Canine Osteosarcoma, Degenerative Myelopathy and Brain Tumours – Three Common Fatal Issues Among Canis Lupus Familiaris

Publishing Author : Jay Gray

Date Published : 07/11/18

 

Introduction

Although different breeds are more prone to different fatal illnesses or syndromes, all breeds, without exception can die from Canine Osteosarcoma, Degenerative Myelopathy and Brain Tumours.  There are several factors to consider with all three of these issues and the paper will be broken down to cover external factors, treatment options, outcomes, symptoms and the such.

It is important to remember that appropriate veterinary care should be taken should a dog be showing any of the symptoms listed in this paper.  Appropriate diagnosis and treatment plans must be thought out and followed through correctly which requires the input of a Veterinary Surgeon to properly oversee and diagnose the issues.

Canine Osteosarcoma is a cancerous growth of the bone.  The tumour on the bone is malignant and not benign.  It tends to affect the limbs of the skeleton but can occur in a multitude of places such as the spine, and the ribs (known as the axial skeleton).

Canine Degenerative Myelopathy is characterised by a non-painful and progressive paralysis of the hind legs.  This tends to occur in older dogs.  Degenerative Myelopathy (DM) was previously registered as a chronic degenerative radiculomyelopathy but the name was later updated.  The disease is ultimately fatal and can be difficult to deal with.

Brain Tumours can occur in a variety of manners, but all consist of malignant cells forming a tumour on or in the brain tissue.  This paper will take the umbrella term for the tumours rather than dealing with the individual types.  Brain tumours can consist of, but are not limited to, Chordoma, CNS Lymphoma, Glioma, Medulloblastoma, Meningioma, Neuroblastoma, Oligodendroglioma, PNET and Schwannoma.

 

Canine Osteosarcoma

Canine Osteosarcoma (OSA) is a malignant tumour of the bone and it is statistically the most common tumour found in any breed of dog.  It tends to affect the limbs (appendicular skeleton) but can also affect the skull, spine or ribs (known as the axial skeleton).  The tumour will most commonly occur in the front legs of the dog, affecting the radius and top of the humerus.  OSA can also form in the femur and tibia of the hind limb but this is less common.  OSA can occur at any age and in any breed but it tends to happen as dogs get older and large or giant breeds are more prone to this disease.

Appendicular OSA is easiest to spot through a localised but firm swelling in the affected limb.  The dog may also develop a lameness which is persistent and is not resolving with rest or even painkillers.  OSA is extremely painful as a condition so dogs can also show signs of stress or restlessness.  Some dogs however, are extremely resilient to pain and may show very few signs because of their bravery.  This means the changes in behaviour that they outwardly show may be small and hard to see.  These could be loss of appetite, change in general demeanour and reduction in activity levels.  Sometimes because of the changes that the cancer causes in the bone architecture, it can weaken the bone which may result in an eventual fracture.  This is known as a pathological fracture.

Axial OSA is dependant on where in the dog’s axial skeleton the tumour is present.  In the mouth, symptoms can present themselves as bad breath, blood in the food or water bowl or a visible mass inside the gums.  Within the skull, the tumour can cause changes in facial appearance or symmetry and ultimately it can grow into the brain cavity causing seizures or other neurological issues.  Spinal OSA can compress the spinal cord or surrounding nerves and present symptoms such as inability to walk or unsteadiness.  These symptoms can be mild or severe depending on the level of compression on the nerves in question.  OSA in the ribs can be observed as a firm swelling beneath the skin but atop the rib cage and is usually found around two thirds of the way down the length of the rib itself.

Currently we have no definitive cause for OSA, but it is assumed and mildly studied to be related to the genes in the dog responsible for the promotion and suppression of cellular growth.  It is certainly more common in certain breeds such as the Greyhound, Mastiff, Doberman and Rottweiler.

