Hypotonia

What is Hypotonia?

Muscle tone refers to the residual tension in a resting muscle. This continuous, partial muscle contraction helps us maintain our posture. It’s measured by the resistance a muscle gives when it’s passively stretched or when a limb is moved at a joint without effort. Hypotonia, otherwise known as poor muscle tone, is a condition that results in a floppy or loose muscle. It’s characterized by abnormally low resistance when a joint is passively moved. Hypotonia should not be confused with weakness, which is a decrease in the maximum power a muscle can generate.

Hypotonia might happen with or without muscle weakness. While it’s easy to notice, figuring out the root cause can be a challenge for doctors. The causes are many, but with a methodical approach that includes a thorough medical history and physical examination, a diagnosis can be achieved. Identifying the cause is fundamental for effective treatment planning and prognosis. While specific treatments exist for some diseases, the general approach usually includes supportive care with services like physiotherapy, nutritional guidance, and respiratory support.

What Causes Hypotonia?

Hypotonia, or reduced muscle tone, is often present at birth and usually diagnosed in early infancy. There are many possible causes for hypotonia in a baby. It can be a result of problems with the muscles, the areas where nerves and muscles connect, or the central and peripheral nervous systems (the brain, spinal cord, and nerves throughout the body). Hypotonia can also be a part of genetic disorders, metabolic diseases (diseases that affect how the body breaks down or uses food), hormone-related problems, or short-term or long-term illnesses.

In about half of the cases, doctors can determine the cause of hypotonia by taking a detailed medical history and doing a physical examination. Central causes, or causes related to the brain and spinal cord, include lack of oxygen to the brain at birth, brain abnormalities or injuries, genetic or chromosomal syndromes, infections present at birth or acquired later, and metabolic disorders. Peripheral causes, or causes related to the nerves and muscles throughout the body, include conditions such as spinal muscular atrophy, myasthenia gravis (a condition that weakens the muscles), exposure to harmful substances, inherited nerve disorders, muscular dystrophies (a group of diseases that cause progressive weakness and loss of muscle mass), congenital or metabolic muscle diseases, and congenital myotonic dystrophies (a group of disorders that affect muscles and other body systems).

Risk Factors and Frequency for Hypotonia

Hypotonia, or reduced muscle tone, isn’t a disease in itself, but rather a symptom of various diseases. Most cases of hypotonia are present at birth, and are most often caused by problems in the brain. This can include things like oxygen deprivation during birth or genetic abnormalities. Around 60% to 80% of all cases fall into this category, with the most common genetic causes being Down syndrome and Prader-Willi syndrome.

Despite being less common, hypotonia can also be caused by issues with the nerves or muscles (called peripheral causes), or by unknown causes. The conditions most often linked to peripheral causes are spinal muscular atrophy, congenital muscular dystrophy, and congenital myopathies. For example, spinal muscular atrophy occurs in about 1 in every 6,000 to 10,000 newborns.

Hypotonia can also develop after birth due to toxins or infections, such as infant botulism. While this condition is rare worldwide, it’s most often diagnosed in the United States, largely due to heightened awareness among doctors. The states with the highest rates of infant botulism are California, Pennsylvania, and Utah, wit California accounting for about half of all cases.

Signs and Symptoms of Hypotonia

The process of diagnosing hypotonia, or low muscle tone, begins with a thorough history. This includes information about the family’s medical history before and during pregnancy. Details about the child’s development and any conditions that may run in the family, like muscular diseases, genetic disorders, or close relative marriages, are considered. Information about the mother’s exposure to toxins or drugs during pregnancy, any infections, how the baby was delivered, and the infant’s health and movements during the pregnancy and at birth are also needed.

Knowledge about when the muscle weakness started and how it’s evolved can be useful in determining the root cause. For instance, weak muscle tone that starts at birth might indicate a brain injury due to inadequate oxygen or blood flow. On the other hand, weak muscle tone that appears 12 to 24 hours after birth could point to a substance created by the body not being processed correctly. Infants with centrally caused muscle weakness normally show more cognitive delays. For older children, a standardized test like the Bayley Scale could be used to assess different aspects of their development like motor skills, cognitive abilities, language, and social-emotional abilities. Evaluating all bodily systems is also key in diagnosing the cause.

The next step of the diagnosis involves a physical and neurological examination, observing any features or birth defects that are not typical. The shape and size of the head should also be examined. The question of whether the weak muscle tone has worsened or remained the same since it was first noticed should be addressed. Four key maneuvers are employed to assess the muscle tone of an infant:

• Vertical suspension: Where the infant is held by their underarms. A hypotonic infant would slip through the examiner’s hands.
• Horizontal suspension: Where the infant is held up in a belly-down position. Typically, an awake full-term baby would flex their arms and legs and lift their head for a bit. A hypotonic infant would instead form an inverted “U” posture.
• “Scarf Sign”: It assesses the muscle tone in the shoulders. The examiner pulls the infant’s hand across their chest until there is resistance. A hypotonic infant’s elbow can go past the midpoint of their body before encountering resistance.
• “Pull-to-sit” maneuver: The examiner tries to help the lying down infant sit up. A significant delay in the baby raising their head, a symptom of hypotonia, should disappear by two months. This maneuver mainly checks the muscle tone in the neck and back muscles.

