Multiple System Atrophy (MSA): This is a neurodegenerative disorder characterized by progressive ataxia, autonomic dysfunction (such as urine retention), and parkinsonism. The absence of significant findings on imaging studies, combined with the clinical presentation, makes MSA a strong consideration.

Spinocerebellar Ataxia (SCA): A group of autosomal dominant inherited disorders that cause progressive ataxia. Some types of SCA can also present with autonomic dysfunction, which could explain the urine retention.
Normal Pressure Hydrocephalus (NPH): Although typically associated with gait disturbance, dementia, and urinary incontinence, some cases might present with ataxia and urinary retention. Negative imaging might not fully rule out NPH, as the diagnosis can be subtle.
Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): An autoimmune disorder that can cause progressive weakness and sensory loss, but in some cases, ataxia and autonomic dysfunction can be prominent features.

Spinal Cord Compression or Tumor: Even with negative initial imaging, the possibility of a spinal cord lesion causing these symptoms cannot be entirely ruled out without thorough investigation, including consideration of repeat or more detailed imaging (e.g., MRI of the spine).
Cauda Equina Syndrome: A condition where the nerves in the spinal canal are compressed, leading to neurological symptoms including urinary retention and potentially ataxia if the compression affects the lower spinal cord.
Vitamin B12 Deficiency: Can cause subacute combined degeneration of the spinal cord, leading to ataxia, weakness, and autonomic dysfunction. This is a treatable condition and thus critical not to miss.

Friedreich's Ataxia: An autosomal recessive inherited disease that causes progressive damage to the nervous system, resulting in ataxia. It typically presents at a younger age but could be considered in the differential diagnosis if the onset is late.
Ataxia-Telangiectasia: A rare, inherited disorder affecting the nervous system, immune system, and other bodily systems. It usually presents in childhood but could be a consideration in unusual cases.
Paraneoplastic Cerebellar Degeneration: A rare condition associated with cancer, where the immune system attacks the cerebellum, leading to ataxia. This diagnosis would be particularly important to consider if there are other signs of malignancy or if the patient has a known history of cancer.