Transient non rate related left bundle branch block in thyrotoxicosis: A case report

Introduction

Thyrotoxicosis is a very common condition that is encountered in daily clinical practice. Thyrotoxicosis leads to multiple complications in multiple organs in the body, especially the central nervous and cardiovascular systems. In cardiovascular system complications, the most common manifestations are heart failure and cardiac arrhythmias. It is very important to recognize the cardiac arrhythmias related to thyrotoxicosis as it will help the attending clinician to manage the patient accordingly.

Case report

A 60-year-old lady with hypertension, diabetes mellitus, and hyperthyroidism presented with fever, shortness of breath, lethargy, palpitation, and heat intolerance for 1 day duration. Further history revealed she had contact with a patient with an upper respiratory tract infection. She was compliant to her medications. Upon clinical examination, blood pressure was 172/94mmHg, and pulse rate was 77 beats per minute and afebrile. There were fine tremors, proximal myopathy, and lid lag on peripheral examination. However, no exophthalmos and lid retraction were observed. Respiratory and clinical examination revealed normal findings. Blood investigation showed hemoglobin was 13 dl/g, total white cell 12.3 × 10⁹/L, and platelet 152 × 10⁹/L. Her renal and liver function test was in the normal range. ECG showed a sinus rhythm with a left bundle branch block, with a QRS duration of 140 msec (Figure 1). An urgent thyroid function test showed elevated free T4 44.67 pmol and suppressed TSH of <0.005 mL U/L. Her cardiac troponin showed 299 pg/mL dl, and proBNP was 2540 pg/dl.OPEN IN VIEWER

Our patient was diagnosed with impending thyroid storm with Burch Wartofsky Socre (BWS) of 25 in view of her clinical presentation consistent with thyrotoxicosis state with the background history of uncontrolled hyperthyroidism. We started her treatment as per thyroid storm protocol (Lugol iodine 10 drops three times daily, T. Carbimazole 25 mg once daily, intravenous hydrocortisone 100 mg three times daily, and T. Propanolol 20 mg twice per day) which she improved markedly. Interestingly our patient return to sinus rhythm and cardiac memory T wave after her thyroid status was improving (Figure 2). An echocardiogram was performed and revealed normal findings. After 1 week of receiving thyroid storm treatment, repeated showed free T4 25.89pmol with TSH < 0.005 mIU/L.OPEN IN VIEWER

After improving her thyroid function, her ECG returned to normal sinus rhythm with no Q wave. She was discharged from the hospital after 2 weeks. During follow-up in the outpatient clinic, she was asymptomatic and was able to taper down her carbimazole to the lowest dose.

Discussion

Thyrotoxicosis is a common condition associated with cardiac rhythm abnormalities, especially sinus tachycardia and atrial fibrillation.^1,2 It has been documented that sinus tachycardia occurs in 42%–73% of thyrotoxicosis cases, followed by atrial fibrillation. ¹ This phenomenon occurs due to an increased systolic and diastolic depolarisation rate with the shortened refractory period of myocytes. Hyperthyroidism negatively affects the cardiovascular system at the cellular and molecular level, such as upregulation of α-myosin heavy chain, prolonged activation of the sodium channel, and affecting the sinoatrial node, directly leading to increased preload and reduced preload. ³ The most common ECG abnormalities are atrial fibrillation, sinus tachycardia, short PR interval, prolonged intra-atrial conduction and intraventricular conduction delay lead to right bundle branch block. ⁴ Reversible left bundle branch block is relatively rare compared to other cardiac conduction abnormalities. Despite extensive studies regarding reversible left bundle branch block, the actual mechanism is unknown.

In our patient, in view of she had a history of hyperthyroidism and clinical examination suggestive of thyrotoxicosis status, she was diagnosed with impending thyroid storm despite her calculated BWS was 25. We started her as per thyroid storm protocol; fortunately, her condition improved. She responded well to carbimazole treatment. On top of that, as we get her to euthyroid state, her cardiac rhythm reverted to sinus rhythm with the presence of T wave inversion at the precordial lead.

Cardiac memory T wave is a phenomenon that results when the normal resumption of normal ventricular activation follows a period of abnormal ventricular activation, which is a transient left bundle branch block in our patient.^5,6 The underlying mechanism is the T-wave ‘remembers the vector of prior abnormal ventricular activation after restoring normal ventricular activation.^5,7 This phenomenon is commonly associated with tachycardia especially supraventricular tachycardia or rate related left bundle branch block.^5,6 Interestingly, in our patient, she developed cardiac memory T wave despite her heart rate was not tachycardic. Furthermore, T wave inversion is commonly associated with hypothyroidism instead of hyperthyroidism. Possible explanation of this event in our patient is due to myocardial stunning as a result from high thyroxine level.^8,9 It has been reported before about two cases of myocardial stunning in hyperthyroidism and toxic effect of thyroxine on the myocardium similar as catecholamine induced cardiomyopathy is the suggestive mechanism. ⁸

In rate-related LBBB, several mechanisms had been proposed, namely physiological block during phase 3 of the action potential, acceleration-dependent block, phase 4 bradycardia-dependent block due to disease of the His-Purkinje system or retrograde concealment in which retrograde penetration of bundle branch block. ¹⁰

Hence, a new onset LBBB is not necessarily due to cardiac ischemia, which leads to a difficult decision when the patient presents with an atypical symptom of cardiac ischemia. Hence, scores such as the Sgarbosaa criteria or Barcelona algorithm play an important role.^11,12 In our case illustration,the criteria for Sgarbossa were not fulfilled for acute myocardial infarction. Besides Sgarbossa criteria, echocardiography is another important modality in detecting regional wall motion abnormalities which commonly present in myocardial ischemia. Our case illustration highlights the important of stringent and careful assessment in patients with new onset of LBBB in order to differentiate between cardiac ischemia or other causes.

In our patient, it was well demonstrated that a combination of Sgarbossa criteria, echocardiography, and thyroid function test showed not all LBBB is due to cardiac ischemia. To our knowledge, this is the first case report demonstrating LBBB in an impending thyroid storm.