What is Hirayama Disease?

Hirayama disease is a rare illness that caused muscles in the forearms and hands to weaken and shrink. This condition, first identified by Dr. Hirayama in 1959, can affect one or both arms and doesn’t impact the sense of touch. Generally, the symptoms get worse over one or two years, then stop suddenly. The cause of Hirayama disease is related to changes in the cells of the neck’s spinal cord due to restricted flexibility of the membrane that covers the spinal cord. This primarily affects nerves responsible for muscle activity.

While this disease is typically not progressive and tends to stop on its own, it can seriously handicap some people. Starting treatment early can help slow the disease’s progression and lessen the disability it causes. It’s known by several names, including benign juvenile brachial spinal muscular atrophy, juvenile asymmetric segmental spinal muscular atrophy, juvenile muscular atrophy of the distal upper extremity, monomelic amyotrophy, and oblique amyotrophy. However, the most common term is Hirayama disease, which was coined in 1991.

What Causes Hirayama Disease?

Several theories have been proposed to explain how this condition develops. One theory, called the “contact pressure theory,” suggests that while bending, spurs (like bony outgrowths) compress the spinal cord. This idea came from studying bodies after death back in the 1960s.

Another theory from 1987, the “tight dural canal theory in flexion,” is now widely accepted. This theory suggests that the outer layer of the spinal cord (the dura mater) becomes more flexible at the top of the neck (C2 and C3). This allows the spine to shift forward when the neck bends. This shifting can lead to small, almost unnoticed injuries and restricts blood flow to the front area of the spinal cord. Over time, this can lead to a condition called myelopathy (damage to the spinal cord) and degeneration, as we can see by the thinning of the lower spinal cord at C7 and C8.

Moreover, it is suggested that this flexibility may be caused by an imbalance in growth between the spine and the dural canal. This imbalance is especially noticeable during growth spurts. As you stop growing, the shifting decreases, and the condition tends to stabilize or pause.

Another theory relates to the ligaments at the back of the spine. Two types of elastic ligaments have been described: thin and thicker ones. The idea is that these ligaments, which support the back of the dura mater, are not evenly distributed. This allows only part of the spinal cord to shift forward.

One more theory proposed by Hirayama suggests that the pressure inside the spinal cord increases when the neck bends, causing stagnant blood flow in the veins at the back of the spinal cord. This stagnation causes pressure on the cord. Also, it’s suggested that irregularities in the immune response of the dura and back ligaments can cause this condition, as these can lead to detachment at the back of the dura. It’s important to remember that these are all theories based on scientific research and observations, and not all may apply to every individual case.

Risk Factors and Frequency for Hirayama Disease

This condition is more common in males and is most frequently seen in the Asian population, particularly in Japan, but also in China, Taiwan, Malaysia, India, and Sri Lanka. There are a handful of cases in Europe and North America as well.

The weakness usually begins sneaky and one-sided around puberty, often in early to mid-teens. The peak onset age is approximately two years later than the height growth peak in Japanese boys. A plateau, or a stage where the condition doesn’t worsen, happens on average 1 to 4 years after onset, but can sometimes last until the early twenties.

Most of the time, the right upper limb is more affected (with a ratio of 2.8 to 1), regardless of which hand the person uses more.
However, a few cases of both upper limbs being involved have been reported, which are considered the most severe forms of this condition. This is more common in the same age group.

Signs and Symptoms of Hirayama Disease

Patients with this condition often report a progressive decrease in the ability of their right hand to function normally. They notice a drop in their hand strength and finesse, which interferes with their ability to perform daily tasks requiring fine motor skills, such as eating, dressing, and grooming. Cold temperatures can further aggravate these symptoms.

As the condition worsens, the patient may initially notice a physical change in their hand, related to muscle loss. Typically, pain isn’t reported until the muscle loss involves the upper arm and shoulder, which happens in more severe cases. Patients could also experience increased fatigue, specific allergies, and symptoms worsening in cold conditions.

This combination of symptoms can lead patients to withdraw or reduce participation in sports and other recreational activities. This can, unfortunately, lead to social isolation.

Commonly, the condition presents as one-sided muscle loss in the lower part of the upper limb. Most muscle loss occurs in the thumb, hand muscles, the muscle on the pinky side of the hand, and the forearm muscles. Depending on severity, there might also be differences in limb length. The muscle loss tends to be slanted due to the larger muscle on the forearm being unaffected.