Diagnosing OSA aims to not only investigate the tumour itself but also determine how extensively the cancer has spread within the body.  This is known as staging.  Taking both blood and urine samples are routine procedures and can provide valuable information regarding the general health of the dog, which in turn can enables vets to develop anaesthetic protocols if they are necessary.  Both blood and urine analysis can also provide information about the cancer itself also.  X Rays are taken of the affected area to rule out other problems like infection or fractures and they also ascertain the extent of the cancer and its infiltration of the bone and neighbouring joints if applicable.  They can also be used to determine the extent of tumour spread to other parts of the body specifically the chest and lungs.  If there is evidence of the cancer in the lungs, then really only palliative care measures can be taken.  The disease will certainly be fatal at this point.

CT scans may also be taken to show virtual slices through organs (known as cross sectional imaging).  This is a much more advanced technique and is able to detect changes in the bone and the long through its cross-sectional plane.  CT can also be used to reconstruct the bone tumour in 3D to help the oncologist make decisions about whether or not the limb can be saved in that specific instance.

A bone biopsy is a further analysis method where the dog is sedated, and a fine needle is pushed through a thin defect in the bone wall in order to collect cells which can be analysed under a microscope to determine whether or not the growth is cancerous.  Sometimes insufficient material is gathered and a further biopsy under GA will be performed where thin cores of bone tissue are taken away to confirm malignancy.

There is no perfect way to treat bone cancer and each of the methods should be discussed at length with the veterinary surgeon and the family of the dog in question.  Ultimately the owners must make the final decision.  It is important to remember that by the time OSA has been diagnosed, cancerous cells have almost definitely left the primary mass and moved elsewhere in the body (metastasised).  These cells can move in clusters or as individuals and are often too small to see before surgery even with the best CT scans.  They often remain dormant for many months or even years, but chemotherapy can slow down the progression of the metastatic cells.

Bisphosphonates are bone-hardening drugs that were once used in human medicine to strengthen the weaker menopausal bone, but their use has evolved to treat a multitude of bone pains but specifically cancerous pain.  The drugs are given IV usually every 3-4 weeks and can be used in conjunction with home care methods and protocol.  Their use will reduce the pain and bone destruction, thus reducing the likelihood of pathological fracture.

Radiation therapy is delivered by a linear accelerator and can be used to treat the cancer itself.  Depending on the protocol decided, 1-4 fractions may be given in conjunction with home care and bisphosphonates.  Radiation only targets the tumour itself and does not address the distant spread throughout the body.

Amputation tends to be well tolerated in many dogs, even in the front legs of giant breeds.  When making this decision to sacrifice the limb, other factors such as concurrent orthopaedic and neurologic disease need to be considered.  Sacrificing the affected limb is an extremely common procedure when OSA is present.  It is the fastest, most reliable way to remove the primary tumour, end bone pain and minimise complications within the leg, further preventing pathological fracture.

Limb sparing surgery is an umbrella term for any technique where the tumour is removed without removal of the limb.  Common examples are Ulnectomy, or Scapulectomy and is used if the tumour is well contained to parts of the bones in question.  If the tumour is in the major long bones that bear the majority of the weight (radius, femur etc) then implants will be added to replace the missing bone.  This could be done through internal prosthesis or external prosthesis.  These techniques are both specialised surgical procedures which are carefully tailored to the individual dog and individual family.

The primary goal in treatment of OSA is the relief of bone pain and re-establishment of a good quality of life.  The secondary goal is longevity and we understand scientifically that chemotherapy extends the life of dogs suffering from long bone OSA.  The treatment is typically IV chemotherapy drugs every three weeks.  The drug most commonly used is carboplatin.  The strictest safety standards must be upheld when reconstituting, delivering and disposing of chemotherapy drugs and oncology clinicians have usually had many years of experience in chemotherapy for OSA.  Their goal is to make patients more comfortable for a longer period of time.

Dogs tend not to experience the same side effects as people with this treatment programme although occasionally transient inappetence, nausea and lethargy can be present.  They often last a day or so after treatment, so the net-net gain is worth it. Where possible Vascular Access Ports will be used to deliver chemotherapy (also commonly used in human treatment.  These are metal discs places beneath the skin with an exit tube directly into a blood vessel.  The disc can stay beneath the skin for extended periods of time if needed which allows animals to receive the drugs without the need of sedation, restraining or shaving.

