craaaazy b12 deficiency has characteristic spinal lesions! they even happen in specific places on the spine! cervical (which can give you the good old lhermitte's sign) and dorsal/thoracic!!!! wow some doctors make my head spin.
mind you 400 is a pretty solid number, though 500 or 600 would be better. what units were your results in? nmol/l or pg/ml? anyway, it can't do any harm to take b12 for a few months to see if it does you some good. it needs the complex too though - b vitamins don't exist in isolation in foods, from what i understand.
http://neuromuscular.wustl.edu/nother/vitamin.htm#B12
Vitamin B12 (cobalamin) deficiency
-Clinical features
--Polyneuropathy
---Sensory change: 2°
spinal or peripheral nerve lesions
-----Early: Paresthesias
-----Loss especially of large fiber modalities
-----Distal
---Motor: Later in course; Distal
---Reflexes
-----Tendon: Reduced or absent at ankles
-----Plantar: Upgoing
---Autonomic: Postural hypotension
--CNS
---
Spinal cord: Earliest locus of involvement
-----Major cause of sensory & motor disability
-----Posterior column fiber loss
-----Spasticity in legs
-----Similar clinical features to N2O toxicity
---Other CNS
-----Cognitive impairment in adults: Leukoencephalopathy on MRI
-----Mental retardation or encephalopathy in childhood syndromes
-----Sensory: Reduced smell & taste
-----Gait ataxia
---Anemia: Megaloblastic; Due to reduced DNA synthesis
---Gastrointestinal: Glossitis; Diarrhea
---Fingernails: Hyperpigmented
-Testing
--Serum
---Low B12
-----Clinically significant: < 100 pg/ml
-----Suspicious: < 200 pg/ml
---High homocysteine & methymalonic acid
-----Confirm biological significance of low B12 levels
--MRI
---
Hyperintense T2 lesions in posterior columns (50%)
---Lesions resolve after 8 to 12 months of therapy
--Evoked potentials
---Somatosensory: Abnormal tibial & median
---Motor: Normal in most
-Pathology
--
Spinal cord
---Multifocal axonal loss & demyelination
---Localization
-----
Cervical & thoracic
-----Posterior column > Anterolateral & Anterior
--Peripheral nerve: Axonal loss; Occasional ± demyelination