Posterior cranial fossa and cervical spine morphometric abnormalities in symptomatic Chiari type 0 and Chiari type 1 malformation patients with and without syringomyelia

Abstract

Background

To better understand how anatomical features of Chiari malformation type 0 (CM0) result in the manifestation of Chiari malformation type 1 (CM1) signs and symptoms, we conducted a morphometric study of the posterior cranial fossa (PCF) and cervical canal in patients with CM1 and CM0.

Methods

This retrospective study had a STROBE design and included 120 adult patients with MRI evidence of a small PCF (SPCF), typical clinical symptoms of CM1, and a diagnosis of CM1, CM0, or SPCF-TH0-only (SPCF with cerebellar ectopia less than 2 mm and without syringomyelia). Patients were divided by MRI findings into 4 groups: SPCF-TH0-only, SPCF-TH0-syr (CM0 with SPCF and syringomyelia), SPCF-CM1-only (SPCF with cerebellar ectopia 5 mm or more without syringomyelia), and SPCF-CM1-syr (CM1 with syringomyelia). Neurological examination data and MRI parameters were analyzed.

Results

All patient cohorts had morphometric evidence of a small, flattened, and overcrowded PCF. The PCF phenotype of the SPCF-TH0-only group differed from that of other CM cohorts in that the length of clivus and supraocciput and the height of the PF were longer, the upper CSF spaces of PCF were taller, and the area of the foramen magnum was smaller. The SPCF-TH0 groups had a more significant narrowing of the superior cervical canal and a smaller decrease in PCF height than the SPCF-CM1 groups.

Conclusions

Patients with SPCF-TH0 with and without syringomyelia developed Chiari 1 symptoms and signs. Patients with SPCF-TH0-syr (Chiari 0) had more constriction of their CSF pathways in and around the foramen magnum than patients with SPCF-TH0-only.

Introduction

Chiari malformation type 1 (CM1) has been diagnosed by MRI using a cerebellar tonsillar herniation (TH) threshold of at least 5 mm below the foramen magnum (FM) [3, 7]. However, some patients with lesser TH and a small posterior cranial fossa (PCF) have CM1-like symptoms, and conversely, many patients with TH of 5–12 mm are asymptomatic [1, 11, 25, 38, 42]. The International Classification of Headache Disorders guidelines permits diagnosis of CM1 with only 3 mm of TH if the CSF spaces are narrowed posterior and lateral to the cerebellum, the supraocciput is short, the tentorium has increased slope, or medulla oblongata is kinked [16]. Small PCF characterizes primary (classic) CM1 and not secondary (acquired) TH [27, 32].

Chiari malformation type 0 (CM0) initially described patients with Chiari I malformation type symptoms, less than 3 mm of TH, and cervical syringomyelia resolving after craniocervical decompression, a CM1 operative procedure [22, 46]. More recently, CM0 has been used to describe patients without syringomyelia, minimal TH, and typical signs and symptoms of Chiari I malformation, including occipital headache, posterior cervical pain, and cerebellomedullary dysfunction [12, 28, 48].

To better understand how small posterior fossa with minimal tonsillar ectopia results in Chiari malformation type 1 signs and symptoms, we conducted a retrospective study identifying morphometric abnormalities of the PCF and cervical canal in patients with CM1, CM0, and small posterior cranial fossa without syringomyelia.

Methods and materials

Participants

This study included adult patients (age ≥ 18 years) with typical clinical symptoms of CM1, a diagnosis of CM1, CM0, or SPCF-TH0-only, and MRI evidence of SPCF (shortened clivus, supraocciput, or both). The study included data from patients examined in 2003–2019 that met the inclusion criteria and had enough high-quality MRI images for MRI morphometry. Patients were divided by MRI findings into 4 age/gender-matched groups: (1) SPCF-TH0-only (SPCF with cerebellar ectopia less than 2 mm and without syringomyelia; 15 M/15F, 43 ± 14 years), (2) SPCF-TH0-syr (CM0 with SPCF and syringomyelia; 15 M/15F, 49 ± 14 years), (3) SPCF-CM1-only (SPCF with one or both cerebellar tonsils extending 5 mm or more below the FM without syringomyelia; 15 M/15F, 46 ± 11 years), and (4) SPCF-CM1-syr (CM1 with SPCF and syringomyelia; 15 M/15F, 48 ± 11 years). Symptoms and signs of CM were compared among the patient groups, including (1) occipital headache; (2) neurologic deficit involving the brainstem, cerebellum, or cranial nerves; and (3) neurologic deficit from syringomyelia-related central cervical myelopathy [26]. The control group (CON) comprised 30 age-/gender-matched subjects (15 M/15F, age 43 ± 11 years) who underwent MRI for any reason and had normal brain and spinal cord imaging.

Radiological evaluation

We used 1.0 and 1.5 T MRI scanners (EXCITE, GE Healthcare, Waukesha, Wisconsin). One trained researcher (A.F.) analyzed the images. Measurements were linear, angular, and calculated (Fig. 1). Evans’ index (EI), a radiological marker of abnormal (values > 0.30) ventricular enlargement, was calculated by dividing the maximum width of the frontal horns (HW) by the entire width between the inner tables of the skull (BTW). Each MRI was qualitatively evaluated and characterized as one of (1) five types of cisterna magna—A, B, C, D, E (A-CM, B-CM, C-CM, D-CM, or E-CM) [51]; (2) three types of PCF: small supraocciput (SmSO pf), small basiocciput (clivus) (SmCL pf), or shortness of both supraocciput and clivus in the midsagittal plane (global smallness, GlSm pf) [36]. The clivus and supraocciput were considered abnormally short if their lengths were ≤ 40 mm [8, 12], which is one standard deviation below the control group mean. These and other control values conformed to those in other studies [20, 26].

Fig. 1.

