ABETALIPOPROTEINEMIA

abetalipoproteinemia_recessive_inherited_medicinestuffDEFINITION:

A disorder of lipid metabolism characterized by fat malabsorption, acanthocytosis, retinopathy, and progressive neurologic disease.

EPIDEMIOLOGY:

- incidence: rare
- age of onset:

* 1st year (gastrointestinal manifestations)
* first 10 years (neurologic and ocular manifestations)

- risk factors:

* familial - autosomal recessive
* chrom. #: ?2p24
* gene: ?
* M > F (3:2)

PATHOGENESIS:

1. Background

* cholesterol and triglycerides are transported in the circulation in macromolecular complexes called lipoproteins
* the protein component of lipoproteins are called apolipoproteins
* abetalipoproteinemia is considered to be a defect in the absorption and transport of lipids
* heterozygotes are asymptomatic

2. Genetic Defect

genetic defect -> decreased availability of apolipoproteins B (apo B-100, apo B-48) -> inhibits the formation of chylomicrons (lipoproteins containing exogenous cholesterol and triglycerides) and very low density lipoproteins (lipoproteins containing endogenous cholesterol and triglycerides) -> inhibits availability of cholesterol and triglycerides -> clinical manifestations:

1. Gastrointestinal

* malabsorption of fats and fat-soluble vitamins:
* anemia and coagulopathy
* night blindness and retinopathy

2. Peripheral Nervous System

axonopathy and demyelination of peripheral sensory and motor nerves and within the posterior column of the spinal cord -> sensory and motor deficits

3. Hematological

decreased levels of cholesterol and triglyceride in the plasma may result in the maldistribution of lipids within the plasma membrane of the RBC’s resulting in RBC membrane deformities -> thorn-like projections (acanthocytosis)

CLINICAL FEATURES:

I. Gastrointestinal Manifestations

initially present from newborn period to 1 year of age

1. Malabsorption Syndrome

* fat malabsorption:
* steatorrhea - pale, foul-smelling, bulky stool
* abdominal distention, vomiting, diarrhea, failure to thrive
* malabsorption of fat-soluble vitamins - E, A, K - (vitamin D absorption does not appear to be affected)

II. Neurological Manifestations

begin in the first or second decade and are progressive
first neurological sign is loss of deep tendon reflexes within the first few years of life

1. Sensory Disturbances

* altered position and vibration senses
* altered pinprick and temperature sensations
* stocking-glove distribution of hypesthesia
* tend to precede ataxia

2. Movement Disorders

* cerebellar ataxia:
* gait disturbances:
* 33% of patients by 10 years of age
* 100% of patients by 30 years of age
* loss of ambulation by 3rd decade
* dysarthria
* dysdiadochokinesia
* dysmetria
* chorea
* intention tremors

3. Others

* muscle weakness
* muscle contractions:
* pes cavus, equinovarus, kyphoscoliosis
* mental retardation
* peripheral neuritis

III. Hematological Manifestations

likely secondary to malabsorption

1. Anemia

* hemolytic etiology with iron and/or folate deficiencies
* may be associated with hyperbilirubinemia

2. Coagulopathy

* prolonged PT

IV. Ophthalmologic Manifestations

1. Night Blindness

* usually initial symptom (Vitamin A deficiency)

2. Pigmentary Retinal Blindness

* most prominent symptom and present by 10 years of age

3. Impaired Visual Acuity

* by 10 years of age and may progress to blindness

4. Ophthalmoplegia

* dissociated nystagmus on lateral gaze
* progressive exotropia

5. Others

* cataracts
* loss of colour vision
* ptosis

INVESTIGATIONS:

I. Serum

* blood smear: acanthocytosis of 50-70% of peripheral erythrocytes (thorny projections from cell surface)
* reduced cholesterol, triglycerides, beta-lipoproteins, chylomicrons, VLDL. vitamins E, A, and K
* lack of apolipoprotein B
* anemia (mild to severe) with slight reticulocytosis and negative
* Coombs test
* low ESR, prolonged PT

II. Biopsies

1. Duodenal

* yellowish discolouration of mucosa (increased lipid content)
* normal villi
* lack of apolipoprotein B on immunofluorescence

2. Peripheral Nerves

* paranodal demyelination

3. Electrodiagnostic Studies

1. Evoked Potentials

* abnormal somatosensory conduction velocity

2. Conduction Velocity

* slowed with decreased amplitude of sensory potentials

3. EMG

* denervation

MANAGEMENT:

1. Diet

* restriction of long-chain fatty acids
* supplement with MCT oil
* tends to alleviate the gastrointestinal manifestations

2. Medical

1. Fat Soluble Vitamins

* Vitamin E: can prevent or retard the neurological manifestations and retinopathy
* Vitamin K: corrects coagulopathy
* Vitamin A: corrects night blindness

2. Iron or Folate Supplementation

* for anemia