Mitochondrial Myopathies (e.g., Kearns-Sayre Syndrome): This condition is characterized by a combination of peripheral neuropathy, sleep disturbances, ophthalmoplegia, dysphagia, and quadriparesis, which matches the patient's symptoms. The lack of response to plasma exchange or intravenous immunoglobulin also points towards a mitochondrial disorder, as these treatments are more commonly effective in autoimmune conditions.

Chronic Inflammatory Demyelinating Polyneuropathy (CIDP): Although the patient did not respond to plasma exchange or intravenous immunoglobulin, which are common treatments for CIDP, some cases can be resistant to these therapies. The presence of peripheral neuropathy and quadriparesis could still suggest CIDP.
Myasthenia Gravis with Thymoma: This condition can cause ophthalmoplegia, dysphagia, and muscle weakness. However, the lack of response to typical treatments and the presence of peripheral neuropathy might make this diagnosis less likely.

Botulism: Although rare, botulism can cause a similar constellation of symptoms, including ophthalmoplegia, dysphagia, and muscle weakness. It is crucial to consider this diagnosis due to its potential for rapid progression and the availability of specific treatments.
Diphtheria: Another infectious cause that, though uncommon in adults, can lead to neurological manifestations including neuropathy and muscle weakness. The diagnosis is critical due to the severity of the disease and the need for specific antitoxin treatment.

Pompe Disease: A genetic disorder that can cause muscle weakness and other systemic symptoms. While it's more commonly diagnosed in infancy, late-onset forms can present in adulthood with a variety of symptoms.
Tick-borne Neuropathies (e.g., Tick Paralysis): Certain tick-borne illnesses can cause rapid onset of neurological symptoms, including paralysis. This diagnosis would be considered rare but is important to rule out due to its potential for severe outcomes if not treated promptly.