Multiple System Atrophy (MSA): This is a progressive neurodegenerative disorder characterized by a combination of symptoms including ataxia, dysarthria, and autonomic dysfunction. The absence of significant findings on imaging studies does not rule out MSA, as it often presents with non-specific or minimal changes on MRI, especially in the early stages.

Spinocerebellar Ataxia (SCA): A group of autosomal dominant inherited disorders that cause progressive ataxia. While imaging might be negative, genetic testing can confirm the diagnosis. The age of onset can vary, and some forms may present later in life.
Idiopathic Late-Onset Cerebellar Ataxia: A condition of progressive cerebellar ataxia without a clear cause, often diagnosed after excluding other known causes of ataxia. It's a diagnosis of exclusion but should be considered in the absence of other findings.
Alcohol-Related Cerebellar Degeneration: If the patient has a history of significant alcohol use, this could be a contributing factor to his symptoms, even if imaging does not show the typical cerebellar atrophy.

Vitamin B12 Deficiency: Although less common, a deficiency in vitamin B12 can cause subacute combined degeneration of the spinal cord, leading to ataxia among other symptoms. This is easily treatable and thus crucial not to miss.
Paraneoplastic Cerebellar Degeneration: An autoimmune response to a cancer (often occult at the time of neurological presentation) can cause subacute cerebellar degeneration. Identifying and treating the underlying cancer can improve neurological symptoms.
Creutzfeldt-Jakob Disease: A rare, fatal brain disorder caused by an abnormal form of a protein called a prion. It can present with ataxia and dysarthria, among other symptoms, and progresses rapidly.

Ataxia-Telangiectasia: A rare, inherited disorder affecting the nervous system, immune system, and other bodily systems. It typically presents in childhood but could be considered in the differential if there's a family history or other suggestive features.
Ataxia with Vitamin E Deficiency (AVED): A rare inherited disorder that leads to progressive ataxia due to a deficiency in vitamin E. It's similar to Friedreich's ataxia but responds to vitamin E supplementation.
Friedreich's Ataxia: An autosomal recessive inherited disease that causes progressive damage to the nervous system. It typically starts in childhood or adolescence but late-onset forms can occur.