Ask a Vet: Is It Normal for Old Dogs to Develop a Bony Head?

Question: 

“Why is my old dog’s head bony all of a sudden? It looks like his skull is caving in! I can touch his head without causing pain and he seems fine in every other way. Is it normal for a senior dog’s head to cave in like that?”

Answer:

Although it appears relatively hard, a dog’s head has a lot more to it than just bone. 

When you pet your dog’s head, you’ll notice that there is a soft layer of tissue over the bones that form the top of the skull. The top of a dog’s skull is covered by the large temporalis muscles. The masseter muscles are the strong muscles overlying a dog’s jaw.

These muscles are actually quite a bit thicker than you might think. When they shrink even a little bit, you’ll start to notice the dog’s head feels and looks bony. 

There are two main causes of a bony head appearance in dogs. First is the interruption of nerve signals to the muscles due to infection, trauma, inflammation, or cancer. Second is the destruction of muscle cells by the muscles may atrophy autoimmune diseases.

Why Your Old Dog Has a Bony Head Appearance

Under these muscles lies the top of a dog’s skull. There are several bumps and ridges that aren’t normally very noticeable because they are covered with those big skull muscles. 

There are several knobs and ridges you can feel when a dog has muscle loss over his skull: 

  • External occipital protuberance: the pointy knob of bone poking out the back of a dog’s skull
  • Sagittal crest: the ridge running down the center top of the skull
  • Nuchal crest: a ridge of bone on either side of the external occipital protuberance
  • Zygomatic arch: ridge of bone under the eye, cheekbone. 
Dog skull anatomy (old dog bony head)
The prominent ridges of a dog’s skull.

Causes of Dog’s Skull Changing Shape & Losing Muscle Mass

There is one set of diseases a veterinarian will consider when a dog’s skull appears to be “caved in” or sunken on both sides. There is another set of diseases veterinarians consider if only one side of a dog’s skull musculature is affected. 

First, I’ll cover common and uncommon causes of bilateral (both sides) skull muscle atrophy in dogs. 

Myositis

Myositis is the medical term used to describe muscle inflammation. The causes include infection, immune-mediated disease or cancer. Here are a few types of myositis that can cause the appearance of a sunken skull in dogs… 

Masticatory Myositis

Masticatory muscles are involved in chewing. Some of the biggest masticatory muscles are located on the top and sides of the skull. Masticatory muscle myositis (MMM) is the most common type of muscle inflammation in dogs. 

A dog’s chewing muscles are made of a special type of muscle fiber that comes under attack from their own immune system in MMM. The trigger of the auto-immune attack is often never discovered. This disease occurs more frequently in some dog breeds including the Labrador Retriever, German Shepherd and Golden Retriever. 

Symptoms of MMM include jaw pain, fever, eye discharge, difficulty opening the mouth, trouble eating, and a bony appearance to the sides and top of the skull. The dog’s head often appears sunken in behind the eye due to the shrinkage of the masseter and temporalis muscles. 

Polymyositis

Polymyositis is a condition in which more than one group of skeletal muscles is inflamed. The cause can be immune-mediated, infection or cancer. Some of the top infectious causes are tick-borne Rickettsial diseases, Leptospirosis and Toxoplasmosis. 

Symptoms of this type of myositis include weakness, lameness, muscle wasting (which can affect head muscles), pain, depression and lethargy. 

The immune-mediated type of polymyositis may be inherited in certain breeds of dogs including Vizslas, Newfoundlands and Boxer dogs. 

Dermatomyositis

Dermatomyositis is an uncommon inflammatory disease that affects a dog’s skin and muscles. Symptoms include skin sores and atrophy of the masticatory muscles involved in chewing. These dogs commonly have a skull that appears bony, amongst other physical changes. 

The cause of this disease is unknown but may be caused by immune-mediated disease. It seems to run in certain dog breeds, especially the Collie, Shetland Sheepdog, Belgian Tervuren, Portuguese Waterdog and German Shepherd. 

Trigeminal Neuritis

Trigeminal neuritis is caused by sudden paralysis of Cranial Nerve V, the trigeminal nerve. Symptoms include an inability to close the jaw, difficulty eating/drinking, drooling and skull/facial muscle atrophy. 

There is no one definitive cause of trigeminal neuritis in dogs. Some believe it is caused by trauma or exposure to viral antigens. The Golden Retriever is overrepresented amongst dogs affected by the disease. (1) 

Anatomical drawing of dog's head muscles.
A cross-section diagram of a dog’s head muscles.