Morphometric measurements for assessing Chiari malformation. Linear, angular, and calculated variables. Linear variables (a–d). C-FM, F-FM, P-FM—distances from FM to corpus callosum, to fastigium, and to pons, respectively [21, 48]. ChD—distance between the Chamberlain line and tip of the dens axis [35]. CL—the length of the clivus [6]. FM—the anteroposterior diameter of the foramen magnum [21]. FV—distance between the fastigium and the floor of the fourth ventricle [46]. h—height of the osseous part PCF—distance from opistion to Twining line [6]. H—height of the skull above the Twining line. h ant.—anterior height of PCF [52]. M-FVV—distance between the M-line (Line M is drawn across the clivus vertex and perpendicular to the C-2 endplate) and fourth ventricle vertex [45]. obex-McR—the distance between the obex and the basion-opisthion line [14, 46]. pB-C2—perpendicular length between a line connecting the basion and the inferoposterior edge of the C2 to the ventral dural [21]. ROST—the anteroposterior distance of the soft tissue, shown as a hypointensity behind the odontoid process [15]. SC-ISCC, VER-ISCC—distance from the uppermost part of the superior colliculi and the uppermost part of the culmen of cerebellar vermis, respectively, to the inferior part of the splenium of corpus callosum [31]. SO—the length of supraocciput [6]. v—the anteroposterior width of the ventral subarachnoid space [6]. Y—the anteroposterior length of the PCF [4]. Z —the height of the PCF [4]. Angular variables (a, b, e). BA (basal angle)—angle formed by the nasion, dorsum sellae, and basion [35]. BoA (Boogaard angle)—angle formed by the dorsum sellae, basion, and opisthion [6]. C2DRv (odontoid retroversion angle)—angle between a line drawn along the inferior margin of C2 vertebral body and a line drawn from the mid-point of the inferior margin through the dens apex [21]. CAA (the clivoaxial angle)—angle between the dorsal surface of the clivus and the posterior surface of the odontoid process [15]. CAAm (the modified clivoaxial angle)—angle between the dorsal surface of the clivus and the posterior surface of the retro-odontoid soft tissue [15]. CLgr (the clivus gradient)—angle of the McRae’s line to the clivus [52]. C-SO—the angle between clivus and supra-occiput [4]. kink—cervicomedullary kinking. TA (tentorium angle)—angle formed by the tentorium and a line from the internal occipital protuberance to the opisthion [21]. TTw (tentorium-Twining angle) [4]—the angle at the intersection of the tentorium and the Twining line. Calculated variables: h/H ratio (d) was used to compare the skull compartments below (osseous part of the PCF) and above the Twining. S fm (f)—FM surface area [43] subdivided into the surface areas of the spinal cord (S sc), of the herniated cerebellar tonsils (S tt = S t₁ + S t₂), and of the CSF (S csf); S fm, S sc, St₁, and St₂ were calculated as the area of the ellipse: (π × semi-minor axis × semi-major axis). Also calculated were relative indicators – S tt/S fm and S csf/S fm ratios. S pf (e)—mid-sagittal PCF area [48] was delimited by the tentorium, the occipital bone, the McRae line, and clivus; subdivided into osseous part (S pf b, located below the Twining line) and extra-osseous part (S pf eb), and calculated as the sum of the areas of right-angled triangles. TR C1-4, TR C4-7, TR C1-7 (c)—the C1-C7, C1-C4, and C4-C7 taper ratio calculation [37]: the antero-posterior diameters of the spinal canal – apsc C1 – apsc C7 (apsc C3-7 not shown)—were plotted against the cervical level; a trend line was fitted by linear regression to the diameters between C1 and C7, and the slope of this line was calculated; the slope of the trend line was recorded as the taper ratio for that spine (mm/level). V cm (Fig. 1a)—volume of the cisterna magna [51] was calculated using formula: [height × width × depth]/6 (width of the cisterna magna not shown). V pf (d) – PCF volume was calculated using formula [4]: [4/3 × π×(X/2 × Y/2 × Z/2)], where X is the width of the PCF (not shown)

Study exclusion criteria

The study excluded patients (1) without symptoms (asymptomatic); (2) with one or both cerebellar tonsils extending 2 to 4 mm below the FM, considered borderline Chiari malformation type 1 or Chiari malformation type 0.5 [5, 12]; and (3) with CM associated with CNS anomalies and with basilar invagination (tip of the dens more than 5 mm above Chamberlain’s line), atlantoaxial dislocation, occipitalization of the atlas, Klippel-Feil anomaly, or other craniovertebral junction (CVJ) bone malformation; with acquired cerebellar tonsillar ectopia from intracranial tumors, craniosynostosis, tethered spinal cord, Ehlers-Danlos syndrome, etc.; with cervical spondylotic myelopathy; and with severe obex descent (> 10 mm inferior to the FM) if associated with other features of “complex” CM1 [35, 40, 41, 47].

Statistical analysis

Measurements from CM patients and control subjects were compared using the U-test and chi-square test (IBM SPSS Statistics 23, Armonk, NY). P values < 0.05 were considered statistically significant. Statistical analysis compared PCF morphometric measurements between cohorts of patients and controls (CON) to 1) discover abnormal morphometric measurements characterizing SPCF-CM1-only, SPCF-CM1-syr, SPCF-TH0-only, and SPCF-TH0-syr patients, (2) find morphometric measures differentiating the SPCF-TH0 (combined SPCF-TH0-syr and SPCF-TH0-only) and SPCF-CM1 (combined SPCF-CM1-syr and SPCF-CM1-only) cohorts from each other and control (CON), and (3) find morphometric measures differentiating syringomyelia patients SPCF-syr (combined SPCF-TH0-syr and SPCF-CM1-syr) from non-syringomyelia patients SPCF-only (combined SPCF-TH0-only and SPCF-CM1-only) and controls (CON). Receiver operating characteristic analysis (ROC-analysis) identified the area under the ROC curve (AUC) and the highest sensitivity, specificity, and diagnostic significance factors.

Results

CM patients vs. CON

Figure 2 a and Tables 3 and 4 present the comparison of the consolidated patient groups with CON. Tables 1 and 2 show comparisons between individual patient groups and CON. ROC analysis identified C-SO angle (AUC 0.834; a value greater than or equal to 71.5 degrees having 79% sensitivity and 67% specificity) and obex-McR (AUC 0.811; a value less than or equal to 6.5 mm having 76% sensitivity and 70% specificity) as the most sensitive and specific measures distinguishing CM groups from control.

Fig. 2.

