The most likely diagnosis for this patient is chronic traumatic encephalopathy. Chronic traumatic encephalopathy is a progressive neurodegenerative disorder triggered by repetitive mild head injury that has most often been described in military combat veterans and athletes with a history of multiple concussions and subconcussions. This patient's career as a professional football player would have made him particularly susceptible to this type of injury. Clinical symptoms typically manifest years or decades after repeated head trauma and present insidiously. Behavioral symptoms are common and include depression, suicidal ideation, apathy, and irritability. Disinhibition, impulsivity, and aggression can also occur in the later stages. Cognitive symptoms include problems with memory, attention, concentration, and executive function. With disease progression, poor judgment and poor insight become more prominent. Parkinsonism, disturbance of gait, and speech abnormalities often occur later in the disease course.

Although this patient exhibits mild parkinsonism on clinical examination, he lacks other features to support a diagnosis of dementia with Lewy bodies, such as fluctuating cognition, visual hallucinations, rapid eye movement sleep behavior disorder, and autonomic dysfunction. In addition, his age at symptom onset is unusual for dementia with Lewy bodies, which typically presents in the sixth decade of life or later.

Depression-related cognitive impairment refers to the cognitive deficits associated with depression and most often is characterized by frontal-subcortical dysfunction and slowed processing speed. Cognitive symptoms improve with treatment of depression. The patient has depression accompanied by suicidal ideation, but his history and constellation of additional neurologic and behavioral symptoms are more suggestive of chronic traumatic encephalopathy.

Parkinsonism tends to occur in chronic traumatic encephalopathy during the later stages of the disease. Comorbid neurodegenerative disease, such as Alzheimer disease, Lewy body disease, and Parkinson disease, can be seen in a percentage of patients with chronic traumatic encephalopathy at autopsy, and some evidence suggests that repetitive head injury is associated with increased risk of these disorders. This patient has slow and shuffling gait and bradykinesia but lacks the cardinal signs of idiopathic Parkinson disease, including resting tremor or rigidity, a unilateral onset, and asymmetric parkinsonism. The most likely diagnosis for his clinical syndrome of cognitive, behavioral, and motor decline is chronic traumatic encephalopathy.