Tetany of vitamin D deficiency is one of the associated symptoms of vitamin D deficiency rickets, most commonly observed in infants younger than six months. Due to the widespread implementation of vitamin D deficiency prevention programs, this condition is now relatively rare.
Etiology and Pathogenesis
Vitamin D deficiency leads to a decline in serum calcium, with the parathyroid gland failing to compensate by increasing secretion. Continued reduction of calcium levels, particularly when total serum calcium falls below 1.75–1.8 mmol/L (7–7.5 mg/dL) or ionized calcium decreases below 1.0 mmol/L (4 mg/dL), can result in heightened neuromuscular excitability, causing convulsions. The underlying mechanism for parathyroid hypofunction during vitamin D deficiency remains unclear; it is hypothesized that during early vitamin D deficiency, the parathyroid gland compensates intensively to maintain normal calcium levels. Prolonged vitamin D deficiency may lead to overcompensation and subsequent exhaustion of the parathyroid gland, resulting in hypocalcemia. Thus, children with tetany of vitamin D deficiency concurrently exhibit features of rickets caused by secondary hyperparathyroidism and manifestations of hypocalcemia due to parathyroid hypofunction.
Clinical Manifestations
The condition primarily presents with seizures, laryngospasm, and tetany, along with varying degrees of manifestations associated with active rickets.
Latent Tetany
In this stage, serum calcium is typically between 1.75–1.88 mmol/L, and no classic symptoms may be observed. However, nerve and muscle provocation can elicit the following signs:
- Chvostek Sign (Facial Nerve Sign): Tapping the cheek at the zygomatic arch and corner of the mouth (location of the seventh cranial nerve) may induce twitching of the eyelid and corner of the mouth, indicating a positive response. This sign may show false positivity in neonates.
- Peroneal Sign: Tapping the peroneal nerve at the head of the fibula with a reflex hammer induces contraction of the foot towards the lateral side, reflecting a positive response.
- Trousseau Sign: Inflating a blood pressure cuff on the upper arm to a pressure between systolic and diastolic levels for five minutes may induce wrist spasm, characterized by wrist flexion, thumb adduction, and finger extension, indicating a positive response.
Typical Tetanic Episodes
Serum calcium levels below 1.75 mmol/L can present with seizures, laryngospasm, and tetany:
- Seizures: These are typically afebrile and manifest as sudden limb jerking, upward eye movement, facial twitching, and loss of consciousness. Episodes can last from a few seconds to several minutes, with prolonged episodes causing cyanosis around the mouth. Following the episode, children regain consciousness, often appearing lethargic or falling asleep, and later return to normal activity. Less severe episodes may involve only brief upward eye movement and facial twitching, with the child remaining conscious.
- Tetany: Tetany is more common in older infants and toddlers, with sudden onset of rigid hand and foot spasms. The hands exhibit wrist flexion, straightened fingers, and thumb adduction into the palm, with strong tonic spasms. In the feet, the ankle joints extend, and the toes bend downward.
- Laryngospasm: This is more common in infants and involves sudden spasms of the laryngeal muscles and vocal cords, causing breathing difficulty or, in severe cases, sudden apnea, significant hypoxia, and even death. Seizures without fever are the most common symptom among these three.
Diagnosis and Differential Diagnosis
A diagnosis is suggested by sudden, recurring seizures without fever, with post-episode consciousness and absence of neurological signs, alongside features of rickets, total serum calcium below 1.75 mmol/L, and ionized calcium below 1.0 mmol/L. The condition must be differentiated from the following:
Other Afebrile Seizure Disorders
Hypoglycemia
This typically occurs during fasting in the early morning and may be associated with inadequate dietary intake or a history of diarrhea. Severe cases may progress to coma following seizures, with blood glucose levels often below 2.2 mmol/L. Symptoms may resolve promptly following oral or intravenous glucose administration.
Hypomagnesemia
This commonly affects neonates or young infants and may involve hyperexcitability to touch or sound, muscle twitching, seizures, and tetany. Serum magnesium levels are often below 0.58 mmol/L (1.4 mg/dL).
Infantile Spasms
This is a form of epilepsy presenting in children younger than one year. Symptoms include sudden, repetitive episodes of head and trunk flexion, clenched fists, and limb flexion toward the abdomen, accompanied by nodding movements and altered consciousness. Episodes are brief but frequent, and the condition is often associated with intellectual impairment. EEG findings demonstrate characteristic high-amplitude abnormal wave patterns.
Primary Hypoparathyroidism
This is caused by intermittent seizures or tetany that recur every few days or weeks, with elevated serum phosphorus, reduced calcium, normal or slightly decreased alkaline phosphatase, and reduced PTH levels. Skull X-rays may reveal calcification in the basal ganglia.
Central Nervous System Infections
Meningitis, encephalitis, or brain abscess may present with fever, signs of infection or toxicity, lethargy, and poor appetite. Young children may show blunted responses, and fever may sometimes be absent. Increased intracranial pressure and cerebrospinal fluid abnormalities are characteristic findings.
Acute Laryngitis
This often accompanies upper respiratory infections and may present suddenly with hoarseness, a barking cough, and inspiratory difficulty. There are no symptoms of hypocalcemia, and calcium supplementation typically proves ineffective.
Treatment
Emergency Management
Oxygen Therapy
Oxygen administration is necessary during seizures. During episodes of laryngospasm, pulling the tongue outward, artificial respiration, or positive-pressure oxygen delivery may be required. Intubation may be necessary in severe cases.
Seizure and Laryngospasm Control
Chloral hydrate (10%) may be used rectally at a dose of 40–50 mg/kg per administration. Diazepam may also be administered intravenously at a dose of 0.1–0.3 mg/kg to control severe seizures or laryngospasms.
Calcium Supplementation
Intravenous administration of 5–10 mL of 10% calcium gluconate diluted in 5–20 mL of 10% glucose solution can quickly elevate serum calcium levels. Oral calcium supplementation should follow seizure cessation. Subcutaneous or intramuscular calcium injection is not used due to the risk of local tissue necrosis.
Vitamin D Therapy
Following emergency interventions, vitamin D supplementation is administered according to the treatment regimen for vitamin D deficiency rickets.