I am posting this in hopes that someone out there may have had this happen to their own pet or have experience with these symptoms.
My 5 year old male chihuahua, Clancy is a healthy, playful little dog. He most recently began displaying a listing and falling over, which appears to be swaying to the left...falling and trouble jumping up the steps. He walks normal, then suddenly begins to lean, sway over. I took him to the vet where he was checked for reflexes, leg pulse, heart rate and observed by the vet as he walked. The vet only can image that it might be neurological, but did a complete blood panel. I do not have the results of this blood work, but Clancy doesn't appear to be in pain, eats normally, drinks and normal bowel and urine functions. I hope that my research will help us to turn over every stone with your input. My vet is not in his office on Thursday, which means we have to wait another day to revisit this problem. I won't even leave my house in fear something might happen to him. Please offer your thoughts on what could be wrong and to have checked out. THanks much!

[CorkyMax]

Quoted from my home vet book:

INTERNAL OTITIS (INNER EAR INFECTION)

Internal Otitis is an inflammation and infection of the inner ear. Most cases are preceded by outer ear infections. Suspect internal otitis if the dog suddenly develops signs of LABYRINTHITIS. TREATMENT: This is an emergency. Take your dog to the vet.

LABYRINTHITIS

Diseases of the inner ear are characterized by labyrinthitis, or dizziness. The
labyrinthitis is part of a complex organ of balance composed of the semicircular canals, the utricle and saccule. The labyrinthitis is like a gyroscope. It's purpose is to synchronize eye movements and maintain posture, balance and coordination. A major cause of labyrinthitis is inner ear infection.
---A dog with labyrinthitis will often assume an abnormal posture, with his head tilted down on the affected side. Dizziness, lack of coordination and loss of balance are evident.The dog circles and leans toward the affected side and may exhibit rapid jerking movements of the eyeballs, a condition called NYSTAGMUS. Vomiting occurs in some cases.
---The prolonged administration of aminoglycoside and neomycin antibiotics can produce labyrinthitis, as well as deafness. Most ear preparations are capable of causing labyrinthitis and ear damage if they make contact with the sensitive structures of the inner ear. This is why the ears should never be flushed or medicated without first making sure that the eardrums are intact.
---Other diseases that can cause labyrinthitis include head trauma, brain tumor, poisoning, drug intoxication and a condition called idiopathic vestibular syndrome. Suspect one of these if your dog develops labyrinthitis without having had a prior ear infection.
---IDIOPATHIC VESTIBULAR SYNDROMEis a disease of unknown cause that affects middle-aged and older dogs. It is the 2nd most common cause of labyrinthitis, after inner ear infection. The onset is sudden. The dizziness, staggering and vomiting can be incapacitating. Vomiting may last for several days, in which case the dog may require intravenious fluids. Signs peak in 24 hrs, but some degree of imbalance persists for 3-6 wks. Recovery occurs in all cases. After recovery, some dogs experience a slight but permanent head tilt.