"Rachi-" is a prefix associated with the spine, as in Rachischisis.
"Amino" is a bit more familiar: it refers to the amino chemical group, or in biomedicine, amino acids.
"Aminorachia" seems to be a rarely used term, I see only 3 hits in Google Scholar.
One of the sources using the term provides some better context:
Selective aminorachia with normal plasma amino acid concentrations was observed in several children . In one child only glycine was elevated in the cerebrospinal fluid with low values for several other amino acids. When analyzed, cerebrospinal fluid concentrations of 5-methyltetrahydrofolate are normal , .
Nicolai, J., van Kranen-Mastenbroek, V. H., Wevers, R. A., Hurkx, W. A., & Vles, J. S. (2006). Folinic Acid–Responsive Seizures Initially Responsive to Pyridoxine. Pediatric neurology, 34(2), 164-167.
It seems to me like this is an attempt to coin a term that hasn't really settled in to the literature, appearing just a few times, but explaining a fairly simple concept that is found much more broadly in literature: epilepsy can be associated with different cerebrospinal fluid levels of certain amino acids. I would not recommend the term, but "selective aminorachia" in these sources means "elevated CSF concentrations of certain amino acids relative to plasma".
This is a bit outside my area of expertise, so I can't comment more specifically on modern expectations of CSF amino acid concentrations with epilepsy or if there is any clinical relevance/utility to measuring them. I've collected a few references that might be a starting point.
Araki, K., Harada, M., Ueda, Y., Takino, T., & Kuriyama, K. (1988). Alteration of amino acid content of cerebrospinal fluid from patients with epilepsy. Acta neurologica scandinavica, 78(6), 473-479.
Devinsky, O., Emoto, S., Nadi, N. S., & Theodore, W. H. (1993). Cerebrospinal fluid levels of neuropeptides, cortisol, and amino acids in patients with epilepsy. Epilepsia, 34(2), 255-261.
Griffith, N. C., Cunningham, A. M., Goldsmith, R., & Bandler, R. (1991). Interictal behavioral alterations and cerebrospinal fluid amino acid changes in a chronic seizure model of temporal lobe epilepsy. Epilepsia, 32(6), 767-777.
Mutani, R., Monaco, F., Durelli, L., & Delsedime, M. (1974). Free amino acids in the cerebrospinal fluid of epileptic subjects. Epilepsia, 15(4), 593-597.
Rainesalo, S., Keränen, T., Palmio, J., Peltola, J., Oja, S. S., & Saransaari, P. (2004). Plasma and cerebrospinal fluid amino acids in epileptic patients. Neurochemical research, 29(1), 319-324.