Evaluating muscle strength in an infant can be difficult. Weakness could show itself as fewer spontaneous movements; this can be examined by observing the baby’s cry, suck reflex, spontaneous movements, facial expressions, anti-gravity movements, and how much effort they put into breathing.

For patients with central hypotonia, there are other signs to look for. These include a decreased level of consciousness, normal muscle strength, normal or exaggerated reflexes, and persistent primitive reflexes and muscle contractions. They can also have unusual physical characteristics and birth defects, which would point towards hypotonia as part of a syndrome. These signs will vary in severity and frequency, depending on what’s causing the hypotonia. As for peripheral hypotonia, the patient may have difficulty moving their limbs against gravity. Though they usually have normal cognitive abilities, they may have delayed motor skills. Their reflexes are often reduced or non-existent, and they might also have feeding and breathing difficulties, and impaired eye and facial movements.

Testing for Hypotonia

To diagnose a newborn baby with low muscle tone (a condition known as hypotonia), doctors have to go through a series of tests. The first tests usually involve a physical examination to check for reflexes and unusual looseness of the ligaments.

If the doctor suspects an infection, they will want to rule this out first. This involves taking samples of blood, urine and fluid from around the brain and spinal cord. They will also do a complete blood count, check for different chemicals in the blood, and screen for any drugs. If the baby’s liver or spleen is bigger than normal, or if there are unusual spots on a head ultrasound, the doctor might suspect a birth infection. In this case, additional testing would be needed, such as testing for certain types of viral and bacterial infections like rubella or herpes simplex.

If the doctor suspects a genetic or metabolic disorder, they might look for physical abnormalities or birth defects. In this case, a specific type of genetic test called a chromosome analysis might be done. If this test points to something like Down syndrome or a condition called PWS, then the doctor might do additional testing to confirm it.

When the doctor suspects a metabolic disorder (a problem with how the body breaks down or uses certain substances), they may do more checks. These include measuring levels of various substances in the blood, which can tell the doctor if the baby’s body is having trouble breaking down carbohydrates, fats or certain acids.

If the doctor thinks that the baby’s low muscle tone might be due to a problem with the nerves or muscles, they may measure a chemical in the blood called creatine kinase (CK). A high level of CK can point to muscle damage, as seen in conditions like muscular dystrophy. But checking CK levels can be difficult in a baby, so the doctor might need to do other tests.

If the readings are abnormally high, the doctor may consider doing a muscle biopsy. This test involves taking a small sample of muscle tissue to be examined under a microscope. Though it’s an invasive procedure, it can be crucial for diagnosing various diseases.

The doctor might also perform an electromyography (EMG), a test that measures electrical activity in the muscles. Depending on the EMG result, the doctor might conduct tests for conditions like spinal muscular atrophy or certain types of neuropathy.

Doctors can also use imaging techniques like MRI and CT scans to look for abnormalities in the brain that might be causing the baby’s low muscle tone. These scans can reveal structural changes, defects or abnormal signalling in the brain. A type of MRI scan called magnetic resonance spectrometry can be useful for identifying certain metabolic disorders.

Treatment Options for Hypotonia

Generally, the main priority in treating hypotonia, or low muscle tone, in infants is managing the symptoms, even before identifying what’s causing the condition. Treatment is customized based on each little one’s specific symptoms and the suspected cause. An approach that involves specialists from different disciplines often leads to the best results.

Support from a range of healthcare professionals may be required. This might include rehabilitation services, dietary guidance, and support for any breathing problems. In cases where the low muscle tone isn’t due to oxygen deprivation to the brain (hypoxic-ischemic encephalopathy), it might be necessary to consult with a specialist in genetics or metabolism. Therapists who specialize in speech, physical movement, and occupational therapy (daily living skills) can be crucial in helping your infant make the most of their muscle function and avoid developmental issues with their body structure.

Children with hypotonia often face nutritional challenges, including being underweight or having imbalances in their intake of important nutrients. Their dietary needs might go up when they’re sick. In severe cases, where the chest muscles are so weak that the baby struggles with eating, a feeding tube inserted through the nose (nasogastric tube) or the stomach (percutaneous gastrostomy tube) might be needed.