Comparative characteristics of the PCF phenotype in control group and CM cohorts. a MRI parameters that differ between all CM group and CON (a down arrow indicates a decrease, and an up arrow indicates an increase in the parameter in all CM group in comparison with CON, p < 0.05), as well as a schematic representation of the main changes in the PCF phenotype (decrease, narrowing is highlighted by a thin arrow, increase by a wide arrow). b MRI parameters that differ between SPCF-syr and SPCF-only cohorts (a down arrow indicates a decrease, and an up arrow indicates an increase in the parameter in SPCF-syr cohort in comparison with SPCF-only cohort, p < 0.05), as well as a schematic representation of the main changes in the PCF phenotype (decrease, narrowing is highlighted by a thin arrow, increase by a wide arrow). c Reduction of the anteroposterior range in the upper part of the cervical spinal canal (SPCF-syr vs. SPCF-only, p < 0.05) and tendency to increase in the lower part leads to the formation of the “crossing” phenomenon. d MRI parameters that differ between SPCF-TH0 and SPCF-CM1 cohorts a down arrow indicates a decrease, and an up arrow indicates an increase in the parameter in SPCF-TH0 cohort in comparison with SPCF-CM1 cohort, p < 0.05), as well as a schematic representation of the main changes in the PCF phenotype (decrease, narrowing is highlighted by a thin arrow, increase by a wide arrow). e, f SPCF-TH0-only patient: woman, 53 years old, for a long time notes complaints of aching headaches, episodic double vision, mainly complicating the descent from the stairs, and attacks of dizziness, tinnitus and instability; on neurological examination—convergent strabismus on the left, left-sided brisk tendon reflexes and Babinski sign, instability in sensitized Romberg’s test. At the brain MRI: (e mid-sagittal T1 MRI) the lower pole of the cerebellar tonsils is located at the level of the foramen magnum (f axial T2 MRI through the plane of the foramen magnum) the cerebellar tonsils occupy the foramen magnum with compression of the brain stem, non-visible cisterna magna (posterior to the cerebellar tonsils); CL h41 mm, SO 32 mm, FM 32 mm, h 24 mm, CAAm 139 grad., pB-C2 10 mm, ROST 5 mm, S fm 509 mm². g Comparative characteristics of the PCF phenotype in SPCF-TH0-only and in other cohorts: (1) the smallest among other forms of CM, but statistically significant in comparison with CON, decrease in CL, SO and Spf; (2) change in the configuration of the PF (decrease in the osseous—below the Twining line—part, and increase in the extra-osseous—above the Twining line—part) while maintaining the total height and volume of the PF (Z and Vpf do not differ from CON); 3) the upper CSF spaces of the PCF (VER-ISCC) are narrower than in CON, but significantly wider in comparison with other forms of CM; (4) a decrease Sfm compared to CON and other forms of CM

Table 3.

The MRI-morphometric findings in CM all, SPCF-only, SPCF-syr, SPCF-TH0, and SPCF-CM1 cohorts: linear (mm) and angular (grad.) variables

Variables	CM all		SPCF-only		SPCF-syr		SPCF-TH0		SPCF-CM1
	M	m	M	m	M	m	M	m	M	m
CL	38.2*	3.8	38.7	3.7	37.8	4.0	38.6	4.0	37.9	3.7
SO	37.9*	4.0	38.0	4.5	37.8	3.5	39.0#	4.7	36.8	2.8
T	4.4*	5.7	4.8	5.6	3.9	5.8	− 0.4#	1.6	9.1	4.2
obex-McR	1.6*	6.6	1.0	7.4	2.2	5.8	5.6#	2.9	− 2.5	6.9
v	6.5	2.3	6.9^	2.3	6.1	2.2	7.1#	2.4	6.0	1.9
pB-C2	7.2*	1.8	7.3	1.8	7.2	1.8	7.4	1.9	7.0	1.7
ChD	1.0*	2.3	0.8	2.7	1.1	1.9	0.7	2.2	1.3	2.4
ROST	3.9*	1.3	3.9	1.4	4.0	1.1	4.0	1.4	3.9	1.2
CAAm	141.1*	11.0	141.5	12.0	140.7	9.9	142.2	11.3	139.9	10.6
CAA	150.0	10.5	150.6	11.4	149.3	9.5	150.8	10.6	149.2	10.5
FM	35.1	2.8	35.4	3.1	34.8	2.4	34.8	2.8	35.4	2.7
h ant	28.8*	4.0	29.6^	4.0	28.1	3.9	29.8#	3.7	27.9	4.2
h	26.4*	4.1	26.4	4.0	26.4	4.4	27.1	4.2	25.7	4.0
H	83.5	7.4	83.9	7.5	83.0	7.3	83.0	7.5	83.9	7.3
BoA	129.8*	8.5	128.3	8.8	131.4	8.0	129.0	8.2	130.7	8.8
CLgr	51.5*	13.2	53.1^	13.0	49.9	13.2	51.0	8.2	52.0	16.8
C-SO	81.0*	10.4	79.8	9.1	82.2	11.6	80.1	12.0	82.0	8.7
apsc C1	17.7	2.7	18.6^	2.7	16.8	2.3	17.0#	2.6	18.5	2.5
apsc C2	16.5	2.0	16.9	2.1	16.2	1.9	16.1#	2.1	16.9	1.9
apsc C3	14.8	1.9	15.1	1.8	14.5	2.0	14.5	1.9	15.1	1.8
apsc C4	13.9	2.1	13.8	2.4	14.1	1.9	13.5#	2.6	14.3	1.5
apsc C5	13.3	2.2	13.1	1.9	13.5	2.4	13.1	2.5	13.5	1.8
apsc C6	13.1	2.1	12.8	1.7	13.4	2.4	12.8#	2.5	13.5	1.6
apsc C7	13.1	2.1	12.7	1.7	13.4	2.3	12.9	2.4	13.3	1.8
HW	36.9*	4.6	36.6	4.3	37.4	4.9	36.5	4.1	37.4	5.0
BTW	137.8	7.2	138.9	7.7	136.5	6.4	137.2	8.2	138.3	6.1
SC-ISCC	5.8	2.2	5.5	2.0	6.1	2.3	5.4#	2.3	6.1	2.0
VER-ISCC	7.4*	2.5	8.0^	2.6	6.8	2.2	7.6	2.1	7.3	2.8
P-FM	35.2*	4.5	35.9	4.8	34.4	4.0	36.3#	4.1	34.1	4.6
C-FM	54.1*	4.1	54.8^	4.7	53.3	3.3	55.1#	3.6	53.0	4.4
F-FM	26.5*	3.5	26.6	3.6	26.4	3.5	27.9#	2.9	25.1	3.6
TA	85.8*	8.8	87.0^	7.5	84.6	9.9	86.9	9.5	84.7	8.0
TTw	38.6	5.7	39.1	5.2	38.0	6.2	38.8	6.1	38.4	5.3
X	103.0	5.1	103.9	5.2	102.0	4.9	102.6	5.2	103.3	5.1
Z	61.4*	5.5	62.7^	5.7	60.1	5.1	61.8	6.1	60.9	5.0
Y	82.5	5.6	82.3	5.5	82.7	5.7	82.1	5.9	82.9	5.3
BA	116.0	7.2	114.9	7.2	117.1	7.1	116.2	7.1	115.8	7.4
kink	156.7	9.7	156.7	10.3	156.6	9.2	156.7	9.6	156.6	9.9
C2DRv	72.8	6.9	72.6	7.2	73.0	6.7	73.9	7.7	71.8	6.0
FV	9.0	2.2	8.6	1.6	9.4	2.7	8.9	2.2	9.1	2.3
M-FVV	29.1	4.0	29.7	3.7	28.6	4.3	29.4	4.1	28.9	4.0

Statistically significant differences (p < 0.05):

^(*)with CON

^{(^)}with SPCF-syr

^(#)with SPCF-CM1

Table 4.

