In human medicine, 80% of diagnoses are made via a clinical exam [http://jamanetwork.com/journals/jamainternalmedicine/fullarticle/1105870], which involves obtaining an in-depth description from the patient of their symptoms. If you are in pain, you will be asked to rate your pain on a scale of 0-10 or bases on a facial expression scale.
 
Dogs don’t have the ability to verbally give us this information so we often have to use our own observation skills and our knowledge of our dogs to work out if our dog could be in pain.  Veterinarians now have a way to apply the same type of pain scale from human healthcare to our pets.  Colorado State University has developed a Canine Acute Pain scale (there is also a Feline version) to help categorize and standardize pain assessment.
 
Pain is subjective and still difficult to measure.  It is not uncommon for two dogs to have the same injuries and while one dog will happily wag its tail and act like nothing is wrong, the other will cry in panic and pain.
 
Any medical or illness that causes pain, discomfort or decreased mobility, such as arthritis, dental disease, hypothyroidism, cancer, impaired sight or hearing, or Cushing’s disease, can lead to increased sensitivity and irritability. There can also be increased anxiety about being touched or approached, decreased responsiveness to commands, reduced ability to adapt to change and an increase in aggression; your dog may choose warn and bite rather than move away.
 
If your dogs’ behavior changes, they maybe communicating to you that they are in pain.

Reverse sneezing can be caused by a variety of irritants and dog allergies.  Dust, pollen, mites, viruses, post nasal drip, nasal inflammation, perfumes and cleaners or chemicals are some of the known causes.  Other triggers can include rapid drinking or eating, pulling on leashes and excitement. 

If the cause is allergy related, your veterinarian may recommend antihistamines.

Gently massaging your dogs throat may help stop the spasms.  Covering the nostrils is sometimes effective because it makes your dog swallow, which can clear out whatever is causing the irritation.

Hypothyroidism is a condition in which the thyroid gland produces less thyroid hormone than normal.  Obvious signs are increased body weight, lethargy and hair loss.
 
Somewhere between normal (euthyroid) and hypothyroid, are dogs whose thyroid levels are lower than necessary for optimal function but whose levels are still technically within normal range.  In this situation, only one or two obvious signs of hypothyroidism may be present and only subtlety.  This can contribute to anxiety and aggression.

If a dog is born with a neurological problem or develops one through injury or illness, its perception and judgment may be affected, causing inappropriate behavior.

Hydrocephalus is a congenital condition in which the fluid filled spaces in the brain (ventricles) become enlarged and the surrounding brain tissue subsequently becomes thinned or compressed. The term “water on the brain” has been used to describe this condition in humans.  The dog breeds most commonly affected are toys and brachycephalics (flat faced dogs).
 
There are few is any signs in mild cases.  However, more severe cases of Hydrocephalus are associated with a variety of neurological signs, sometimes including aggression.  The definite test for Hydrocephalus is a Computed Tomography (CT) scan or a Magnetic Resonance Imaging (MRI).

Encephalitis is the inflammation of the brain with several causes and can have a variety of neurological signs which may include aggression.

• Idiopathic (unknown)
• Immune-medicated disorders
• Postvaccinal complications
• Viral Infections (canine distemper, rabies, parvovirus)
• Bacterial Infections (anaerobic and aerobic)
• Fungal Infections (aspergillosis, histoplasmosis, blastomycosis)
• Parasitic Infections (Rocky Mountain Spotted Fever, Ehrilchisis)
  
• Foreign Bodies

 
Diagnosis is made by observation of clinical signs and through evaluation of the Cerebrospinal fluid (CSF) within the brain and spinal cord.

A tumor is an abnormal growth of cells and may be classified as primary or secondary.  A primary brain tumor originated from cells normally found within the brain and its surrounding membranes.  A secondary brain tumor, is either cancer that has spread to the brain (metastasis) from a primary tumor elsewhere in the body, or is a tumor that affects the brain by extending into brain tissue from an adjacent non-nervous system tissue, for instance bone or nasal cavity.
 
Brain tumors are diagnosed from clinical signs and by specific neurological testing, with or without ancillary diagnostic aids such as CT scans or MRIs.

Partial seizures occurring in a region of the brain that controls aggression (hypothalamus or limbic system) can result in sudden unprovoked aggression. Breeds known for this sudden aggression are Springer Spaniels, Cocker Spaniels, Chesapeake Bay Retrievers, Bull Terriers, Poodles and Golden Retrievers.  Clinical signs of seizure related aggression are:
 

• A mood change just before the seizure
• Sudden violent aggression for trivial or no reason
• Signs of autonomic discharge (salivation, dilated pupils, and evacuation of anal sacs)
• Aggressive posturing, more or less continuous during an attach lasting several minutes, hours or even days

 
Following an attack, affected dogs often appear depressed and lethargic, unresponsive to commands and may stare at a wall or simply sleep.  Diagnosis is made by observation of clinical signs, Electroencephalogram (EEG) or MRI.