Multiple Sclerosis (MS): Given the history of decreased white matter, which is indicative of demyelination, and the sudden shock-like sensations in the head, known as Lhermitte's sign, MS is a highly plausible diagnosis. Lhermitte's sign is a classic symptom of MS, although it can occur in other conditions as well.

Vitamin B12 Deficiency: This condition can lead to demyelination in the spinal cord and brain, potentially causing symptoms similar to MS, including shock-like sensations. Vitamin B12 deficiency is known to cause subacute combined degeneration of the spinal cord, which might present with similar neurological symptoms.
Cervical Spondylosis: This condition involves wear and tear on the disks in the neck, which can lead to compression of the spinal cord or nerve roots, potentially causing shock-like sensations in the head or neck upon certain movements.
Chiari Malformation: A structural defect in the cerebellum that controls balance can cause a range of symptoms, including sudden shock-like sensations, especially with changes in posture.

Spinal Cord Infarction or Stroke: Although less common, a sudden vascular event affecting the spinal cord can present with acute neurological deficits, including shock-like sensations. Early recognition is crucial for potential intervention.
Meningitis or Encephalitis: Infections of the meninges or brain can present with a wide range of neurological symptoms, including sudden onset of shock-like sensations, and require urgent medical attention.
Subarachnoid Hemorrhage: A life-threatening condition that involves bleeding into the space surrounding the brain, which can cause sudden, severe headache and other neurological symptoms.

Adrenoleukodystrophy: A rare genetic disorder that affects the brain and spinal cord, leading to demyelination and potentially causing a variety of neurological symptoms, including vision loss, hearing loss, and muscle weakness.
Leukoencephalopathy with Vanishing White Matter: A rare genetic disorder characterized by progressive white matter rarefaction, leading to severe neurological deterioration. It's an extremely rare condition but should be considered in the differential diagnosis of progressive white matter diseases.
Neuromyelitis Optica Spectrum Disorder (NMOSD): An autoimmune condition that primarily affects the optic nerve and spinal cord, leading to symptoms like vision loss and spinal cord dysfunction. While not as common as MS, it's a critical diagnosis to consider due to its distinct treatment approach.