What is the most likely diagnosis for a patient with diplopia, unsteadiness, and absent tendon reflexes?

The most likely diagnosis for a patient presenting with diplopia, unsteadiness, and absent tendon reflexes is indeed Miller Fisher syndrome. This condition is characterized by a clinical triad of symptoms: ataxia (which relates to unsteadiness), ophthalmoplegia (leading to diplopia), and areflexia (reflected in the absent tendon reflexes).

Miller Fisher syndrome is considered a variant of Guillain-Barré syndrome and is often associated with a preceding upper respiratory infection, commonly caused by Campylobacter jejuni. The autoimmune response to the infection leads to the demyelination of peripheral nerves, resulting in the neurological symptoms observed.

The presence of diplopia and ataxia, combined with absent reflexes, strongly points toward this diagnosis. The absence of tendon reflexes is especially indicative because other conditions like brainstem stroke and multiple sclerosis may present with different neurological signs and typically do not isolate these three features. Similarly, Wernicke encephalopathy usually presents with a classic triad of symptoms, including confusion, ocular abnormalities, and ataxia, but it does not typically highlight absent reflexes in the same manner. Therefore, the specific combination of symptoms in this scenario aligns most closely with Miller Fisher syndrome.