Motor neurone disease presenting with raised serum Troponin T

Introduction

Myocardial damage indicated by a rise in cardiac Troponin (cTn) may not necessarily be due to a cardiac event. Many diseases such as sepsis, pulmonary embolism, heart and renal failure can also be associated with an elevated cTn level. ¹ The European Society of Cardiology and the American College of Cardiology Committee, in 2000, redefined myocardial infarction (MI) as an elevation of cardiac Troponin T in conjunction with clinical evidence of myocardial ischaemia. Since this definition was presented, cTn has replaced the previously used creatine kinase-MB as the preferred biochemical marker for the diagnosis of MI. The inclusion of cTn in the diagnostic pathway was made due to its high sensitivity for detection of myocardial damage/necrosis.^2,3 Despite, however, its excellent specificity, there are increasing numbers of both non-ischaemic cardiac and non-cardiac conditions associated with elevated levels of cTn.^1,4,5 There are also published strategies on how to assist in distinguishing the differences between ischaemic and non-ischaemic cTn elevation. ⁶

This short communication discusses the rare event of a patient with motor neurone disease, where the possible diagnosis of acute MI arose due to an elevated cTn. There was little reference to motor neurone disease as a potential cause, and most lists of causes of raised Troponin concentrations in the literature do not include this possibility.

Case study

Our patient was a 69-year-old gentleman with a medical history including hypertension, ischaemic heart disease with a previous history of cardiac stenting done one year prior to admission and motor neurone disease. His medications at the time of review were as follows: ranitidine 150 mg, twice daily; atorvastatin 10 mg, once daily; aspirin 75 mg, once daily; clopidogrel 75 mg, once daily; candesartan 16 mg, once daily; isosorbide mononitrate modified-release 60 mg, twice daily; nebivolol 5 mg, once daily and furosemide 40 mg, once daily.

He presented to the accident and emergency department with a history of a central chest ache of sudden onset, commencing whilst asleep. This ache was described as being of mild intensity, lasting a total of 2 h prior to complete resolution. The pain seemingly resolved after a single dose of sublingual nitrate spray. He had no other presenting complaint.

According to hospital guidelines, a request for a 6-h serum Troponin T concentration was automatically generated and was found to be raised at 194 ng/l (serum Troponin T >27 ng/l is consistent with myocardial damage). An echocardiogram was conducted and, despite being a reportedly difficult assessment due to the motor neurone disease, did not show any acute changes of myocardial damage apart from a longstanding ‘odd wall motion’ which was put down to an old left bundle branch block.

The patient was admitted to the cardiac ward for further investigations. Subsequently, his electrocardiogram was normal and a further repeat Troponin T remained raised 36 h later (164 ng/l). The results were discussed with the on-call medical consultant, who noted reports in the literature describing raised concentrations of cardiac markers, including inflammatory muscular and motor neurone disease.^7–10 No further tests were conducted on this gentleman, and repeat cTn done two months post discharge were also found to be elevated in the same range as the in-patient results. This patient’s raised cTn was therefore explained on the basis of his active motor neurone disease.

Discussion

The above described patient suffering from motor neurone disease presented with raised concentrations of cardiac markers (cTn) in the absence of acute myocardial damage. The elevated cTn levels were taken to be caused by the motor neurone disease; however, the exact source of the raised cardiac markers cannot be confirmed; they could possibly indicate an element of myocardial involvement through a background inflammatory process related to the motor neurone disease.

A literature search does show other muscle diseases such as polymyositis to have baseline elevated Troponin T. Dhir and Jiang ¹¹ showed that other assays such as Troponin I may be a better marker for confirmation (or exclusion) of myocardial damage in these patients. Creatinine kinase (CK-MB), Troponin T (cTnT) and Troponin I (cTnI) have been reviewed with regard to their sensitivities and specificities. ¹² The overall finding is that the specificity of cTnI, as an assay, is documented to be slightly superior to those of cTnT in renal and muscular disease.

At that time, cTnT was the assay available in the hospital at which the patient was admitted, and decisions on treatment had to be made on the results available at the time. There is definitely further discussion to be had on submitting cTnI for similar patients in the future considering the improved specificity of this assay in patients presenting with a muscular disease that may have raised the baseline cTnT levels.

This rare case outlines the importance of considering motor neurone disease as a cause of elevated cTn in the absence of clinical evidence of an acute coronary event. This case underlines the need to recognise the shortfalls of cTn testing and the importance of not using cTn assays in isolation outside of the clinical decision-making process. Serum cTn levels must therefore be interpreted in conjunction with the patient’s clinical context and not in isolation. The issues with the use of cardiac markers in the management of patients with seemingly cardiac events highlight the role of involving the clinical biochemistry laboratory in the education of clinical colleagues regarding the appropriate use of cardiac markers and their interpretation.