs, and other medications. But when medication adjustment doesn’t resolve hyponatremia, the next step should be investigating CNS causes, not trying different blood pressure medications.

The shoulder pain described as “lancinating” (sharp, stabbing) and localized to one area is highly suggestive of radiculoneuritis. In Lyme-endemic areas, radicular pain — especially when severe, unilateral, and nocturnal — should prompt Lyme testing and consideration of early neuroborreliosis even before meningeal signs develop.

The cognitive difficulties (“difficulties in concentrating”) and unsteadiness represent early CNS involvement. These aren’t vague complaints in an 83-year-old — they’re objective signs of neurologic dysfunction requiring investigation.

The weight loss and poor appetite are nonspecific but consistent with chronic infection and systemic inflammation. Combined with neurologic symptoms, they should have raised suspicion for infectious etiology.

The SIADH diagnosis itself should have triggered broader differential thinking. SIADH from medications typically resolves within days to weeks of stopping the offending agent. Persistent SIADH despite medication changes suggests alternative causes: CNS infection, malignancy, lung disease, or adrenal/thyroid disorders.

The complete resolution of SIADH after antibiotics validates the diagnosis and demonstrates the reversibility of Lyme-induced metabolic derangements. Had she been treated at her first ED visit for shoulder pain, she likely would never have developed meningitis or hyponatremia.

Frequently Asked Questions

Can Lyme disease cause low sodium levels?

Yes. Hyponatremia from Lyme disease occurs when neuroborreliosis causes SIADH (syndrome of inappropriate antidiuretic hormone secretion). Meningeal inflammation affects ADH regulation, causing water retention and dilutional low sodium.

What is SIADH?

SIADH is excessive antidiuretic hormone (ADH) secretion causing water retention despite low serum sodium. The kidneys fail to excrete free water, diluting blood sodium concentration. Causes include CNS disorders (including Lyme meningitis), medications, lung disease, and malignancy.

Why was her hyponatremia blamed on medication?

Drug-induced SIADH is common, particularly with diuretics, SSRIs, and antihypertensives. When no obvious cause was found, medication was assumed. But persistent hyponatremia after medication changes should have prompted investigation for CNS infection.

What is Bannwarth syndrome?

Bannwarth syndrome is painful radiculoneuritis (nerve root inflammation) occurring in early neuroborreliosis. Characterized by severe, stabbing pain often affecting shoulder girdle or trunk, worse at night. Common in European Lyme disease presentations.

How quickly did SIADH resolve with antibiotics?

SIADH resolved after a 3-week course of antibiotics for Lyme meningitis. This proves the hyponatremia was infection-driven, not from medication, and demonstrates the reversibility of Lyme-induced metabolic complications.

Should unexplained hyponatremia be investigated with lumbar puncture?

When hyponatremia is persistent, unexplained, and accompanied by neurologic symptoms (back pain, cognitive changes, unsteadiness), lumbar puncture should be considered to evaluate for CNS infection including Lyme meningitis, particularly in endemic areas.

Can hyponatremia from Lyme disease be severe?

Yes. This patient had sodium of 125-126 mmol/L (normal 135-145 mmol/L). Severe hyponatremia can cause confusion, seizures, coma, and even death if untreated. Recognizing Lyme as the cause allows treatment of underlying infection.