In some situations, there are specific treatments available. For example, in Pompe disease, early treatment by replacing a missing enzyme can significantly improve the child’s health. For spinal muscular atrophy (SMA), there are a few extremely expensive treatments approved by the US Food and Drug Administration. These include Spinraza, a gene therapy that fixes the SMN2 gene; Zolgensma, which replaces a missing or faulty SMN1 gene; and Risdiplam, a new oral treatment. Both Spinraza and Zolgensma are administered by injection into the spinal canal.

Genetic counseling is also very important. It helps parents understand the risks and choices for future pregnancies and informs any family members who might also be at risk. Parents should be involved from the start, learning about the disease and the steps of the treatment process.

What else can Hypotonia be?

Neonates, or newborns, can experience a condition known as hypotonia, which refers to reduced muscle strength or tone. Several reasons can lead to this situation, including issues with the central nervous system, genetic and chromosomal problems, or certain diseases. Some possible causes include:

• Systemic diseases affecting the nervous system like congestive heart failure, particularly in infants born with a heart defect.
• Sepsis, a severe infection that can also cause hypotonia.
• Metabolic disorders, where the newborns may appear septic, but a definite diagnosis will only become clear once the metabolic tests are complete.
• A hypoxic-ischemic insult, where a newborn has suffered from a lack of oxygen. This can lead to a depressed consciousness along with hypotonia.
• Down syndrome, a genetic disorder characterized by certain facial features which cause hypotonia.
• Prader-Willi Syndrome (PWS), another genetic disorder which can lead to unexplained hypotonia in newborns.
• Peroxisomal disorders, which are due to mutations in PEX genes and may present with significant hypotonia.
• Benign congenital hypotonia – an unprogressive disorder of the neuromuscular system where muscle tone improves with age.
• Pompe disease – a rare disorder caused by a deficiency of a certain enzyme that leads to hypotonia.

It’s important to note that the presence of hypotonia does not immediately indicate one specific condition or disease. Medical professionals should be careful to evaluate each case on an individual basis to arrive at an accurate diagnosis. Depending on the cause, treatment plans will vary.

The severity of each disease could vary. Hypotonia may pretense several challenges, such as feeding difficulties and failure to thrive in the infant stage; however, with appropriate medical attention and treatment, these can usually be managed effectively to ensure the overall well-being and development of the child.

What to expect with Hypotonia

The outlook for a patient can vary greatly depending on the root cause of the condition. In cases like transient myasthenia (a temporary weakness of the muscles) and infantile botulism (a rare but serious illness that causes weakness and loss of muscle tone), full recovery with little to no complications is generally expected.

For conditions like hypotonia (low muscle tone) connected to sepsis (a severe infection that spreads in the body) or other diseases like congestive heart failure (a chronic condition where the heart doesn’t pump blood as well as it should), the prognosis usually improves once the underlying disease is treated.

However, there are certain conditions, such as spinal muscular atrophy (a disease that causes weakness and shrinking of the muscles used for movement) and Pompe disease (a disorder that disables the body’s cells to break down a type of sugar leading to damaging build-up), which can unfortunately be fatal early in life.

It’s important to understand that working with a team of different healthcare professionals can greatly influence the patient’s outcome. This team might include rehabilitation services, nutritional support, and home care services among others. These kinds of support systems can greatly improve the prognosis, leading to better results for the patient.

Possible Complications When Diagnosed with Hypotonia

Hypotonia, or decreased muscle tone, often brings about several complications. Most notably, the risk of death is particularly high during the first year of life. After this point, however, those who survive often exhibit developmental delays.

A majority of individuals with hypotonia require dietary support to compensate for various micro and macro-nutrient deficiencies. Also, hypotonic patients necessitating lung support might be susceptible to frequent respiratory infections including pneumonia.

In certain scenarios, patients confined to a wheelchair due to their condition are likely to suffer from fractures, weakened bones, muscle contractures (permanent shortening of muscles or joints), and bedsores. This is the case even when adequate physiotherapy is administered.

Key Complications:

• High mortality rate within the first year of life
• Developmental delays in survivors over one year old
• Deficiencies in both micro and macro-nutrients
• The susceptibility to frequent respiratory infections and pneumonia
• Increased likelihood of fractures and weakened bones in patients using wheelchairs
• Muscle contractures and bedsores

Preventing Hypotonia

It’s really important for the parents to understand everything about their child’s illness and what it means for their child’s health. Parents play a key role from the start in helping to choose the best treatment options suited for their child. It’s also vital for parents to understand the need for regular check-ups with the doctor.

If the child needs to use a breathing machine at home, it’s essential for parents to learn how to use it correctly. This can help avoid unnecessary trips to the hospital. It’s also important for parents to know if there are any specific things their child should avoid due to their illness, as this can prevent serious problems from occurring.

When parents are actively involved and educated about their child’s health, it not only leads to better health outcomes for the child, but it also helps to build a strong and positive relationship between the doctor and the family.