Vitamin D deficiency is a global health issue that affects the bone as well as neural health. Among these severe manifestations, tetany- a condition characterised by involuntary muscle contraction and spasm, primarily due to hypocalcemia .. This article dives into the pathophysiology linking vitamin D deficiency to tetany, exploring the clinical manifestation, diagnosis, therapeutic management and preventive measure. In order to mitigate the consequences of this preventable condition, it is crucial to maintain awareness and early interventions. 

Introduction 

Vitamin D, often referred to as the “sunshine vitamin” is vital for physiological processes, particularly calcium and phosphorus homeostasis. Deficiency in vitamin D disrupts these processes leading to a range of complications resulting from low calcium levels in the blood.¹ 

While vitamin D deficiency is often overlooked, it remains one of the most significant contributors to hypocalcemia-induced tetany globally, particularly in populations with limited exposure to sun, poor dietary intake or underlying medical conditions. 

This article aims to provide a comprehensive overview of the association between vitamin D deficiency and tetany, focusing on its cause, clinical implication and management strategies.

Pathophysiology of vitamin D and calcium regulation 

Vitamin D metabolism 

Vitamin D exists in two forms, one of which is vitamin D2 (ergocalciferol) which is found in fortified foods and supplements and the other is vitamin D3 (cholecalciferol) which is synthesised in skin on exposure to ultraviolet B rays. It is also found in some dietary sources.
Both forms are metabolised in the liver to 25-hydroxyvitamin D [25(OH)D], the major circulating form used to assess vitamin D status. Subsequently, 25(OH)D is converted in the kidneys to its active form, calcitriol (1,25-dihydroxyvitamin D).

This active form helps to regulate the calcium homeostasis and phosphate balance due to its action on the intestine, kidney and bone. The synthesis and activity of calcitriol is tightly controlled by a feedback mechanism involving the parathyroid hormone (PTH), serum calcium and phosphate levels.

Regulation of vitamin D activation

Vitamin D is either synthesised in the skin upon exposure of UV light or obtained from dietary sources. It is initially inactive and must undergo two hydroxylation steps to become active. The first hydroxylation occurs in the liver, converting vitamin D into 25-hydroxyvitamin D, which is a stable precursor of vitamin D in the body. The second hydroxylation takes place in the kidney where it gets converted into calcitriol by the enzyme 1α-hydroxylase. This step is regulated by PTH in response to hypocalcemia and serum phosphate levels. Elevated PTH stimulates 1α-hydroxylase activity, increasing the production of 1,25-dihydroxyvitamin D which in turn enhances calcium and phosphate absorption in the intestine.^2,3 

Role of Vitamin D in calcium homeostasis 

The body maintains calcium levels through the combined effects of the intestine, kidney and bones. Vitamin D, especially in its active form (i.e. calcitriol), plays a crucial 

role in this process: 

• In the intestine: it boosts calcium levels by absorption as well as by increasing the levels of proteins and pumps in the intestinal cell involved in the transport of calcium
• In the kidney: Parathyroid hormone through controlled feedback mechanism reduces calcium loss via the urine and vitamin D helps in balancing the phosphates
• In the bones: Vitamin D with PTH work together to release the calcium from the bones by stimulating cells (osteoclasts) that break down the bone tissue

What happens when calcium regulation fails?

Disruption in the vitamin D or calcium levels can lead to various health issues:

• Low Calcium (Hypocalcemia): Often caused by vitamin D deficiency, it leads to increased PTH levels, which can weaken bones and cause conditions like rickets (in children) or osteomalacia (in adults)
• High Calcium (Hypercalcemia): Can occur in conditions like sarcoidosis, where excess vitamin D leads to exaggerated calcium absorption and release from bones
• Nerve and Muscle Effects: Low blood calcium affects nerve and muscle function, causing the characteristic symptoms of tetany

Clinical importance

Proper vitamin D and calcium regulation are crucial for diagnosing and treating mineral imbalances. For example:

• Chronic Kidney Disease: Reduces the production of active vitamin D, disrupting calcium and phosphate balance
• Other Factors: Medications, liver disease, and digestive issues can also interfere with vitamin D levels⁴ 

What is Tetany 

Tetany refers to a condition where muscle contracts involuntarily due to low calcium levels in the blood. It can cause:

• Tingling or numbness ( especially in the hands, feet and face)
• Muscle cramps or spasms
• Signs like Chvostek’s signs (facial twitching when tapping the cheeks) or Trousseau’s sign (hand spasms when a blood pressure cuff is inflated)
• Severe cases may involve breathing difficulties like throat spasm which can be life threatening⁵ 

This happens because low calcium disrupts nerve function, triggering hyperactivity. Severe hypocalcemia, especially when calcium levels drop below 7 mg/dL, significantly increases the risk of tetany.⁶ 

How does Vitamin D deficiency cause tetany? 