The MRI-morphometric findings in CM all, SPCF-only, SPCF-syr, SPCF-TH0, and SPCF-CM1 cohorts: calculated variables (M ± m) and qualitative parameters (n, %)

Cohorts	CM all		SPCF-only		SPCF-syr		SPCF-THO		SPCF-CM1
CalcV	M	m	M	m	M	m	M	m	M	m
EI	26.8*	3.1	26.4	2.9	27.4	3.3	26.6	2.7	27.0	3.4
V cm, mm3	55.4*	114.3	44.1	50.2	66.7	153.6	78.5#	149.3	32.3	54.5
h/H	0.3*	0.1	0.3	0.1	0.3	0.1	0.3	0.1	0.3	0.1
V pf, cm3	274.3	38.1	282.2^	40.5	266.4	34.1	274.1	41.1	274.5	35.3
S pf, mm2	2858.9*	301.0	2924.1^	325.1	2793.6	261.5	2883.2	307.1	2834.5	295.4
S pf b, mm2	1659.7*	211.0	1692.8	225.1	1626.6	192.0	1705.9#	203.7	1613.5	209.5
S pf eb, mm2	1199.2*	221.0	1231.3	227.0	1167.0	211.8	1177.3	229.4	1221.0	211.9
S fm, mm2	820.9	199.8	787.6	178.7	854.3	215.3	774.7#	226.0	864.0	162.3
S sc, mm2	130.9	32.7	134.6	34.8	127.1	30.3	123.0#	33.6	138.2	30.3
S tt, mm2	275.3*	180.7	250.3	169.3	300.3	189.8	189.6#	150.2	354.9	170.9
S csf, mm2	414.8*	154.2	402.7	127.3	426.8	177.4	462.0#	170.1	370.8	123.8
S csf/ S fm	51.7*	17.4	52.5	16.3	50.9	18.5	60.3#	16.1	43.7	14.5
S tt/S fm	17.6*	13.2	16.7	11.9	18.5	14.4	11.2#	8.8	23.6	13.8
TR C1-4	− 1.3	0.9	− 1.6^	0.8	− 1.0	0.9	− 1.2	0.9	− 1.4	0.8
TR C4-7	− 0.2	0.7	− 0.3	0.8	− 0.2	0.6	− 0.2	0.8	− 0.3	0.6
TR C1-7	− 0.7	0.5	− 0.9^	0.4	− 0.6	0.5	− 0.7	0.5	− 0.8	0.5
QualP	n	%	n	%	n	%	n	%	n	%
A-CM	2*	2.0	1	2.0	1	2.0	2	3.0	0	0.0
B-CM	3*	3.0	1	2.0	2	3.0	3	5.0	0	0.0
C-CM	43*	36.0	21	35.0	22	37.0	24	40.0	19	32.0
D-CM	21	18.0	10	17.0	11	18.0	16#	27.0	5	8.0
E-CM	51*	43.0	27	45.0	24	40.0	15#	25.0	36	60.0
SmSO pf	49	41.0	27	45.0	22	37.0	24	40.0	25	42.0
GlSm pf	27	22.0	13	22.0	14	23.0	8#	13.0	19	32.0
SmCL pf	44	37.0	20	33.0	24	40.0	28#	47.0	16	26.0

Statistically significant differences (p < 0.05):

^(*)with CON

^{(^)}with SPCF-syr

^(#)with SPCF-CM1

CalcV calculated variables, QualP qualitative parameters

Table 1.

The MRI-morphometric findings in CM patients’ groups and control subjects: linear (mm) and angular (grad.) variables

Variables	SPCF-TH0-only		SPCF-CM1-only		SPCF-TH0-syr		SPCF-CM1-syr		CON
	M	m	M	m	M	m	M	m	M	m
CL	39.0*	2.9	38.4*	4.3	38.1*	4.8	37.4*	3.1	42.7	3.0
SO	39.4*	5.5	36.6*	2.8	38.6*	3.9	36.9*	2.9	43.6	3.3
T	0.1*	0.5	9.5*^	4.2	− 0.8*#	2.1	8.7*	4.3	− 3.6	2.9
obex-McR	5.3*	2.7	− 3.3*^	8.1	6.0#	3.1	− 1.7*	5.5	7.5	1.7
v	7.5	2.6	6.3	1.8	6.6	2.2	5.7*	2.0	7.2	2.0
pB-C2	7.4*	1.9	7.1	1.7	7.4	1.9	6.9	1.7	6.5	1.8
ChD	0.4	2.5	1.2*	2.8	0.9*	1.8	1.3*	2.0	− 0.8	2.5
ROST	4.0*	1.4	3.8*	1.4	4.0*	1.3	4.0*	1.0	3.0	1.1
CAAm	142.1	12.0	140.9*	12.1	142.4*	10.7	138.9*	8.9	148.4	7.6
CAA	151.3	10.9	150.0	12.1	150.2	10.4	148.4*	8.7	154.3	8.1
FM	34.9	3.3	35.9	2.8	34.7	2.4	34.9	2.6	35.8	2.9
h ant	30.3*	3.2	28.8*	4.7	29.2*#	4.0	27.0*	3.5	34.8	3.9
h	26.8*	4.2	26.1*	3.7	27.5*	4.3	25.3*	4.3	33.4	4.4
H	83.5	8.0	84.2	7.1	82.3	6.9	83.6	7.7	81.4	6.3
BoA	127.9*	7.8	128.7*	9.7	130.1*	8.5	132.7*	7.4	123.1	5.0
CLgr	52.1*	7.8	54.0*	16.8	49.9*	8.5	49.9*	16.8	56.9	5.0
C-SO	79.9*	10.4	79.7*	7.7	80.3*	13.5	84.2*	9.1	69.0	7.3
apsc C1	18.0	2.8	19.3*^#	2.4	15.9*^#	2.0	17.7	2.3	17.4	2.9
apsc C2	16.6	2.3	17.2	1.8	15.7*	1.7	16.7	1.9	16.9	1.5
apsc C3	14.7	1.8	15.4	1.7	14.2	2.1	14.8	1.8	15.0	1.5
apsc C4	13.2	2.9	14.4	1.4	13.9	2.1	14.3	1.7	14.1	1.9
apsc C5	12.7	1.9	13.6	1.7	13.4	2.8	13.5	1.9	13.0	2.0
apsc C6	12.1	1.6	13.5^	1.6	13.5	2.9	13.4	1.7	13.0	2.0
apsc C7	12.3	1.8	13.2	1.6	13.4	2.8	13.4	1.9	13.3	2.4
HW	36.2*	3.9	37.0*	4.7	36.8*	4.3	37.8*	5.3	33.9	3.3
BTW	139.1	9.0	138.6	6.3	134.9	6.5	137.9	6.0	137.6	5.3
SC-ISCC	4.9	2.0	6.1^	1.9	6.0	2.5	6.1	2.2	5.6	1.5
VER-ISCC	8.5*	1.9	7.5*^	3.2	6.6*^	1.9	7.0*	2.4	9.9	2.6
P-FM	36.8*	4.1	35.0*	5.4	35.7*#	4.1	33.2*	3.6	39.2	3.2
C-FM	55.9*	4.0	53.7*	5.2	54.3*^#	2.9	52.3*	3.3	58.4	4.0
F-FM	28.0	2.7	25.3*^	3.9	27.8#	3.0	25.0*	3.4	28.5	2.6
TA	86.7	8.3	87.3#	6.7	87.1	10.7	82.1*	8.5	89.0	6.7
TTw	38.9	4.9	39.3	5.5	38.6	7.2	37.4	5.1	37.4	3.9
X	104.0	5.5	103.7	5.1	101.2^	4.5	102.9	5.2	102.7	4.5
Z	63.0	6.3	62.4*#	5.2	60.7*	5.6	59.5*	4.4	65.3	4.8
Y	82.6	5.6	81.9	5.5	81.6	6.3	83.8	4.9	82.6	4.0
BA	116.1	5.8	113.8*	8.4	116.3	8.4	117.8*	5.7	114.0	4.7
kink	156.9	9.0	156.5	11.7	156.4	10.3	156.8	8.1	159.9	7.1
C2DRv	73.4	8.7	71.9*	5.4	74.4	6.5	71.7*	6.7	75.1	5.5
FV	8.8	1.9	8.4#	1.3	9.1	2.4	9.7	2.9	8.9	1.2
M-FVV	29.7	3.9	29.6	3.6	29.1	4.3	28.1	4.3	29.9	3.5

