Lyme Meningitis Leading to Hyponatremia (SIADH)

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 does not mean hyponatremia from Lyme disease is rare — it may simply be under-recognized. When patients present with low sodium, clinicians often focus first on medications or endocrine disorders rather than infection.

One Month of Shoulder and Back Pain

One month prior to hospital admission, the 83-year-old woman 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 consistent with early neurologic Lyme disease, where radicular pain and neurologic symptoms may appear before meningitis is recognized.

Admitted for Unexplained Weight Loss

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

Her sodium level was low (125 mmol/L), consistent with syndrome of inappropriate antidiuretic hormone (SIADH) secretion.

SIADH causes the body to retain water despite low sodium levels, diluting the blood sodium concentration.

Medication Blamed for Low Sodium

Physicians initially suspected medication-related hyponatremia. Her blood pressure medication was discontinued and replaced with amlodipine.

However, one month later, her sodium remained low (126 mmol/L), indicating the medication change had not corrected the problem.

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.

The spinal tap confirmed Lyme meningitis.

Her SIADH resolved completely after a three-week course of antibiotics.

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 explained.

Bannwarth syndrome is a form of early neuroborreliosis characterized by severe radicular pain that often worsens at night.

Why Lyme Meningitis Causes SIADH

Several mechanisms may explain SIADH in Lyme meningitis:

• Central nervous system inflammation
• Increased intracranial pressure
• Hypothalamic involvement affecting ADH regulation
• Systemic inflammatory cytokine effects

The result is excessive ADH secretion despite low sodium levels, causing water retention and dilutional hyponatremia.

The Diagnostic Delay

This case illustrates a one-month delay between the patient’s initial symptoms and the final diagnosis of Lyme meningitis.

1. Initial emergency visit for shoulder pain
2. Progression to back pain and neurologic symptoms
3. Discovery of hyponatremia
4. Medication adjustments without improvement
5. Lumbar puncture confirming Lyme meningitis

Clinical Insight

Frequently Asked Questions

Can Lyme disease cause low sodium levels?

Yes. Lyme meningitis can cause SIADH, which leads to dilutional hyponatremia.

What is SIADH?

SIADH is excessive secretion of antidiuretic hormone, causing water retention and low sodium levels.

What symptoms may accompany Lyme meningitis?

Symptoms may include radicular pain, headache, cognitive changes, balance problems, and meningitis.

Does hyponatremia improve with Lyme treatment?

Yes. In this case, the patient’s sodium levels normalized after antibiotic treatment for Lyme meningitis.