Vitamin D is crucial for calcium absorption therefore any deficiency can lead to:

Reduced calcium absorption

The body struggles to absorb calcium from the food, causing low blood calcium (hypocalcemia).

Hypocalcemia: The leading cause of Tetany

Hypocalcemia occurs when calcium levels in the blood are too low. Calcium is essential for several critical body functions:

• Supporting nerve function
• Enabling muscle contractions
• Clotting factor (IV)
• Contributing to the heart electrical signals and pumping functions

While many health conditions can lead to hypocalcemia, the most common causes are low parathyroid hormone levels (hypoparathyroidism) and vitamin D deficiency.

Overactive nerves and muscles

Low calcium promotes high sensitivity to nerves and muscles leading to spasms. 

Long-term problems

Prolonged deficiency causes bone to lose calcium worsening the condition.⁷

Mechanisms of Tetany in Vitamin D deficiency

Calcium ions are vital to keep the neuron (nerve cells) stable and prevent it from activating on its own. When there is a shortage of calcium in the body, a condition known as hypocalcemia makes the neurons more excitable. This means the neuron needs less stimulation to become active. As a result, they can start firing off signals spontaneously leading to involuntary muscle contractions. These uncontrolled muscle contractions can eventually cause a condition called tetany where muscles become stiff and spasm. Severe hypocalcemia, which is often defined as having serum calcium levels below 7 mg/dL, highly increases the risk of developing tetany.⁸

In simpler terms, calcium helps keep our nerve cells calm. Without enough calcium, these cells can get overly excited and cause muscle to twitch and cramp uncontrollably. This is why maintaining calcium level is important for muscle and nerve function.⁹ 

Diagnosis and management 

Tetany is often suspected when individuals experience muscle cramps, tingling sensations in the hands and feet. These symptoms can be neurological examinations that assess both sensory and motor functions. In some cases, the symptoms of tetany may not be as obvious, requiring a high level of clinical suspicion for diagnosis. Additionally, certain clinical signs, such as Trousseau’s sign (a specific type of muscle spasm), can indicate hypocalcemia-related tetany.

To confirm a diagnosis of tetany, doctors typically look for low calcium levels in the blood. They may also check the levels of parathyroid hormone (PTH), vitamin D, magnesium, and phosphorus to help determine the underlying cause of the condition. This approach ensures that the diagnosis is accurate and that the appropriate treatment can be administered.⁹ 

Treatment

Vitamin D supplementation is crucial for treating tetany caused by vitamin D deficiency, especially when solar radiation and dietary intake are insufficient due to malabsorption. Conventional oral replacement therapy effectively increases 25(OH)D (calcidiol) levels, regardless of fat malabsorption. In cases of malabsorption, calcidiol, calcitriol, and alfacalcidol are more effective than vitamin D3. 

The recommended safe dose for vitamin D3 is 1000–2000 IU daily, and for calcitriol, it is 0.05 μg/kg/day. Higher doses may be necessary for malabsorption, and if oral supplements are ineffective, intramuscular vitamin D injections (100,000 IU/month) should be considered. Severe hypocalcemia or tetany can be treated by IV calcium gluconate followed by oral supplements once symptoms improve.¹⁰ Vitamin D supplementation is often advised along with calcium to enhance absorption when hypocalcemia is due to vitamin D deficiency.¹¹

Summary

Vitamin D deficiency is a significant global health issue with implications for both bone and neural health. Among its severe manifestations, tetany stands out as a condition characterised by involuntary muscle contractions and spasms, primarily due to hypocalcemia. The pathophysiology links vitamin D deficiency to tetany which involves disruptions in calcium homeostasis which is crucial for nerve and muscle function. Vitamin D through its active form calcitriol, plays a vital role in calcium absorption and regulation. When vitamin D levels are insufficient, calcium absorption is impaired leading to hypocalcemia and increased risk of tetany. Clinical manifestations of tetany include tingling, muscle cramps, and severe cases can involve life-threatening breathing difficulties. Diagnosis involves assessing blood calcium levels and related markers. Management strategies focus on vitamin D supplementation, which is key to restore calcium balance and prevent tetany. Early intervention and awareness are essential to mitigate the effects of this preventable condition, highlighting the importance of maintaining adequate vitamin D levels for overall health.