The disease most often causes weakness in the wrist and fingers due to the nerve roots in these areas being affected. Fine motor actions and grasp are limited, and patients might struggle with tasks like pinching or using a key. The strength and finesse in the other hand and fingers are normally unaffected. Arm movements like flexion, extension, pronation, and supination are also typically unaffected. The muscles closer to the body are mostly spared as well.

The patient’s ability to sense vibration, temperature, sharpness, dullness, and light touch on all limbs is usually unaffected. Muscle stretch reflexes are generally normal and symmetrical.

Heart function and blood flow are also generally normal. The limbs are warm, and blood flow to the skin is good.

The set of clinical diagnostic criteria for the disease includes:

Weakness and muscle loss in the upper limb
Onset between 10-20 years of age
Affected area is one-sided or mostly asymmetric
Gradual onset and progression, followed by a halt
No signs of nerve cell damage or sensory disturbance
Other similar conditions have been ruled out

Testing for Hirayama Disease

To differentiate between this particular disease and other similar conditions, doctors often utilize a combination of scans and tests like radio-imaging, electromyograms (EMG), and nerve conduction studies. Blood tests may not contribute significantly to the diagnosis. However, an increase in the level of serum IgE, a type of antibody in the blood, has been observed. Traditional x-ray findings are usually not specific, although sometimes, they might reveal degeneration at the C2 area (the second bone in the spine).

In terms of MRI imaging, a crescent-shaped lesion (an area of tissue that has been damaged due to injury or illness) may be visible in the posterior epidural space of the lower cervical cord, or an atrophic spinal cord (a condition often seen with aging, when the spinal cord becomes thinner), minor uneven cord flattening, and increased signal intensity. Specialized flexion MRIs (where the neck is flexed during the scan), may provide a confirmatory diagnosis as they can pick up distinct changes in the alignment of soft tissue structures, indicating chronic microtrauma (small repeated injuries) and ischemia (a decrease in blood flow).

Moreover, some specific signs, like the ‘snake eye’ sign, have been linked with poorer health outcomes. MRI can also help in determining instability in the neck areas by highlighting reduced cross-sectional areas of the superficial neck muscles.

As for nerve conduction studies (tests that measure the speed of conduction of electrical signals in nerves) and electromyograms (tests that detect the electrical potential generated by muscle cells), they typically indicate significant findings in the C7, C8, and T1 innervated muscles (muscles controlled by these specific spinal nerves). More specifically, these studies might reveal decreased compound muscle action potential (CMAP) amplitude, especially in the median nerve, which helps differentiate this disease from others.

Finally, “Reverse split hand syndrome” could be observed in Hirayama disease (a rare neurodegenerative disease), which helps further differentiate it from similar conditions such as amyotrophic lateral sclerosis. This syndrome is characterized by decreased/absent CMAP amplitude in the abductor digiti minimi muscle (small muscle of the hand) while preserved in the abductor pollicis brevis (a muscle in the thumb), which is not the case in amyotrophic lateral sclerosis.

Treatment Options for Hirayama Disease

Hirayama disease (HD) is a condition that generally resolves itself, but it requires treatment because of its long-term effects. One of the treatment approaches involves wearing a cervical collar. This collar limits the movement of the neck, and it’s worn for 3-4 years to prevent the condition from worsening as the body grows. You are also given physical therapy to strengthen the back of your neck, which can improve the position of your neck.

For severe cases of the disease, doctors might suggest surgical options. In these cases, typically you have unusual muscle involvement and severe shrinking of the spinal cord. There are different procedures, one of which is called ‘duroplasty with tenting.’ The benefits of this surgery can be observed immediately, with decreased heartbeat-like movement of your spinal cord and increased electrical activity in your spinal cord after the protective layer is cut. Afterwards, you might also notice an increase in your hand strength as you go through rehabilitation.

To diagnose and manage Hirayama disease, doctors use something called the “Huashan classification system.” HD is grouped into three types based on the muscles affected and the presence of damage to the spinal tract:

Type 1: atrophy or shrinkage of hand and forearm muscles, or unequal shrinkage in both upper limbs. This further divides into type 1a (stable) and type 1b (progressive).

Type 2: atrophy combined with spinal tract damage.

