case files neurology
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case files neurology


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because of unsteadiness. On
examination the patient appeared older than his stated age. His hair was nearly
completely gray. There was slight limitation of head movement to either side,
but no pain with neck extension. His tongue was red and depilated. His gait was
broad based, and he was unable to walk a straight line. He was able to stand with
his feet together with his eyes open, but he nearly fell when his eyes were
closed. He had normal arm coordination but was ataxic on the heel-knee-shin
maneuver. Deep tendon reflexes (DTRs) were 3+ in the arms, trace at the
knees, and absent at the ankles. Both plantar responses were extensor. There
was a 2+ jaw jerk and a positive snout reflex. There was a stocking decrease
in sensation and a marked decrease in vibration and joint position sense in the
toes and ankles. Cranial nerves were normal, and there were mild problems
with memory and calculation. T2-weighted MRI of the brain demonstrated
extensive areas of high-intensity signal in the periventricular white matter.
MRI of the spine showed a hyperintense signal along the posterior column of
the spinal cord.
\u25c6 What is the most likely diagnosis?
\u25c6 What is the next diagnostic step?
\u25c6 What is the next step in therapy?
ANSWERS TO CASE 22: Subacute Combined
Degeneration of Spinal Cord
Summary: This is a 48-year-old patient with a progressive gait disorder char-
acterized by sensory ataxia caused by impaired position sense and spasticity.
His examination is significant for both peripheral and central nervous system
involvement, primarily affecting the white matter fibers of the posterior
columns of the spinal columns and pyramidal tracts and large myelinated
peripheral nerve affecting coordination and muscle tone.
\u25c6 Most likely diagnosis: Vitamin B12 deficiency
\u25c6 Next diagnostic step: Vitamin B12 level and if positive, subsequent
testing to determine the source of B12 malabsorption
\u25c6 Next step in therapy: Intramuscular vitamin B12
Analysis
Objectives
1. Understand the range of pathologic and clinical manifestations of
vitamin B12 deficiency.
2. Know the differential diagnosis of vitamin B12 deficiency.
3. Understand the types of tests to confirm the diagnosis and etiology of
vitamin B12 deficiency.
4. Be aware of the proper mode of repletion of vitamin B12.
Considerations
The pertinent features of this case include the presentation, unsteadiness of gait,
and numbness and stiffness. The physical examination helps localize the
pathology. There was a stocking decrease in sensation, specifically vibration
and joint position sense, which strongly suggests a neuropathy involving myeli-
nated fibers. Involvement of the dorsal columns of the spinal cord, at or above
the lumbar level is also a possibility. The pathologically increased reflexes in
the arms along with the presence of primitive reflexes are \u201cupper motor neuron
signs\u201d and suggest involvement of the corticospinal tract above the level of the
cervical spinal cord. In this case, one would expect increased reflexes in the
legs also, unless there is also a coexistent neuropathy. The ataxic heel knee to
shin maneuver, also points to aberrant input to the cerebellum, which comes
through large fibers. The mild problems on mental status examination indicate
a cortical disorder. All of these findings suggest involvement at multiple levels
of the nervous system. The imaging study confirms involvement of myelinated
regions in the spinal cord, specifically the dorsal columns and in the brain.
190 CASE FILES: NEUROLOGY
Assuming all these signs/symptoms are manifestations of a single entity, a sys-
temic disease should be considered, such as HIV-1 associated vacuolar
myelopathy, Lyme disease, multiple sclerosis, neurosyphilis, or vitamin B12
deficiency. Neuropathic conditions would not be expected to give upper motor
neuron signs. Another clue on physical diagnosis is the abnormal tongue and
prematurely graying hair.
APPROACH TO SUBACUTE COMBINED
DEGENERATION OF THE SPINAL CORD
Spinal cord diseases are common, and many are treatable if discovered early.
The spinal cord is a tubular structure originating from the medulla of the brain
and extending through the bony spine to the coccyx. Ascending sensory and
descending motor white matter tracts are located peripherally; posterior
columns govern joint position, vibration and pressure, lateral spinothalamic
tracts pain and temperature, and ventral corticospinal tracts carry motor signals.
Vitamin B12 Deficiency
Vitamin B12 deficiency usually presents as paresthesias in the hands and feet
and loss of vibratory sense. There is a diffuse effect on the spinal cord, prima-
rily the posterior lateral columns, explaining the early loss of vibratory sense.
Late in the course, optic atrophy and mental changes as well ataxia can occur.
The macrocytic anemia is common.
Cyanocobalamin is a compound that is metabolized to a vitamin in the
B complex commonly known as vitamin B12. Vitamin B12 is the most chemi-
cally complex of all the vitamins. The structure of B12 is based on a corrin ring,
which is similar to the porphyrin ring found in heme, chlorophyll, and
cytochrome. The central metal ion is cobalt (Co). Once metabolized, cobal-
amin is a coenzyme in many biochemical reactions, including DNA synthesis,
methionine synthesis from homocysteine, and conversion of propionyl into
succinyl coenzyme A from methylmalonate. Dietary cobalamin (Cbl),
obtained through animal foods, enters the stomach bound to animal proteins.
Absorption requires many factors including stomach acid, R-protein, and
intrinsic factor from parietal cells, and the distal 80 cm of the ileum for trans-
port. Interference in any of these points can lead to malabsorption of vitamin
B12. In addition there are a number of inborn errors of metabolism that can
both interfere with the absorption and action of vitamin B12. The most com-
mon cause of vitamin B12 deficiency is malabsorption because of pernicious
anemia, a condition where antibodies are generated to the parietal cells of the
stomach, and the necessary proteins are not available. There are many other
causes, however, that should be considered.
Pathologically, in experimental subacute combined degeneration (SCD),
there is edema and destruction of myelin. Thus, the clinical presentation of SCD
CLINICAL CASES 191
is caused by dorsal column, lateral corticospinal tract, and sometimes lateral
spinothalamic tract dysfunction. The initial symptoms are usually paresthesia in
the hands and feet. This condition can progress to sensory loss, gait ataxia, and
distal weakness, particularly in the legs. If the disease goes untreated, an ataxic
paraplegia can evolve. Specific findings on examination are loss of vibratory
and joint position sense, weakness, spasticity, hyperreflexia, and extensor plan-
tar responses. The syndrome of sensory loss as well as spastic paresis associated
with pathologic lesions in the dorsal columns and lateral corticospinal tracts is
referred to as subacute combined degeneration. There are also effects on other
body systems, most conspicuously hematologic with the macrocytic anemia.
Differential Diagnosis
The manifestations of vitamin B12 deficiency are noted in Table 22\u20131. The dif-
ferential diagnosis for progressive spastic paraplegia includes degenerative,
demyelinating, infectious, inflammatory, neoplastic, nutritional, and vascular dis-
orders. HIV-1 associated vacuolar myelopathy, Lyme disease, multiple sclerosis
neurosyphilis, toxic neuropathy, Friedreich ataxia, and vitamin E deficiency.
192 CASE FILES: NEUROLOGY
Table 22\u20131
CLINICAL MANIFESTATIONS OF VITAMIN B12 DEFICIENCY
Neurologic
\u2022 Paresthesia
\u2022 Peripheral neuropathy
\u2022 Combined systems disease (demyelination of dorsal columns and corticospinal
tract)
Behavioral
\u2022 Irritability, personality change
\u2022 Mild memory impairment, dementia