Hi & Welcome to HCM.
I appreciate your concern for your cousin.
Anterior horn disease is a type of motor neurone disease.
Motor neuron disorders (MNDs) are characterized by progressive degeneration of motor neurons; they include both sporadic and hereditary diseases.
Either or both of the following 2 sets of motor neurons can be affected:
1. Upper motor neurons: which originate from the primary
motor cortex of the brain
2. Lower motor neurons: which originate in the brain stem, motor nuclei and
spinal cord (anterior horn cells) and directly innervate skeletal muscles.
Apparently your cousin is likely to be having the second type of the disease mentioned above (i.e, motor neurons in the anterior horns) characterized by spinal muscular
atrophy.
This condition is genetically determined, with autosomal recessive (most common), autosomal dominant, and X-linked recessive modes of inheritance.
Hope to have answered your query.
A feed back is highly appreciated.