Despite best efforts, dogs with appendicular OSA will almost certainly be lost to the disease, either from the primary tumour itself causing uncontrollable pain, or the metastases affecting the quality of life for the dog.  The most common site of spread is the lungs and advanced disease here most commonly causes weight loss and weakness, not breathing problems as one may expect.

Median survival times have been reported for the following therapies.

Amputation and Chemotherapy – 10-12 months

Limb spare and Chemotherapy – 10-12 months

Radiation and Chemotherapy – 8-10 months

Amputation alone – 4-5 months

Palliative care – 1-3 months.

 

Canine Degenerative Myelopathy (DM)

Canine Degenerative Myelopathy is characterised by a non-painful, gradual and progressive hind limb paralysis which usually occurs in older dogs.  DM was previously known as chronic degenerative radiculomyelopathy and is a progressive generative spinal cord disease.  Ultimately, the condition is fatal leaving devastating and difficult to deal with consequences for the dogs and owners who are caring for them.

DM is associated with genetic abnormalities in dogs and the most common form is due to a genetic mutation in the gene coding for superoxide dismutase, a protein that’s responsible for destroying free radicals in the body.  Free radicals are part of the natural defence mechanism but become harmful when they are produced in excessive quantities causing cellular death and a multitude of degenerative diseases.  The same gene mutation can be responsible for a form of motor neuron disease in humans.  Genetic testing for this abnormality is available through laboratories in order to identify dogs that are at risk of DM.  Usually these tests are done in a breeding prospect to ensure that the trait is not passed on to the offspring. Test results identify dogs that are clear (very unlikely to develop DM), those who are carriers (less likely to develop the disease) and those at risk of developing DM.  It is important to remember that this genetic test in no way confirms DM.  In addition, some dogs that might be destined to ultimately develop the disease do not so in their lifetime and pass away from other causes before DM presents itself.

In the past, it was regarded as a disease of the German Shepherd, but in recent years the disease has been identified in a multitude of other breeds and is no longer considered a ‘large breed’ issue. DM is particularly a concern in Boxers, Corgis, Bernese Mountain Dogs, Borzoi, Retrievers, Poodles, Rhodesian Ridgebacks and Shetland Sheepdogs.  As the disease develops after the recommended breeding age, breeders of predisposed breeds should always practice responsible breeding with genetic screening prior to breeding to reduce the prevalence of the gene and thus reducing the chances of DM affecting the line moving forwards.

Degenerative Myelopathy tends to affect dogs over five years of age, but typically no older than eight years of age.  The dog may begin to show early clinical symptoms such as a non-painful and subtle weakness of one hind limb. This is often misinterpreted as a chronic orthopaedic disease in the hip or stifle.  The clinical signs can be mild and the onset insidious, but signs progress over months leading to an ataxic walk (commonly known as a drunken sailor walk) in the hind limbs.  The dog may also drag their paws along the ground or cross their hind limbs when walking causing stumbling or falling over completely.  Similar clinical signs can also be seen in other spinal cord or neurological diseases so it is important to make sure that the dog has a comprehensive examination to make sure misdiagnosis is avoided.  During consultation, a thorough history should be obtained and a neurological examination should be performed, providing vital information to the clinician and enabling them to begin further investigations such as radiographs, MRI or blood tests.

DM always progresses to complete paralysis resulting in the dog being completely unable to support their own bodyweight.  The progression of the disease results in further weakness, muscle atrophy and even faecal and urinary incontinence.  The disease can progress up the length of the spinal cord and eventually affect the forelimbs as well as the rear end.

Diagnosis of DM is based on clinical signs, breed and age, supported by the absence of other clinical diseases.  It is a diagnosis of exclusion, meaning that other diseases with similar clinical signs must be excluded by the neurologist before a diagnosis is made.  A blood sample may be taken to rule out metabolic causes of spinal cord dysfunction, for example cobalamin deficiency and also for genetic testing for the DM associated genetic mutation.  It is likely that advanced diagnostic imaging will be recommended by the clinician and an MRI scan of the spinal cord will be performed.  A sample of cerebrospinal fluid, which surrounds the spinal cord, may also be obtained to allow analysis and exclusion of other disease processes. MRI and CSF analysis requires the dog to have a general anaesthetic so often, both these procedures will be performed at the same time.

