[2][3] José Mariano de Conceição Vellozo,[4] the botanist who first formally described the species, named it after the narrow neck (Latinized forms of Greek ἰσθμός, isthmós) connecting the two sections of its fruit.

Its hairless, membranous to slightly leathery, elliptical to egg-shaped leaves are 7–11.5 by 2.5–4 centimeters.

The midvein and secondary veins are sunken on the upper surfaces and raised on the undersides of the leaves.

The tertiary veins on the underside of the leaves form a networked pattern.

The branches of the inflorescences have thin, colorless, egg-shaped to triangular bracts that are fringed with long hairs.

The 5 cream-colored to orange petals are fused at their base to form a 1.5–1.8 by 1–1.5 millimeter funnel-shaped tube topped with lobes.

The petal lobes have strap-shaped appendages on their left margins that are 2–3 by 0.5–0.8 millimeters.