A sunken or concave appearance on your dog’s head happens when the muscles on top of the skull lose mass, making the bone underneath more visible. The large chewing muscles that sit on either side of the head and across the top of the skull are the ones that typically waste away, creating hollowed-out areas above the eyes or along the temples. This can range from a normal part of aging to a sign of a serious immune or neurological condition that needs veterinary attention.
The Muscles That Shape Your Dog’s Head
Your dog’s head gets its rounded, full shape largely from two groups of chewing muscles: the temporalis muscles on top of the skull and the masseter muscles along the sides of the jaw. These are collectively called the masticatory muscles. When they’re healthy and well-developed, they fill in the space between the skin and the skull bone, giving the head a smooth, muscular profile.
When these muscles shrink, the skull’s bony ridges and contours become visible. You might notice indentations above the eyes, a ridge running along the top of the head that wasn’t there before, or a generally “skeletal” look to the face. The change can happen on both sides evenly or, in some cases, only on one side.
Masticatory Muscle Myositis (MMM)
The most well-known cause of a sunken head in dogs is masticatory muscle myositis, an autoimmune condition where the body’s immune system attacks the chewing muscles specifically. The masticatory muscles contain a unique type of muscle fiber (called type 2M) that isn’t found anywhere else in the body. In MMM, the immune system produces antibodies that target these fibers, creating inflammation that destroys the muscle tissue over time.
MMM typically progresses through two phases. In the early acute phase, the chewing muscles may actually swell and become painful. Your dog might have trouble opening their mouth, resist chewing hard food, cry out when yawning, or drop food while eating. Up to 44% of dogs with acute MMM develop bulging eyes because the swollen muscles behind the eye sockets push them forward.
If the acute phase goes unrecognized or untreated, it transitions into a chronic phase. This is when the sunken appearance becomes obvious. The inflamed muscle tissue is replaced by scar tissue (fibrosis), and the muscles visibly shrink. At this point, many dogs have permanently restricted jaw movement, sometimes barely able to open their mouths at all. This “locked jaw” effect makes eating difficult and can lead to weight loss and other complications.
The gold-standard diagnostic test is a blood test that detects antibodies against those type 2M muscle fibers. It’s a simple, non-invasive way to confirm the diagnosis. One important caveat: in very advanced cases where the immune system has already destroyed most of the 2M fibers, the test can come back negative because there’s nothing left for the antibodies to target.
Treatment involves suppressing the immune system with corticosteroids, typically over a course of several months with a gradual tapering schedule. Early treatment, before significant scar tissue forms, gives the best chance of preserving jaw function and muscle mass. Once fibrosis has set in, the muscle loss is largely permanent.
Trigeminal Nerve Problems
The trigeminal nerve controls the chewing muscles. When this nerve is damaged or stops functioning properly, the muscles it supplies waste away from disuse, similar to how a leg muscle shrinks when it’s in a cast for weeks.
In a study of 63 dogs with muscle wasting on only one side of the head, researchers found several underlying causes. About 48% had a nerve sheath tumor pressing on the trigeminal nerve. Another 21% had other types of masses affecting the nerve. In roughly 29% of cases, no specific cause was ever identified.
A benign form called pure trigeminal motor neuropathy causes the chewing muscles to weaken or become paralyzed without affecting sensation in the face or involving other nerves. Dogs with this condition can still feel their face normally but lose the ability to chew with full strength. The jaw may hang slightly open, and the muscles gradually shrink.
One-sided wasting is a particularly important clue. If the sunken appearance is noticeably worse on the left or right side of your dog’s head, nerve damage or a mass pressing on the nerve becomes a more likely explanation than a systemic condition like MMM, which tends to affect both sides.
Cushing’s Disease and Hormonal Causes
Cushing’s disease (hyperadrenocorticism) causes the body to produce too much cortisol, a stress hormone. Chronically elevated cortisol breaks down muscle tissue throughout the body, but the effect is often most visible on the head because the skull’s bony structure makes even small amounts of muscle loss obvious.
The muscle breakdown in Cushing’s is specific: cortisol preferentially destroys the same type of fast-twitch muscle fibers that make up a large portion of the chewing muscles. Over time, the lost muscle is replaced by fat tissue, and the remaining fibers shift toward a weaker, slower type. This is why dogs with Cushing’s often look weak and wasted even though they may be gaining weight elsewhere.
If your dog has a sunken head along with other signs like increased thirst and urination, a pot-bellied appearance, thinning skin, or hair loss, Cushing’s disease is worth investigating. Hypothyroidism (an underactive thyroid) can cause a similar pattern of selective muscle fiber loss, though it’s generally less dramatic.
Normal Aging
Not every sunken head signals disease. Like humans, dogs lose muscle mass as they age, a process called sarcopenia. Research on aging dogs has found that older animals show measurable muscle atrophy at the cellular level, with changes including fiber shrinkage and deterioration of the energy-producing structures inside muscle cells. The body’s recycling system for damaged cells ramps up with age, contributing to the gradual loss of muscle tissue.
Age-related muscle loss tends to happen slowly and symmetrically. You might notice your senior dog’s head looking bonier over months or years, along with a general loss of muscle along the spine, hips, and legs. The dog otherwise feels fine, eats normally, and can open their jaw without pain. This gradual, painless thinning in a dog over eight or nine years old is often just part of getting older, though it’s worth mentioning at your dog’s next checkup to rule out underlying conditions.
What to Look For at Home
A few observations can help you and your vet narrow down the cause:
- Speed of onset. Muscle loss that appears over days to weeks is more concerning than changes that develop over many months. Rapid wasting points toward an active disease process like MMM or nerve damage.
- One side vs. both sides. Symmetrical wasting suggests a systemic cause (autoimmune disease, hormonal imbalance, or aging). One-sided wasting raises suspicion for a nerve problem or mass on that side.
- Jaw function. Try gently opening your dog’s mouth. If they resist, cry out, or physically can’t open wide, that suggests MMM or another condition affecting the jaw muscles directly. A dog with age-related muscle loss will still open their mouth normally.
- Eating behavior. Dropping food, preferring soft food, chewing only on one side, or drooling more than usual all suggest the chewing muscles or the nerves controlling them aren’t working properly.
- Eye changes. Bulging eyes alongside a swollen or painful head point toward the acute phase of MMM. Sunken eyes alongside a wasted head can occur in chronic disease.
How Treatment Affects Recovery
Whether the sunken appearance can be reversed depends entirely on what’s causing it and how far it’s progressed. In MMM caught early, before scar tissue replaces the muscle, immunosuppressive treatment can halt the destruction and allow some muscle to rebuild. Dogs typically need months of medication with a slow taper, and relapses are possible if treatment is stopped too quickly.
For nerve-related causes, the outcome depends on whether the underlying problem can be addressed. In cases of benign trigeminal neuropathy with no identifiable mass, the condition may stabilize on its own, though the lost muscle doesn’t always return. When a tumor is involved, the prognosis depends on the tumor’s type and location.
Hormonal causes like Cushing’s disease respond well to treatment in many cases. Once cortisol levels are brought under control, the body stops breaking down muscle at an accelerated rate, and some dogs regain a degree of muscle mass over time. Age-related sarcopenia can be slowed with good nutrition and appropriate exercise, but some degree of muscle thinning in a senior dog is expected and not reversible.
The single most important factor across all these conditions is timing. Muscle that has been replaced by scar tissue or fat doesn’t regenerate. The sooner the cause is identified and addressed, the better the chance that your dog’s head can fill back out.