Statistically significant differences (p < 0.05):

^(*)with control group

^{(^)}with SPCF-TH0-only group

^(#)with SPCF-CM1-syr group

Table 2.

MRI-morphometric findings in CM patients’ groups and control subjects: calculated variables (M ± m) and qualitative parameters (n, %)

Groups	SPCF-TH0-only		SPCF-CM1-only		SPCF-TH0-syr		SPCF-CM1-syr		CON
CalcV	M	m	M	m	M	m	M	m	M	m
EI	26.0*	2.2	26.7*	3.4	27.3*	3.0	27.4*	3.5	24.6	2.1
V cm, mm3	57.1*	47.9	31.2*^	50.0	100.0*#	205.3	33.4*	59.6	241.6	198.4
h/H	0.3*	0.1	0.3*	0.1	0.3*	0.1	0.3*	0.1	0.4	0.1
V pf, cm3	285.0	43.6	279.4	377.7	263.1*^	35.8	269.6*	32.5	291.0	28.0
S pf, mm2	2961.6*	318.1	2886.6*	333.2	2804.8*^	279.2	2782.5*	246.8	3174.9	289.5
S pf b, mm2	1728.0*	217.8	1657.7*	230.4	1683.9*#	189.8	1569.4*	179.5	2094.1	251.3
S pf eb, mm2	1233.7*	238.1	1229.0*	219.4	1120.9	209.3	1213.1*	207.6	1080.8	144.9
S fm, mm2	717.6*	172.9	855.0^	159.6	833.9	260.8	872.6	167.3	876.6	208.3
S sc, mm2	123.9	30.3	145.0^	36.2	122.1#	37.3	131.6	22.0	136.5	30.7
S tt, mm2	183.0	124.7	315.2*^	182.8	196.5#	175.2	393.4*	152.0	175.9	165.0
S csf, mm2	410.8*	127.6	394.9*	128.8	515.2#	193.6	347.5*	116.2	564.2	149.5
S csf/S fm	58.0	15.1	47.3*^	15.8	62.7#	17.2	40.3*	12.5	65.5	15.1
S tt/S fm	12.5	8.8	20.7*^	13.1	9.8#	8.7	26.3*	14.1	9.2	8.0
TR C1-4	− 1.6	0.8	− 1.6	0.7	− 0.8^	0.9	− 1.2	0.8	− 1.2	1.1
TR C4-7	− 0.2	1.0	− 0.3	0.5	− 0.1	0.6	− 0.3	0.6	− 0.4	0.6
TR C1-7	− 0.9	0.4	− 0.9	0.5	−0.5*^#	0.5	− 0.7	0.4	− 0.8	0.4
QualP	n	%	n	%	n	%	n	%	n	%
A-CM	1*	3.0	0*	0.0	1*	3.0	0*	0.0	19	63.0
B-CM	1	3.0	0	0.0	2	7.0	0	0.0	4	13.0
C-CM	13*	43.0	8	27.0	11*	37.0	11*	37.0	3	10.0
D-CM	8	27.0	2	7.0	8	27.0	3	10.0	4	13.0
E-CM	7*	23.0	20*^	67.0	8*	27.0	16*	53.0	0	0.0
SmSO pf	13	43.0	14	47.0	11	37.0	11	37.0	0	0.0
GlSm pf	4	14.0	9	30.0	4	13.0	10	33.0	0	0.0
SmCL pf	13	43.0	7	23.0	15	50.0	9	30.0	0	0.0

Statistically significant differences (p < 0.05):

^(*)with control group

^{(^)}with SPCF-TH0-only group

^(#)with SPCF-CM1-syr group

CalcV calculated variables, QualP qualitative parameters

CM patients with syringomyelia vs. CM patients without syringomyelia

Figure 2 b, c and Tables 3 and 4 present the comparison of the SPCF-syr cohort with the SPCF-only cohort. Tables 1 and 2 show the subgroup analyses of the SPCF-syr and SPCF-only cohorts. ROC analysis identified apsc C1 (AUC 0.694; value less than or equal to 18.5 mm having 77% sensitivity and 53% specificity) and TR C1-7 (AUC 0.690; value less than or equal to −0.84, having 70% sensitivity and 58% specificity) as the most sensitive and specific measures differentiating SPCF-syr and SPCF-only cohorts.