Type 3: abnormal shrinkage of upper limb muscles, same level of shrinkage in both limbs, or presence of sensory problems.

Doctors usually suggest a non-surgical approach for patients with type 1 and 3 diseases. However, surgery may be recommended for those with type 2 condition. The most successful surgical procedure involves going through the front of the neck, then removing the disc and fusing the bones together. The angle your neck forms when seen from the side (the ‘lordosis angle’) is critical to maintaining good health of nearby segments after this surgery. This approach is preferred over fusing the neck because it reduces the risk of harmful effects on nearby bone segments after the disease stabilizes.

The principle behind duroplasty with tenting sutures and laminoplasty, a surgical procedure, is based on the theory of ‘tight dural canal theory in flexion.’ But you must be aware that the procedures carry few risks such as bleeding and meningitis, particularly when the network of veins at the back of the spine is coagulated or clotted.

Finally, guidelines on how to diagnose and treat the condition have also been produced using the modified Delphi technique, a method involving a panel of experts to reach a consensus.

When it comes to conditions which mimic Hirayama disease, there are quite a few. These can be broadly divided into Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) afflictions. Some UMN disorders that can resemble Hirayama disease include amyotrophic lateral sclerosis, spinal muscular atrophy, cervical spondylotic myelopathy, ossified posterior longitudinal ligament, and toxic neuropathy.

As for LMN conditions, there are several that can appear similar to Hirayama disease, such as:

Myotonic dystrophy: This is a genetically inherited condition, often seen in middle-aged men. It’s associated with muscle rigidity in the upper arms, accompanied by broad changes in the body including cataracts, respiratory disorders, hormonal issues, fertility problems, and alterations in heart rhythm. The distinguishing factor from Hirayama disease is the systemic involvement and muscle biopsy findings.
Tephromalacie Anterior: An anterior horn cell damage leading to muscle wasting in the distal upper limbs. It’s caused by blockage of spinal arteries, usually seen in middle-aged adults, and associated with hardening of the arteries. This condition affects both limbs.
Syringomyelia: This is a long-term, progressive degeneration disorder linked to motor and sensory modifications seen in the upper limbs of both sides. It’s often seen in elderly people following an injury. Structural changes in the spinal cord can be seen on radiographic imaging.
Pronator Syndrome: It usually affects one arm and is connected with a past injury. Symptoms get worse with certain activities. Pain is a key symptom of this condition.
Peripheral Nerve Entrapment Syndrome: A condition where a peripheral nerve is compressed causing pain, numbness and weakness.

Being aware of these conditions which mirror Hirayama disease can help in diagnosing the correct illness and applying the appropriate treatment.

What to expect with Hirayama Disease

Hirayama disease naturally tends to stop getting worse on its own after a certain period of time, which can be anywhere from 1 month to 5 years. Overall, people with Hirayama disease usually have a better outcome compared to other diseases that affect the motor neurons, with a longer life expectancy and less severe health problems. Wearing a cervical collar, which helps support the neck, has been shown to be effective in improving symptoms in about 58% of cases.

However, please keep in mind that some symptoms might last for many years. It’s estimated that around 70% of people with Hirayama disease will experience some mild disabilities.

Tests such as nerve conduction studies and electromyography can be used to see how much and what kind of nerve damage is present and how well the nerves are healing. In some cases, these tests might show more extensive nerve involvement than was initially expected based on the person’s symptoms. This information can help doctors predict their patient’s outcome.

Possible Complications When Diagnosed with Hirayama Disease

In Hirayama disease, most complications arise because of ongoing problems with muscle movement. This could lead to weakness in the hand, forearm, or arm, resulting in long-term muscle shrinkage due to disuse and permanent tightening of muscles. The disease can also cause muscle stiffness, or it might come back even after being stable for several years. Physical therapy has shown varying levels of success in different reported cases, and surgeries to relocate tendons may also be worth considering.

The complications of Hirayama disease may include:

Weakness in the hand, forearm, or arm
Long-term muscle shrinkage from disuse
Permanent muscle tightening
Muscle stiffness
Recurrence of the disease after a stable period

Some of the potential treatments could involve:

Physical therapy, with varying levels of success
Surgery to relocate tendons, if appropriate

Preventing Hirayama Disease

Helping patients understand the likely outcome and how to prevent long-term movement problems can empower them to make better choices about their personal and professional lives.