Is there a difference between terminal lucidity and paradoxical lucidity?

In his recent letter to the Journal, Nahm proposes a conceptual distinction between terminal lucidity and paradoxical lucidity [1]. Terminal lucidity, he asserts, “refers to any kind of unusually enhanced mental clarity before death,” whereas paradoxical lucidity is mental clarity that occurs in persons with neurodegenerative diseases, such as in Alzheimer’s disease. “Terminal,” Nahm claims, “is a time- and process-related attribute that […] says nothing about the potential paradoxicalness of a given lucid episode […]. The designation ‘paradoxical’, however, is a qualitative and state-related attribute that… [implies] a pathophysiologic, neurodegenerative brain condition.”

This distinction contrasts with our assertion made in the Journal that all instances of terminal lucidity are paradoxical, but not all instances of paradoxical lucidity are also instances of terminal lucidity [2]. Nahm believes we are incorrect. Why? Because “terminal lucidity doesn’t always involve severe neurodegenerative conditions that would render its occurrence paradoxical.” Many conditions—neurological and non-neurological—grant a person entry into the house of the dead. Terminal lucidity, Nahm suggests, can account for all such cases, even in otherwise healthy individuals, since it is not attached to a specific brain condition.

We welcome this dialogue. Scrutinizing the definitions of paradoxical lucidity and terminal lucidity improves the science and may enhance clinical care down the road. Yet we remain skeptical of Nahm’s argument. Below, we engage his assertions with particular attention to the evidence he presents and to the concept of terminal lucidity.

Nahm claims that, in the course of his own work, he has observed cases of terminal lucidity in persons without severe neurodegeneration or other brain conditions. This would support his assertion that terminal lucidity is condition agnostic. Yet this doesn’t appear to be borne out by the evidence. Of the studies Nahm cites—all his own—terminal lucidity is described in persons with underlying brain conditions, including persons with “chronic schizophrenia and dementia” [3], “mental illness or a mental disability” [4,5], and those “suffering from brain abscesses, tumors, strokes, meningitis, dementia or Alzheimer’s disease, schizophrenia, and affective disorders” [6,7]. In two of Nahm’s most recent cited articles, terminal lucidity is even defined as, “the unexpected return of mental clarity and memory shortly before the death of patients suffering from severe psychiatric and neurologic disorders” [5,6; emphasis added].

While Nahm’s case collections are not exclusively of persons with severe neurodegeneration, they all appear to examine lucidity in those with underlying brain conditions. Moreover, Nahm regards terminal lucidity as “unexpected” [5] or “unusual” [1] seemingly for the same reason he regards paradoxical lucidity as paradoxical: because the episodes defy the pathophysiologic situation. Thus, Nahm’s own studies and assertions appear to be at odds with his argument. His work supports—rather than contradicts—our claim that terminal lucidity is a special sub-type of paradoxical lucidity that occurs near death.

Evidence notwithstanding, let us suppose that Nahm is correct that terminal lucidity should be regarded as distinct from paradoxical lucidity. To support this claim, the concept of terminal lucidity, defined as “any kind of unusually enhanced mental clarity before death” [1], must be sufficiently clear for it to be distinguished from other types of lucidity. Has Nahm accomplished this conceptual work?

First, consider how scientists should interpret “before death.” In previous work, Nahm provides greater clarity by specifying this as “shortly” before death [5]. But what is the correct temporal threshold of shortly before death? An hour? A day? A week? A puzzle arises when we consider that classification of terminal lucidity occurs retrospectively, after a person dies. This could leave classification open to a subjective line-drawing exercise as to when a person entered the dying process.

Second, terminal lucidity seems to imply that a lucid episode is causally related to death, not just temporally related. This, however, doesn’t seem to be captured by Nahm’s definition. Suppose a person with a long-term mental illness displays a moment of “unusually enhanced mental clarity,” but moments later is hit by a bus and dies. It doesn’t seem like this should count as an instance of terminal lucidity. But, as a case of unusual mental clarity occurring shortly before death, it falls under Nahm’s definition. What is needed is a causal story relating the dying process to the lucid moment. This would exclude cases that are temporally related but not causally related. Such a theory, however, is currently underdetermined.

We agree with Nahm that some—if not many—episodes of lucidity occur within days or weeks of death. His pathbreaking work demonstrates as much. But these facts alone do not distinguish the concept of terminal lucidity from paradoxical lucidity. Without further evidence and conceptual work to substantiate this point, our original assertion that terminal lucidity is a type of paradoxical lucidity still stands [2].