SPCF-TH0 patients vs. SPCF-CM1 patients

The CM and SPCF-TH0 cohorts had a small, flattened, and overcrowded PCF. However, the SPCF-TH0 group had less PCF size and greater narrowing of the upper cervical canal than the SPCF-CM1 group (Fig. 2d, Tables 3 and 4). ROC analysis identified that for obex-McR (AUC 0.861), a value greater than or equal to 2.5 mm was 88% sensitive and 75% specific in differentiating SPCF-TH0 from SPCF-CM1. VER-ISCC was increased in the SPCF-TH0-only group but not the SPCF-CM1-only group. TR C1-7 was reduced in the SPCF-TH0-syr group but not the SPCF-CM1-only group (Tables 1 and 2). The SPCF-TH0-only PCF phenotype differed from other CM cohorts (SPCF-TH0-syr, SPCF-CM1-only, SPCF-CM1-syr) (Fig. 2g; Tables 1 and 2) because (1) CL, SO were longer and S pf was greater, (2) Z (total height of the PF) was taller, (3) VER-ISCC (the upper CSF space of PCF) was greater, and (4) S fm was lower. ROC analysis identified that a value of S fm (AUC 0.725) less than or equal to 777 mm² was 70% sensitive and 63% specific in differentiating SPCF-TH0-only from the other CM cohorts and CON.

Clinical findings

The age of onset of clinical symptoms did not differ significantly between groups: SPCF-TH0-only (28 ± 12 years), SPCF-CM1-only (29 ± 15 years), SPCF-TH0-syr (30 ± 19 years), and SPCF-CM1-syr (30 ± 13 years). Tables 5 and 6 show the clinical findings in the patient cohorts. Clinical symptoms and signs did not differ significantly between the SPCF-TH0-only and SPCF-CM1-only groups, the SPCF-TH0-syr and SPCF-CM1-syr groups, nor the combined SPCF-TH0 group and combined SPCF-CM1 group. Figure 2 e and f show MRI and clinical data of the SPCF-TH0-only group.

Table 5.

Clinical findings in SPCF-TH0-only, SPCF-CM1-only, SPCF-TH0-syr, SPCF-CM1-syr patients' groups

Clinical signs and symptoms	SPCF-TH0-only (n = 30)		SPCF-CM1-only (n = 30)		SPCF-TH0-syr (n = 25)		SPCF-CM1- syr (n = 30)
	n	%	n	%	n	%	n	%
CSF obstruction
Valsalva- or strain-induced occipital/upper cervical pain/headache	15	50	14	47	7	28	12	40
Hydrocephalus	1	3	3	10	6*	25^	7**	24
Compression of brainstem, cerebellum, or cranial nerves
Swallowing difficulty/choking/aspiration, dysphagia	1	3	5	17#	7	28^	12	40
Hoarseness/dysarthria	3	10	3	10	7	28	6	20
Absent gag reflex	1	3	2	7	4	16	4	13
Central sleep apnea/snoring	0	0	0	0	1	4	1	3
Downbeat nystagmus	0	0	2	7	1	4	3	10
Truncal ataxia	15	50	16	53	13	52	22	73
Tinnitus	8	27	9	30	6	24	9	30
Vertigo/dizziness	16	53	16	53	10	40	15	50
Autonomic symptoms (syncope, drop attacks, sinus bradycardia)	6	20	4	13	3	12	5	17
Trigeminal/glossopharyngeal neuralgia	2	7	0	0	1	4	2	7
Trigeminal sensory loss	6	20	2	7	2	8	6	20
Tongue weakness/deviation	1	3	0	0	2	8	0	0
Palatal weakness	1	3	0	0	1	4	2	7
Spinal cord dysfunction (syringomyelia)
Upper motor neuron signs	3	10	4	13	7	28	8	27
Lower motor neuron signs	5	17	7	23#	19	76^	23	77
Pain and temperature sensory loss	6	20	9	30#	20	80^	25	83
Spasticity	0	0	0	0#	1	4	6	20
Scoliosis (primarily thoracic levoscoliosis)	4	13	3	10	9	36	8	27
Motor weakness	5	17	4	13#	18	72^	14	47

^(*)6 from 24

^(**)7 from 29; statistically significant differences (p < 0.05):

^{(^)}with SPCF-TH0-only group

^(#)with SPCF-CM1-syr group

Table 6.

Clinical findings in CM all, SPCF-only, SPCF-syr, SPCF-TH0, and SPCF-CM1 cohorts

Clinical signs and symptoms	CM all (n = 115)		SPCF-only (n = 60)		SPCF-syr (n = 55)		SPCF-TH0 (n = 55)		SPCF-CM1 (n = 60)
	n	%	n	%	n	%	n	%	n	%
CSF obstruction
Valsalva- or strain-induced occipital/upper cervical pain/headache	48	42	29	48	19	35	22	40	26	43
Hydrocephalus	17*	15	4	7^	13**	25	7*°	13	10*~	17
Compression of brainstem, cerebellum, or cranial nerves
Swallowing difficulty/choking/aspiration, dysphagia	25	22	6	10^	19	35	8	15	17	28
Hoarseness/dysarthria	19	17	6	10	13	24	10	18	9	15
Absent gag reflex	11	10	3	5	8	15	5	9	6	10
Central sleep apnea/snoring	2	2	0	0	2	4	1	2	1	2
Downbeat nystagmus	6	5	2	3	4	7	1	2	5	8
Truncal ataxia	66	57	31	52	35	64	28	51	38	63
Tinnitus	32	28	17	28	15	27	14	25	18	30
Vertigo/dizziness	57	50	32	53	25	45	26	47	31	52
Autonomic symptoms (syncope, drop attacks, sinus bradycardia)	18	16	10	17	8	15	9	16	9	15
Trigeminal/glossopharyngeal neuralgia	5	4	2	3	3	5	3	5	2	3
Trigeminal sensory loss	16	14	8	13	8	15	8	15	8	13
Tongue weakness/deviation	3	3	1	2	2	4	3	5	0	0
Palatal weakness	4	3	1	2	3	5	2	4	2	3
Spinal cord dysfunction/syringomyelia
Upper motor neuron signs	22	19	7	12^	15	27	10	18	12	20
Lower motor neuron signs	54	47	12	20^	42	76	24	44	30	50
Pain and temperature sensory loss	60	52	15	25^	45	82	26	47	34	57
Spasticity	7	6	0	0^	7	13	1	2	6	10
Scoliosis (primarily thoracic levoscoliosis)	24	21	7	12^	17	31	13	24	11	18
Motor weakness	41	36	9	15^	32	58	23	42	18	30

^(*)17 from 113

^(**)13 from 53

^(*°)7 from 54

^(*~)10 from 59

statistically significant differences (p < 0.05):

^{(^)}with SPCF-syr

Discussion

Our study identified PCF morphometrics that differentiated cohorts of patients with TH < 2 mm and TH > 5 mm and with and without syringomyelia from each other and a control (CON) group.

