Multiple Sclerosis (MS): This is a chronic autoimmune disease that affects the central nervous system, leading to demyelination, inflammation, and axonal damage. The symptoms of MS can be highly variable but often include intermittent episodes of neurological dysfunction, such as weakness, numbness, and vision problems, which can progress over time. The progression from lower extremity symptoms to involvement of one arm could be indicative of a relapsing-remitting course of MS.

Transient Ischemic Attacks (TIAs): TIAs, or "mini-strokes," are temporary periods of symptoms similar to those of a stroke, usually lasting only a few minutes and don't cause permanent damage. They can occur in any part of the brain and might initially present with intermittent lower extremity weakness and numbness, potentially progressing to involve other areas such as an arm, depending on the cerebral vasculature involved.
Spinal Cord Lesions: Lesions in the spinal cord, whether due to trauma, infection, or tumors, can cause a variety of neurological symptoms, including weakness and numbness. The symptoms can start in the lower extremities and progress to involve the upper limbs if the lesion expands or if there is a secondary injury.
Peripheral Neuropathy: Certain types of peripheral neuropathy, especially those with a progressive course, can initially present with symptoms in the lower extremities and then spread to the upper limbs. Conditions like diabetic neuropathy or neuropathies associated with vitamin deficiencies can have this presentation.

Spinal Epidural Abscess: This is a medical emergency that requires prompt diagnosis and treatment. It can present with progressive neurological deficits, including weakness and numbness, starting in the lower extremities and potentially progressing to involve the upper limbs. The key to diagnosing this condition is recognizing the acute progression of symptoms and the presence of pain.
Cervical Spondylotic Myelopathy: This condition involves compression of the spinal cord in the cervical region due to degenerative changes. It can lead to a wide range of neurological symptoms, including weakness, numbness, and difficulty with coordination and balance. The progression of symptoms can be gradual, and it's crucial not to miss this diagnosis due to its potential for significant and irreversible neurological damage if left untreated.
Acute Disseminated Encephalomyelitis (ADEM): ADEM is a rare autoimmune disease characterized by a sudden, widespread attack of inflammation in the brain and spinal cord. It can present with a variety of neurological symptoms, including weakness, numbness, and vision problems, which can evolve rapidly over hours or days.

Neuromyelitis Optica Spectrum Disorder (NMOSD): This is an autoimmune disorder that predominantly affects the optic nerves and spinal cord, leading to symptoms like vision loss, weakness, and numbness. While less common than MS, it's a critical diagnosis to consider, especially if there are optic neuritis symptoms or a longitudinally extensive spinal cord lesion.
Subacute Combined Degeneration (Vitamin B12 Deficiency): This condition results from a deficiency in vitamin B12 and can lead to demyelination in the spinal cord, causing symptoms like weakness, numbness, and gait difficulties. It's a rare but treatable cause of neurological deterioration.
Intramedullary Spinal Cord Tumors: These are rare tumors that arise within the spinal cord itself. They can cause a variety of neurological symptoms, depending on their location and size, including progressive weakness, numbness, and loss of bladder and bowel control.