Jack Garnham

Brilliant piece of work by the venerable Dachling Pang on AARF and normal craniocervical junction physiology.

AARF is defined by pathological "stickiness" of the C1 - C2 relationship. It cannot be diagnosed on a single scan; in normal children, there can be substantial physiological rotation of C1 on C2. We need multi-position CT to evaluate the dynamic C1 - C2 relationship. Early diagnosis is crucial. In chronic, untreated AARF, conservative management is less likely to be successful; many patients will need prolonged traction, open reduction, or C1 – C2 (sometimes C0 – C2) fusion.

The normal motion curve (the second image) is key to understanding AARF. The C1 angle is essentially where the head is pointing. The C1 - C2 angle is the difference between the degrees of rotation of C1 and C2. Using these metrics, we can see that there are three broad "phases" of movement:

1⃣ At 0 - 25 degrees of rotation, C1 rotates on C2, while C2 remains stationary. The angle between C1 and C2 increases linearly.
2⃣ From 25 to 65 degrees of rotation, the increasingly taught ligaments pull C2 into rotation with C1. The C1 - C2 angle continues to increase but more slowly.
3⃣ Above 65 degrees of rotation, the C1 - C2 angle cannot increase any further and additional degrees of rotation are achieved by movement of the subaxial vertebrae.

When the head rotates back, and eventually to the opposite side, C1 should "cross" C2 at 0 degrees of rotation (i.e., in neutral positioning).

The third image shows type I AARF, the most severe form, in which C1 and C2 are "locked". The C1 - C2 angle does not change at all with rotation. When these children turn, you are seeing the skull spin on a stuck atlas, and the whole C1 – C2 unit twist on the rest of the spine.

Uraemic encephalopathy with the lentiform fork sign! Results from accumulated uraemic toxins and acid – base / electrolyte derangement in the context of renal failure. There are three general (and overlapping) MRI patterns:

1⃣ Basal ganglia (most “classic”).
2⃣ Cortical - subcortical (PRES-like, more parieto-occipital).
3⃣ White-matter (centrum semiovale).

Patient with end-stage renal failure and frequent non-attendance at dialysis presenting with confusion and seizures. Bilateral swollen, T2 hyperintense lentiform nuclei with a brighter rim tracing the internal and external capsules.

What is the sign and the diagnosis?

This is NMOSD!

⭐️ NMOSD is an autoimmune astrocytopathy. AQP4-IgG targets aquaporin-4 on astrocyte endfeet; hence, lesion distribution tracks AQP4 density.

⭐️ It is rare (0.5 – 10 / 100,000), with a marked female predominance (as high as 9:1). Peak incidence is in middle age, not the classic young-adult MS demographic. Coexistent autoimmunity (e.g., SLE, Sjögren's, myasthenia) is common.

⭐️ When presenting with optic neuritis, long-segment posterior disease involving the chiasm and tracts is typical.

⭐️ Radiology "sweet spots":

- Periependymal surfaces, hypothalamic-diencephalic lesions and involvement of the area postrema in particular.
- Corticospinal tracts.

⭐️ On your axial T2, look for dot-like foci of hyperintensity in areas of spinal cord involvement, so-called "bright spotty lesions".

⭐️ Some MS treatments (e.g., interferon-β, natalizumab, fingolimod) can worsen NMOSD, so differentiation is important. B-cell depletion and complement / IL-6 blockade are the NMOSD path, not interferon or natalizumab.

This is a 40 year-old female presenting with a severe, rapidly progressive myelopathy and intractable nausea, vomiting, and hiccups. On MR there's FLAIR hyperintensity in the area postrema and a longitudinally extensive cervical spinal cord lesion.

What's the diagnosis and required test? This disease also commonly presents with optic neuritis; what's the typical radiological pattern of optic pathway involvement?