What causes fishmouthing of the vertebral bodies on lateral radiography of the spine?
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Answer:
Fishmouthing, or fish vertebra, describes vertebral bodies whose superior and inferior endplates are concave, so that the intervertebral disc spaces become biconvex and the paired endplates outline the shape of a fish mouth. Two mechanisms produce it and they look different. In generalised bone softening, osteoporosis, osteomalacia, hyperparathyroidism, renal osteodystrophy and thalassaemia, the turgid nucleus pulposus presses on weakened bone and the endplates deform in a smooth, gently curved arc that involves the whole endplate. In sickle cell disease the depression is different: it is a squared-off, step-like central depression with sharply angular margins, sparing the peripheral endplate, and this configuration is called the H-shaped or Lincoln log vertebra and is highly suggestive of a haemoglobinopathy. Recognising which pattern is present is the point of the sign, because the sickle pattern reflects focal microvascular occlusion rather than global softening. Supporting findings differ accordingly: coarsened trabeculae and marrow expansion with widened diploe in chronic haemolysis; a rugger jersey spine in renal osteodystrophy; and generalised cortical thinning with vertebral wedging in osteoporosis. Bone densitometry, biochemical screening for calcium, phosphate, alkaline phosphatase and parathyroid hormone, and haemoglobin electrophoresis are the relevant confirmatory tests, and MRI distinguishes acute vertebral infarction and marrow reconversion from tumour.
Why is it called so?
The opposed concave endplates of adjacent vertebrae outline a shape resembling the open mouth of a fish.
Pathophysiology
The vertebral endplate is supported by the underlying trabecular bone and loaded through the hydrostatic pressure of the nucleus pulposus. When trabecular bone is weakened diffusely, as in osteoporosis or osteomalacia, this pressure indents the endplate progressively and evenly, so the deformity is smooth and symmetric. In sickle cell disease the central portion of the endplate is supplied by end arteries that are especially vulnerable to sickling and sludging; occlusion causes localised infarction and growth arrest of that segment while the peripheral endplate, supplied by the metaphyseal equivalent circulation, continues to grow. The result is a sharply demarcated square central depression rather than a smooth curve, which is why the two patterns can be distinguished radiographically.
Alternative names: Fish vertebra; codfish vertebra; biconcave vertebra
Other associated named signs: H-shaped vertebra or Lincoln log vertebra of sickle cell disease, rugger jersey spine of renal osteodystrophy, and the hair-on-end skull of chronic haemolysis
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