Using the CM1 MRI diagnostic standard of 5 mm or more of TH, a radiologic diagnosis of CM1 could be made in 0.8–0.9% of normal adults and 1.0–3.6% of children undergoing MRI [25, 42, 49]. The prevalence of symptomatic CM1 is much lower, only 0.01–0.04% [19]. The CM0 term was introduced to describe cases of cervical syringomyelia with less than 3 mm of cerebellar TH that resolved after PCF decompression [22, 46]. Some authors argued that CM0 was a CM1 variant [2, 24] because patients with CM0-syringomyelia had an SPCF and caudally located obex like CM1-syringomyelia [4, 6, 46]. The CM0 term was later applied to patients with Chiari-like symptoms, an SPCF, abnormal PCF configuration, and less than 3 mm of TH, even in the absence of syringomyelia [12, 28, 48]. A short clivus and wider angle of the tentorium to Twining line was found in 22 patients with CM1-like signs and symptoms, less than 3 mm of TH, and no syringomyelia (SPCF-TH0-only) [38]. In another study, eight PCF morphometric measures predicted which patients with CM0 (9 patients with and 5 without syringomyelia) and CM1 would be symptomatic [28]. Another study demonstrated that CM0 patients with (3 patients) and without (4 patients) syringomyelia had smaller PCF area, PCF width, and clivus length compared to controls [48]. In another study of 98 patients with MRI findings of cerebellar tonsils within but not below the FM, craniocervical decompression significantly reduced CM1-like symptoms and signs [18]. Another study of patients with CM1-type symptoms, no syringomyelia, and less than 3 mm of TH, showed that PCF underdevelopment around the FM led to mild hindbrain ptosis [32]. These studies used the term “CM0” to describe patients with CM signs and symptoms, less than 3 mm of TH, and no syringomyelia.

In one surgical series, CM-related cervical syringomyelia arose from CM1 in 96.3% and CM0 in 3.7% of cases [9]. One study of the prevalence of small PCF without syringomyelia reported that 8% of people who underwent head MRI had tight cisterna magna and short clivus length [51]. About 25% of adults 21–50 years of age who underwent head MRI had at least 2 mm of TH [39]. Minimal TH can occlude the FM and predispose to acute development of CM1 symptoms [17].

Our results agree with earlier reports comparing classical CM1 (TH ≥ 5 mm) to CM0 (TH < 2 mm) morphometrics [6, 12, 38, 48]. In our study, underdeveloped PCF was present in all 4 CM groups [8, 26, 33, 34, 48] (Tables 1 and 2). Similar MRI morphometric features were reported in five CM0 with S and seven CM1 subjects from families affected by CM [24]. In seven families with multiple members with CM, 46 subjects had MRI-findings of small PCF, 17 of which had CM1 (TH 5 mm and more) with or without S, 9 had “intermediate” CM1 (ranging from 3 to 5 mm) with or without S, and 20 had CM0 (TH < 3 mm) with or without S (Suppl. Table 1 in [30]). Patients with CM1 and CM0 related to PCF hypoplasia manifest identical symptoms and signs consistent with a CM phenotype [6, 24, 38].

Angular measurements in all our CM groups demonstrated horizontal inclination of the clivus and PCF flattening compared to CON. An increase in EI, but not into the hydrocephalus diagnostic range, was seen in all CM groups compared to CON. Constriction of the fourth ventricular outlets or PCF subarachnoid space could have obstructed CSF flow sufficiently to expand the lateral ventricles in some CM patients. The PF total height (Z) was the same as in CON only in the SPCF-TH0-only cohort, suggesting that enlargement of the superior part offset the underdevelopment of the inferior part of the PCF in that cohort.

Abnormal PCF morphometric features in the CM cohorts

Consistent with other morphometric studies of classical CM1 and CM0 with and without syringomyelia, our patient groups had shorter CL, SO, h, and S pf b than CON [6, 21, 37, 38, 43]. Reduction of S pf b reflects hypoplasia of the inferior part of the PCF [32]. The SPCF-TH0 and SPCF-CM1 cohorts compared to control also had (1) reduced P-FM and C-FM, measures of PCF shallowness and reduced distance between neural and bony structures [21, 48]; (2) reduced distance between the cerebellar vermis and splenium of the corpus callosum (VER-ISCC) [31] characteristic of crowded PCF in CM1 [44]; (3) reduced volume of the cisterna magna (V cm) typical of classical CM1 [26] and CM0 [23]; (4) ventriculomegaly involving the lateral and 3^rd ventricles; (5) retro-odontoid tissue hypertrophy constricting the ventral CSF space at the cervicomedullary junction (ROST) [15], presumed to develop from atlantoaxial ligament strain and atlantoaxial hypermobility; (6) narrowing of anatomical angles (C-SO, CLgr., BoA) and flattening of the PCF [4, 50, 52]; and (7) reduced CSF space within the FM (S csf/S fm) [43] due to CSF displacement by the presence of the tips of the tonsils in CM0 and the full tonsils in CM1 [43].

All patients had a small, flattened, and overcrowded PCF and TH, and some had syringomyelia. The anteroposterior diameter of the FM in our study and Botelho’s [8] was not elongated, which was proposed [1, 10, 21] to facilitate TH in CM [1]. We noted and others noted a trend toward reduced FM anteroposterior diameter in the SPCF-TH0 groups [38] (Table 1). The CM0 and CM1 groups had similar M-FVV distance, signifying cerebellar deformity within a crowded PCF [45], and included patients with cervicomedullary kinking. We identified five measurements (P-FM, C-FM, F-FM, BA, CAA) that differentiated SPCF-CM1 (TH ≥ 5 mm) from SPCF-TH0 (TH < 2 mm) and controls (Table 1). The SPCF-TH0 groups had normal P-FM and C-FM (Table 1), as previously reported [48].

