Orthostatic headache: This condition is characterized by headaches that worsen with standing and improve or resolve with lying down. The most common cause is a cerebrospinal fluid (CSF) leak, which can be spontaneous or secondary to a procedure like a lumbar puncture. The orthostatic nature of the headache is due to the decrease in CSF pressure when standing, which leads to brain sagging and stretching of pain-sensitive structures.

Postural orthostatic tachycardia syndrome (POTS): While primarily known for causing tachycardia and other autonomic symptoms upon standing, some patients with POTS may experience headaches that improve with recumbency. The exact mechanism is not fully understood but may relate to autonomic dysfunction and changes in blood flow.
Low CSF pressure headache (not due to a known leak): Similar to orthostatic headache but without an identifiable cause of CSF leak. The symptoms are the same, with headache worsening with standing and improving with lying down.

Subarachnoid hemorrhage: Although the classic presentation is a sudden, severe headache (often described as "the worst headache of my life"), some cases might present with postural worsening due to blood in the subarachnoid space affecting CSF dynamics. Missing this diagnosis could be fatal.
Cervical artery dissection: This condition can cause a headache that might be postural due to the involvement of the cervical spine and its blood supply. It's a medical emergency because it can lead to stroke.

Chiari malformation: A structural defect in the cerebellum that controls balance can cause headaches that worsen with standing due to increased pressure on the cerebellum and brainstem when upright. However, this is less common and typically associated with other neurological symptoms.
Spontaneous intracranial hypotension: Similar to orthostatic headache but can be due to a variety of causes leading to low intracranial pressure. The presentation can be variable, including postural headache.