How Common are Nerve Sheath Tumors in Dogs?
We know our dogs can’t literally tell us when something is wrong so pet owners who want to make sure they’re being responsible may worry about any underlying health problems our furry friends may have. Typically, there are some warning signs that may indicate your beloved pup has a tumor which can develop into dog cancer. These may include a lump or a bump, swelling or a wound that doesn’t heal. Some pup parents may immediately fear the cause is a cancerous tumor and more specifically they may wonder if it’s a dreaded nerve sheath tumor. So, how common are nerve sheath tumors in dogs?
As you would imagine, nerves are super-important because they transport signals between the brain and spinal cord and various parts of the body. Nerves are involved in essential bodily functions like muscle movements, while others (sensory nerves) are involved in transmitting information such as touch, temperature, pain, and a sense of the position of the legs.
Let’s take a look at what scientists have discovered about nerve sheath tumors and the level of likelihood that your dog might develop any of these firm, white nodules beneath the skin.
What is a Nerve Sheath Tumor?
A nerve sheath tumor is an abnormal growth or mass of the skin or soft tissue. The tumor is made up of Schwann cells which are found in connective tissue known as the myelin, which surrounds the nerve. Myelin acts as insulation and a conduit for the transmission of nerve signals. Peripheral nerve sheath tumors (PNSTs) in dogs develop from the supporting cells that surround, protect and insulate the nerves. These tumors usually grow along the nerve but do not typically spread to other parts of the body. Whether it’s the type that spreads or not, early detection of these tumors is crucial.
Causes of a Nerve Sheath Tumor
Nerve sheath tumors are thought to develop in areas around a former injury. In the repair process for tissues and cells that were damaged during injury, Schwann cells normally appear. Nerve sheath tumors originate from these Schwann cells. Admittedly, there is no published information to support this.
Symptoms to Look For
The most common clinical signs of nerve sheath tumors include:
- Lameness (possibly a chronic, progressive lameness of one leg)
- Severe, unexplained pain
- A mass that can be felt by touch examination
- Partial loss of movement in a limb
- Lack of coordination
- Lack of awareness of movement and posture
- Muscle atrophy
- Absence of reflexes
Other symptoms may include decreased pupil size and droopy eyelids. Horner’s syndrome may also occur. This symptom is caused by damage to the sympathetic nervous system.
The clinical signs depend on the nerve involved. It should be noted that it is very difficult to diagnose malignant peripheral nerve sheath tumors of the forelimb in dogs. As the tumor grows, the animal may be unable to use the leg, and adjacent nerves may be affected. If the cancer develops close to the spinal cord and grows into the spinal canal, you may detect weakness and an uncoordinated gait in other legs.
Peripheral nerve sheath tumors in dogs can also affect the nerves of the head and face. The fifth cranial nerve (V) is the one most commonly affected. This nerve activates the muscles involved in chewing and transmits signals related to feeling in the face. If a PNST develops in this nerve the result is likely to be shrinkage or atrophy of the muscles on the top and side of the head. This may cause the bony areas of the skull to become more conspicuous. If the tumor develops in one of the other cranial nerves, there may be other symptoms such as an inability to blink.
The bad news is that sometimes there are little or no signs of the presence of a tumor.
Diagnostic Tests Related to Lameness
There are other conditions that are more common in dogs than nerve sheath tumors and these other conditions can also cause lameness. Orthopedic problems in pets need to be investigated first by x-rays and laboratory tests. If these tests are normal, the signs continue to worsen despite symptomatic medications and the neurologic examination reveals a weak limb with atrophy of the muscles, a PNST may become the prime suspect.
Magnetic resonance imaging (MRI) is the best tool for accurate diagnosis in these cases. An MRI can pinpoint the specific nerve that is affected. It may also show the extent of involvement of the nerve and it can determine whether the tumor has invaded the spinal cord. Other diagnostic imaging studies, such as an ultrasound, CT scan and myelography, can also be used to evaluate affected animals. However, a definitive diagnosis is obtained through a biopsy of the tumor. In addition, X-rays of the chest are usually performed to look for any spread or metastasis of the cancer.
Treatment of Nerve Sheath Tumors
Whether the tumor is benign or malignant it needs to be removed to reduce discomfort and prevent it from developing further or getting larger. Surgical removal of the tumor is the treatment of choice for peripheral nerve sheath tumors in dogs but unfortunately, amputation becomes inevitable in some cases.
If the tumor cannot be completely removed or if surgery cannot be performed, radiotherapy may be an alternative, depending on the size of the tumor and its location. Radiotherapy treatments last three to four weeks and the aim is to stop the division of cells at the site of the tumor to deter further growth. Thanks to technological advances over the years, most dogs respond well to this procedure, and it has a good success rate in deterring the growth of future tumors. Side effects are minimal and may include temporary or permanent hair loss at the site of treatment.
If none of these treatments seems appropriate, then palliative care may be considered in the form of medications to keep your faithful friend comfortable and mobile for as long as possible.
The prognosis for your pet depends on the location and severity of changes caused by the tumor and the treatment chosen. The closer the tumor is to the spinal cord, the worse the prognosis. Pets with tumors close to the spinal cord may live only a few months, while those with tumors outside the spinal cord may live up to one year.
Long-term monitoring is required after treatment because this type of tumor is difficult to remove or kill. You need to keep a close eye on the progression of the disease and how it is responding to treatment. Unfortunately, in 72% of cases, malignant peripheral nerve sheath tumors recur after surgery. Since these lesions typically are not detected early, the limbs have to be amputated in most of the cases. The median survival time for dogs with malignant peripheral nerve sheath tumors is two years. The closer the tumor is to the paw, the better the chances are of recovery. However, reports also suggest that benign peripheral nerve sheath tumors have an excellent prognosis.
What Are the Odds of My Dog Being Affected?
In the beginning of the article we asked the question, how common are nerve sheath tumors in dogs? Chin up, nerve sheath tumors have a low rate of occurrence in dogs. When they are diagnosed, they are most commonly found in older animals.
The average age of onset for most breeds is nine years of age. There is a higher chance of the following breeds to be affected:
If you notice any symptoms in your pet that concern you, before rushing to conclusions, take your dog to be examined by a veterinarian so you can get an accurate diagnosis.
Check out our cancer library for more detailed information on Nerve Sheath Tumors in dogs.