To identify the condition associated with atrial myxoma, it's important to understand the clinical manifestations of this condition. Atrial myxoma is a type of primary heart tumor, often found in the atria, and has certain characteristic symptoms and signs due to its mechanical effect within the heart and systemic effects from cytokine production.

1. Fever and Weight Loss: These are constitutional symptoms often seen in patients with atrial myxomas. They occur due to the release of cytokines like interleukin-6 from the tumor, which can cause systemic inflammation and symptoms similar to chronic infection or malignancy.
2. Subungual Splinter Haemorrhages: These are small areas of bleeding underneath the fingernails or toenails and can be associated with systemic embolization from parts of the myxoma breaking off and traveling in the bloodstream. This is a known feature of atrial myxoma.
3. Systolic Murmur at Apex: This is generally not associated with atrial myxoma. Myxomas can cause obstruction of blood flow within the heart, potentially leading to a diastolic murmur, but a systolic murmur at the apex is typically more associated with mitral regurgitation or stenosis due to other causes.

Conclusion: Considering the information above, the feature not typically associated with atrial myxoma is "Systolic murmur at apex." This option is generally more indicative of valvular heart disease rather than a myxoma.