Unfortunately, degenerative myelopathy is both irreversible and progressive, so the prognosis is poor as no specific treatments are available, although many dogs can be supported and maintain a reasonable quality of life for several months up to several years.  Progression can eventually result in the forelimbs and the hind limbs being affected although in some dogs this can take many years and the dog may die of another issue before the occurrence of the front legs being affected.  Many owners choose a palliative care pathway before opting for euthanasia when the dog is no longer able to walk using the hind legs, or faecal/urine incontinence occurs.

 

Brain Tumours

A brain tumour is a form of cancer affecting the covering of the brain tissue or the brain itself.  Unlike Osteosarcoma which can affect the skull, a brain tumour must affect the tissue itself rather than the bone surrounding it.  A pet can develop symptoms that relate to the location of the tumour and the function in the given area of the brain in that region.  The most common symptom occurring in dogs is epileptic seizures and usually occurs when the dog is over seven years of age.  Other signs can include various behavioural changes such as vacancy, loss of normal training, frequent disorientation, wobbliness and weakness and even blindness.  Although these signs can point towards a brain tumour they can also be symptoms of a multitude of other conditions.

Brain tumours are usually the result of age and time resulting in the unfortunate necessity for cells to divide.  It is also accepted that certain dog breeds may be more likely to develop brain tumours such as Boxers and Golden Retrievers.  There is no study correlating dietary influences or lifestyle to brain tumours, so it is generally accepted that the causal factors are genetic.

An MRI scan is the only option for radiologic diagnosis of a brain tumour.  A complete diagnosis further requires a piece of the tumour to be looked at under a microscope.  A good neurologic examination can be extremely helpful, when for a reason, this is not possible.  Dogs with brain tumours often have slowly progressive asymmetric neurological abnormalities.  (Abnormalities that affect one side of the body.)

Tumours can originate from the outer lining of the brain (the meninges), the inner lining, the blood vessels of the brain, the glands inside and beneath the brain, or the deep tissue of the brain itself.  Although there are a multitude of types of brain tumours, the most common in dogs is called a meningioma which is typically a benign tumour of the meninges, although malignant variants do also exist.  Tumours of the deeper brain tissues are also common and are grouped together in a term known as glioma.  Glioma tumours are typically malignant rather than benign.  Other rarer brain tumours include choroid plexus tumours, ependymal tumours, and embryologic tumours.  Aside from actual brain tumours, other cancers such as lymphoma or histiocytic sarcoma can be found in the brain.

Like all other cancers, brain tumours can be definitively treated with chemotherapy, radiotherapy and surgery.  Which of these treatments (if any) are appropriate depends entirely on the type of brain tumour.  Some tumours are very responsive to medication, such as white blood cell cancers.  Most tumours will cause local blood vessels to leak and the build up of fluid can cause issues so the use of medications such as corticosteroids are used to help relieve this build up of fluid. Surgery is possible in some instances, particularly if the tumour is growing on, or near the surface of the brain where it is readily accessible with minimal trauma and invasion to the rest of the brain.  Deeper brain tumours in dogs are not typically operated on as the negative aspects fare outweigh the long-term benefits to the patient.

Radiotherapy can be effective as surgery in some cases of brain tumours but not all.  Radiotherapy is particularly useful for tumours that cannot be accessed easily with surgery, or tumours that are especially large.  Pituitary gland tumours of a certain size are a good example of a tumour that can respond well to radiotherapy.

The prognosis is dependant on the type of tumour and location of the tumour.  Unfortunately, the overall prognosis for bran tumours in dogs is extremely poor and full recovery is fairly rare.  There are several good cases where one or more treatment method has cured the tumour completely or at least allowed a very prolonged remission period.

 

 

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