Too Much Steroid in the Body

In this case, steroid refers specifically to corticosteroids. These are hormones naturally produced in a dog’s adrenal gland to regulate many metabolic functions. We also have synthetic corticosteroid medications prescribed to treat inflammation and immune-mediated disease. 

One of the possible side effects of steroid medication in dogs is muscle wasting. Some dogs are very sensitive to steroid treatment and develop marked symptoms including weakness and shrinkage of their muscles. You’ll usually notice this the most over the top part of a dog’s body–the skull, spine and pelvis become more prominent and the muscles over them shrink. 

Cushing’s disease (hyperadrenocorticism) also causes steroid excess in a dog’s body. This condition produces way too much cortisol due to a tumor in the pituitary gland or on the adrenal gland. The symptoms are the same as if the dog was taking synthetic steroids: weight gain, muscle wasting, increased thirst and appetite, pot-belly and panting. 

It’s not unusual for a dog taking steroid medicines like prednisone or a dog with Cushing’s disease to have a bony head. 

Cancer-Related Muscle Wasting

Cancer puts a dog’s metabolism into overdrive (cachexia) by using up many more calories, vitamins and minerals than a healthy body. Owners of dogs with cancer notice what vets call “dorsal muscle wasting.” The shrunken muscle mass makes it easy to feel a dog’s bones and is especially noticeable over the spine, pelvis and skull. 

Coonhound Paralysis

Similar to Guillain-Barré Syndrome in humans, “Coonhound paralysis” affects the ventral nerve roots of the spinal cord. The medical term for this condition is polyradiculoneuritis.

Symptoms include muscle atrophy, weakness, trouble swallowing and even paralysis. The cause cannot be identified in many cases. The good news is that most dogs start to recover with a few weeks with good medical and nursing care. 

Old Age Muscle Loss

Elderly dogs may be affected by generalized muscle wasting, also called sarcopenia. The condition is common in humans and there is a lot of research on it. Changes in activity level, hormones and changes in protein metabolism are believed to be the cause for age-related muscle changes.

If Your Dog’s Head Looks Like It’s Caving in On One Side

Dogs with the appearance of their skull caving in on only one side look pretty bizarre. Following is a list of some of the diseases that can cause unilateral (one-sided) head muscle wasting in dogs. 

Horner Syndrome

Horner Syndrome is a group of symptoms that occur when certain nerves supplying the eye are compromised. It can be caused by tumors, infections, or trauma and sometimes the cause is never found.

Horner Syndrome usually affects only one side of the head. It can be caused by tumors or inflammatory conditions anywhere between the side of the face to the base of the chest. 

Symptoms of Horner Syndrome in dogs include consistently small pupil, sunken eye and raised third eyelid on the affected side. The muscles of the face and skull often appear sunken. 

Trigeminal Nerve Sheath Tumor

Tumors that grow in or around the head can affect the muscles and nerves on the outside of the skull. Trigeminal nerve sheath tumor is the most common tumor to cause one-sided skull muscle atrophy in dogs. Your vet will also want to look for other tumors inside the ear, skull and neck. 

But any tumor can affect the many nerves that travel from the brain to the muscles of the face. If it compromises a significant portion of the nerve tissue, the muscle will become flaccid and atrophied. 

Diagnosing the Cause of Dog Head Shape Changes

It’s not always possible to identify the cause of a dog’s head deformity. Your veterinarian will start the investigation with a thorough physical exam, including looking deep in the ear canals. Radiographs/x-rays may identify tumors or trauma in the head/neck/chest area. 

Special imaging like MRI or CT is often needed to find soft tissue tumors, fluid or trauma because these don’t show up very well on plain x-rays. Sometimes nerve and muscle biopsies are necessary to identify cancerous, immune and infectious diseases. 

Is Head Muscle Atrophy in Dogs Treatable?

The muscle may expand back to normal size if you can treat whatever is making your dog’s head look like it is collapsing. In practice, even after the underlying disease is under control, head and facial muscle atrophy usually persist.

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References

  1. Mayhew, P. D., Bush, W. W., & Glass, E. N. (2002). Trigeminal neuropathy in dogs: a retrospective study of 29 cases (1991–2000). Journal of the American Animal Hospital Association, 38(3), 262-270.
  2. Melmed, C., Shelton, G. D., Bergman, R., & Barton, C. (2004). Masticatory muscle myositis: pathogenesis, diagnosis, and treatment. COMPENDIUM ON CONTINUING EDUCATION FOR THE PRACTISING VETERINARIAN-NORTH AMERICAN EDITION-, 26(8), 590-605.