Characteristic PCF morphometric features of CM1

By definition, CM1 patients have more TH than CM0 patients. The SPCF-CM1 cohort had significantly greater odontoid retroversion (C2DRv) and a lower obex position (obex-McR) [14, 46] than the SPCF-TH0 cohort. The obex was positioned below McRae's line (FM plane) in the SPCF-CM1 cohort, slightly above it in the SPCF-TH0 cohort, and significantly superior to it in controls. The SPCF-CM1 cohort compared to the SPCF-TH0 cohort had (1) smaller depth of ventral CSF spaces (v) at the FM (especially SPCF-CM1-syr patients) and (2) more pronounced reduction of SO, h, P-FM, C-FM, F-FM, S pf b area, and especially V cm (Tables 3 and 4). Reduced supraocciput length (SO) was reported previously in CM1 [33, 36].

Characteristic PCF morphometric features of SPCF-TH0

The SPCF-TH0-only group had reduced FM (S fm) area, increased pB-C2 distance, and constriction of the ventral CSF space compared to CON. The obex (Obex-McR) and medulla in the SPCF-TH0 groups were caudally positioned compared to CON. The SPCF-TH0-only group compared to SPCF-CM1-only had (1) longer VER-ISCC and F-FM distances, (2) larger cisterna magna volume (V cm), (3) less frequent absence of the cisterna magna (E-CM), and (4) significantly reduced S fm, apsc C1, and apsc C6 (Tables 1 and 2). The SPCF-TH0-only cohort had narrowing of CSF pathways, increased pB-C2 distance, reduced FM area, reduced apsc C1 diameter, and slightly less reduction of PCF volume compared to SPCF-CM1-only. The FM area was smaller, narrower, and tighter (S fm, S sc, S tt) in the joint SPCF-TH0 group than the collective SPCF-CM1 group. The location of the hypoplasia, tightness, and neural crowding in the SPCF-TH0 group was in the PCF immediately above and within the FM and cervical spinal canal. In contrast, the superior PCF was less crowded in the SPCF-TH0 than the SPCF-CM1-only group. Patients with syringomyelia (SPCF-TH0-syr) had the least cervical spinal canal anterior-to-posterior depth (apsc C1, apsc C2, apsc C4, and apsc C6).

The caudal PCF, FM, and upper cervical spinal canal form a bony funnel around the medulla, tonsils, and CSF lumina. Axial MRI of the PCF, FM, and upper cervical spinal canal can detect bony and soft tissue structures constricting CSF pathways and promoting development of syringomyelia in CM0 and CM1 [17, 32, 37, 43]. Odontoidal retroflexion and retro-odontoidal tissue hypertrophy can constrict ventral CSF pathways in both CM0 and CM1. In CM0 patients, axial MR-images can show abnormal wrapping of the cerebellar tonsils around the medulla when the tonsils appear normal on midsagittal MR-images [7, 17, 29].

Characteristic PCF morphometric features of CM patients with syringomyelia

Previous studies of PCF and upper cervical spinal canal morphometrics and CSF flow in CM1 patients with and without syringomyelia identified factors associated with syringomyelia [37, 42, 52]. In our study, syringomyelia in SPCF-TH0 patients was associated with more significant tapering of the cervical spinal canal (TR C1-7) [37]. Several factors that reduced PCF volume and increased crowding were associated with syringomyelia, including (1) shorter distance from basion to the peak of the tentorium (Z), from the culmen of cerebellar vermis to the splenium of the corpus callosum (VER-ISCC), and from the corpus callosum to the FM (C-FM); (2) decreased PCF area (S pf); (3) narrower ventral CSF space at the FM (v) [6, 20]; (4) lower clivus gradient (CLgr) [52]; and (5) smaller PCF volume. Comparing the SPCF-TH0-syr group with SPCF-TH0-only group, we found that the SPCF-TH0-syr group had significantly shorter anterior–posterior diameter of the cervical canal at C1 (apsc C1), greater tapering of the cervical spinal canal (TR C1-4, TR C1-7), and greater PCF “crowdedness” (VER-ISCC, S pf), underscoring that upper cervical spinal canal narrowing and PCF crowding predisposes to syringomyelia in CM0 patients. The hindbrain was caudally located in SPCF-TH0-only and CM patients. SPCF-TH0 narrowed the inferior PCF around the FM area while SPCF-CM1 reduced PCF volume more broadly. The spinal canal cross-sectional area caudal to the FM (S fm, apsc C1) was smaller in SPCF-TH0 than SPCF-CM1. SPCF-CM1 had greater TH, more inferior obex positioning, and greater odontoid retroversion than SPCF-TH0. Reduced PCF volume and neural element crowding associated with syringomyelia in both SPCF-TH0 and SPCF-CM1. Our study's type, frequency, and severity of clinical signs and symptoms replicated those of other studies of patients with a small PCF and CM1 or CM0 [13, 17, 26, 38]. Only patients with syringomyelia had signs and symptoms of cervical central myelopathy (Table 5).

Our study has some limitations. Patients were diagnosed as having a small PCF if their clivus and supraocciput lengths were less than 40 mm, not by confirming a small PCF by volumetric measurements. We also excluded patients with complex Chiari, basilar invagination > 5 mm, evidence of spinal instability, or atlantooccipital assimilation, so our morphometric values could vary from those of other studies in which complex Chiari patients were included.

In conclusion, reliable radiographic criteria to diagnose SPCF-TH0 and SPCF-TH0-syr (CM0) cases are essential. Symptomatic cases of SPCF-TH0 without syringomyelia have Chiari 1 symptoms and signs, tonsillar ectopia less than 2 mm, and morphometric evidence of a small PCF. Patients with SPCF-TH0-syr (CM0) often have signs of cervical myelopathy from syringomyelia.

Abbreviations

BI

Basilar invagination

CM

CM1 and CM0

CM0

Chiari type 0 malformation

CM1.5

Chiari malformation type 1.5

CM1

Chiari type 1 malformation

CON

Control group

CVJ

Cranio-vertebral junction

FM

Foramen magnum

PCF

Posterior cranial fossa

S

Syringomyelia

SPCF

Small posterior cranial fossa

SPCF-TH0-only

Patients with SPCF, Chiari 1 symptoms, tonsillar ectopia less than 2 mm, and without S

SPCF-TH0-syr

CM0 with SPCF and with S

SPCF-CM1-only

CM1 with SPCF and without S

SPCF-CM1-syr

CM1 with SPCF and with S

SPCF-TH0

Joint group SPCF-TH0-only and SPCF-TH0-syr

SPCF-CM1

Joint group SPCF-CM1-only and SPCF-CM1-syr

SPCF-only

Joint group SPCF-TH0-only and SPCF-CM1-only

SPCF-syr

Joint group SPCF-TH0-syr and SPCF-CM1-syr

TH

Tonsillar herniation