The difficulty in writing numbers is related with a certain types of agraphia present in Broca's aphasia, Wernicke's aphasia and Conduction aphasia.
Nonfluent agraphia in Broca's aphasia
These individuals have trouble in transcoding tasks where substitutions and omissions are present in the conversion of the number in digit form to its written verbal form. In addition, writing number sequences in both directions but particularly in reverse, proves to be extremely troublesome.
Fluent agraphia in Wernicke's aphasia
These individuals are prone to lexical errors. When dealing with numbers there is often incorrect production of words in the oral form (paralexias) and incorrect writing of words when another is intended (paragraphias). Also, writing quantities from dictation is impaired due to trouble understanding language in these people.
Apractic agraphia in Conduction aphasia
Apractic agraphia is an impairment in writing where the written production is irregular even though there is normal sensorimotor function, visual feedback and written language knowledge (Alexander et al., 1992). This impairment also applies to writing quantities where self-corrections and approximations are found. Also, patients do not convert verbally heard numbers to the right graphic form.