Orthostatic Headache: This condition is characterized by headaches that worsen in upright positions and improve or resolve when lying down. It can be primary (without an identifiable cause) or secondary to other conditions such as dehydration, blood loss, or certain medications. The normal MRI suggests that there might not be a structural cause for the headache, making orthostatic headache a plausible explanation.

Spontaneous Intracranial Hypotension (SIH): Although an MRI is normal, SIH can sometimes present with normal imaging. It is characterized by low cerebrospinal fluid (CSF) pressure, often due to a spontaneous leak. Positional headaches are a hallmark, typically worsening with standing and improving with lying down.
Cervicogenic Headache: This type of headache is referred from the cervical spine and can be exacerbated by certain positions or movements of the neck. While an MRI of the brain might be normal, imaging of the cervical spine could reveal abnormalities contributing to the headache.
Migraine: Some migraine headaches can have a positional component, although this is less common. Migraines can present with a wide range of symptoms, and their triggers can sometimes include positional changes.

Subarachnoid Hemorrhage (SAH): Although an MRI is sensitive for detecting SAH, very early or small bleeds might not be visible. A lumbar puncture (LP) is crucial in this scenario if SAH is suspected, as it can detect blood in the CSF.
Pituitary Apoplexy: This is a medical emergency where a pituitary tumor undergoes sudden hemorrhage or infarction. It can present with a severe headache, often with a positional component, and visual disturbances. An MRI of the pituitary gland might be necessary for diagnosis.
Cerebral Venous Sinus Thrombosis: This condition involves a blood clot in the brain's venous sinuses and can present with headache. While an MRI might appear normal, specific sequences (like MR venography) are needed to diagnose this condition.

Chiari Malformation: This condition involves the cerebellum extending into the spinal canal, which can cause positional headaches among other symptoms. However, it is less likely if the MRI does not show any obvious structural abnormalities.
CSF Leak: Similar to SIH, a CSF leak can cause positional headaches due to low intracranial pressure. The leak might not always be visible on a standard MRI, requiring additional imaging or diagnostic procedures for detection.