Introduction to the PTA
The importance of the PTA
On emerging from coma, head-injured patients experience what is called post-traumatic amnesia (PTA). It represents a stage of recovery during which one's orientation and memory for ongoing events remains poor. Knowing whether a patient is experiencing PTA is important for 4 reasons:
(1) PATIENT CARE - the behaviours associated with PTA include agitation, irritability and restlessness, therefore measuring PTA monitors the patient's mental state during this acute recovery stage.
(2) REHABILITATION - as new learning ability is impaired during PTA, conventional forms of therapy which require the patient to retain information over time are not viable.
(3) NEUROPSYCHOLOGICAL TESTING - the neuropsychological testing of patients in PTA will have little validity.
(4) INDEX OF SEVERITY - the duration of PTA is used as an index of severity for prognosis, medico-legal and scientific purposes.
Development of the Westmead PTA Scale
Researchers at Westmead Hospital have extended the work of the Oxford group to produce the Westmead PTA Scale. It consists of 7 orientation questions and 5 memory items designed to objectively measure the period of post-traumatic amnesia (PTA).
The Westmead PTA Scale is a standardised and prospective measure of PTA and represents a move away from the subjective and retrospective estimates of the past.
A key feature of the Westmead PTA Scale is that it uses an operational definition of PTA. That is, a person is said to be out of PTA if they can achieve a perfect score on the Westmead PTA Scale for 3 consecutive days. PTA is judged to have ended on the first of the 3 consecutive days of perfect recall.
The work of W. Ritchie Russell (1903 - 1980) and colleagues is the basis of all PTA research. Their studies have shaped much of what we know about PTA today. Russell et al. used subjective and retrospective estimates of PTA based on the careful questioning of patients. In 1943, Symonds & Russell set out 2 warnings regarding this method. In a nutshell, they said beware of: (1) 'islands' of memory; and (2) the oriented amnesic. These warnings still apply today.
In their words:
'Care is necessary to avoid two sources of error. One arises from accepting the patient's first memory of his surroundings as the end-point, when it has in fact been followed by a further period of clouded consciousness and amnesia. Such 'islands' of memory are not uncommon and may be followed by further amnesia for a day or two; it is therefore the beginning of continuous memory which we prefer as our measurement. The second, though less common, error is to assume that because a patient is aware of what is happening around him he will be able to recall this later. This may lead to underestimation of the duration of post-traumatic amnesia in a patient who is under observation in the acute stage of his symptoms.' [p. 7]
The Westmead PTA Scale's operational definition was designed with these warnings in mind.