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Comparison of the tendon and plantar strike methods of eliciting the ankle reflex
  1. C E Clarke,
  2. P Davies,
  3. T Wilson,
  4. T Nutbeam
  1. Department of Neurology, City Hospital, Birmingham, UK
  1. Correspondence to:
 Dr C E Clarke, Department of Neurology, City Hospital, Dudley Road, Birmingham B18 7QH, UK; 
 c.e.clarke{at}bham.ac.uk

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Little work has evaluated the various ways of eliciting the ankle reflex. A previous study of elderly patients with normal/absent reflexes found greater intraobserver and interobserver agreement with the plantar compared with the tendon strike method.1 Other studies showed that the reflex was best elicited in the kneeling position but moving comatose patients can be impossible or lengthy.2,3 We compared the reliability of the plantar and tendon strike methods of eliciting the ankle jerk in different disease states by examiners with different skill levels.

Four patients with pathologically brisk reflexes, five with reduced/absent reflexes, and nine subjects with normal ankle reflexes, as judged by an experienced neurologist, were recruited. All subjects had symmetrical signs and gave written informed consent. Subjects were screened from examiners so that only their legs were visible. None had identifying scars, wasting, or pes cavus. Subjects were examined by 15 third year medical students and five experienced neurologists. Initial training in both methods was given: in the tendon strike method the Achilles tendon of the supine patient was struck with the leg flexed at the knee and externally rotated at the hip; in the plantar technique the examiner’s hand was struck while placed on the plantar aspect of a supine patient’s foot. Reinforcement was permitted at examiners’ discretion. Each examiner saw half of the subjects (that is, nine patients and controls) on four occasions. On each occasion they examined both ankles. Examiners evaluated the reflexes four times using each technique twice. The order of bed, subject, and method allocation to examiners was according to a randomised partially balanced incomplete block design. Examiners rated the reflexes as normal, pathologically brisk, or reduced/absent and stated whether or not they were confident in their result. The study had local ethical approval.

Table 1A shows the sensitivities for the first of examiners’ encounters with each subject/method. Sensitivity was high for the reduced/absent category for both experience levels, but low for normal or increased reflexes. The tendon method for students on brisk reflex patients was particularly inaccurate compared with plantar. This was not true for clinicians. Despite low sensitivity, examiners sometimes declared confidence in their incorrect classifications: 81% declared confidence when incorrectly classifying a brisk reflex using plantar compared with 63% using the tendon method. Intraobserver agreement for reduced reflexes was reasonable but lower for normal or brisk (table 1B). These were not always small misclassification errors: 23% of clinicians’ test pairs using tendon strike classified an increased reflex correctly once but as absent on the other test. In several instances the reflex was incorrectly classified on both tests. All 13 students who declared a preference preferred the plantar strike but no clinician stated a preference.

We found poor sensitivity and reproducibility for both techniques with normal and brisk reflexes for both examiner types. Results for reduced/absent reflexes may be inflated as most patients had absent reflexes which are easier to detect. Also they may have had other subtle lower motor neurone signs giving clues to examiners. The low sensitivities show that the ankle reflex should be interpreted in the light of other physical signs. Experienced clinicians had similar results with both techniques. This conflicts with previous findings in elderly patients with normal or reduced reflexes of better interobserver and intraobserver agreement with the plantar method.1 The disparity may reflect this study including patients with brisk reflexes or that the clinicians were neurologists, or both. Medical students did better with the plantar method for brisk reflex patients. They have insufficient experience to differentiate normal from brisk with the tendon method. This suggests that students should be taught the plantar method in preference to the tendon strike method.

Table 1

(A) Sensitivity of tendon and plantar strike methods including κ coefficient of interobserver agreement. (B) Intraobserver agreement between two tests of each method shown as the percentage (95% CI) of times that identical results were obtained. The κ coefficient (standard error) shows response agreement for the two test sessions for each method

References

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Footnotes

  • Competing interests: none declared.

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